Old Fields, Adam Clark, Department of Ecology, Evolution, and Behavior, University of Minnesota

Introduction General Overviews Journals Historical Understanding of Succession Concepts of Old Fields Origination Mechanisms Long-Term Successional Trends Potential Successional End States Soil Formation and Dynamics Global Extent of Old Fields Common Study Methods and Topics Chronosequences and Time-series Analysis of Community Dynamics Non-Plant Communities Theoretical FrameworksExamples of Long-Term Studies Drivers of Contingency Species Interactions and Invasion Grazing Fire and Litter Feedbacks Dispersal Limitation Broader Implications Ecological Understanding Ecosystem Management Introduction Old fields are ecosystems that form on previously human-managed land after management has ceased. Traditionally, the term “old field” is reserved for abandoned agricultural or pasture land, though many other kinds of management (e.g. strip mines, landfills) lead to similar systems. Old fields are therefore unique among ecosystems in two major ways. First, they form exclusively as a result of human interventions, and second, they are almost always transient in nature, meaning that in the absence of subsequent management, old fields will eventually transform into some other kind of ecosystem. Because of this, old fields are often discussed in terms of their “age”, which describes how long ago management ceased, and their “trajectory”, which describes how they change over time. This has meant that the study of old fields is intimately related to the concept of “succession”, which broadly concerns the response of natural systems to disturbances (see Oxford Bibliographies article on *Succession*). As with succession, historical notions of how old fields develop have followed a broad conceptual arc. In the beginning of the twentieth century, it was generally assumed that any abandoned landscapes in similar locations and climates would follow identical and predictable successional trajectories, and would ultimately proceed towards a “climax” that resembled the pre-disturbance state. However, this model was abandoned relatively quickly, and replaced with theories that suggested that multiple climax states were possible depending on differences in initial conditions and, potentially, stochastic events. Depending on the interpretation, predicting the specific trajectory that any particular old field might be expected to follow could therefore either be difficult, or entirely impossible. Since the second half of the twentieth century, there has been a rapid expansion of empirical data on old field succession, both through repeated observations taken from individual old fields over time, and through “chronosequences”. Chronosequences, also called “chronoseries “ or “space-for-time substitution”, approximate temporal

dynamics by stitching together records from many separate old fields of different ages. Studies based on chronosequences are generally more common than are long-term studies of individual fields, though because stochastic events may lead to different successional trajectories among the different fields in the chronosequence, there is a longstanding debate questioning their utility as an indicator of true temporal dynamics. In general, empirical studies have led to support for both predictability and stochasticity in successional dynamics, though it seems that at least some aspects of old fields, such as species richness or the relative abundance of functional groups, often follow predictable trajectories. Contemporary research on old field succession is particularly active in the field of restoration ecology (see Oxford Bibliographies in Ecology article *Restoration Ecology[obo-9780199830060-0109]*), where it has been shown that a mixture of active management and natural successional dynamics might be employed as a cost-effective method for restoring landscapes after intensive land use. General Overviews A recent compilation in a book edited by Cramer and Hobbs (2007) includes excellent summaries of the history of the study of old fields, and a broad set of case studies from research sites around the world. A much briefer review and synthesis was also published in Cramer et al. 2008 as a journal article. Old fields, and in particular studies of succession in old fields, have also been a major component of many empirical and theoretical advances in ecology. An extensive bibliography including over one thousand such studies sorted by subject and region is available in Rejmánek and Van Katwyk 2005. An older study in Odum 1960 focuses on a specific site in South Carolina, but is notable as an early application of rigorous quantitative methods for tracking the flow of energy and materials over the course of succession, and strongly influenced much of the subsequent work in the field. Additionally, a number of general reviews of plant succession include specific discussions about old field succession. Whittaker 1953 reviews evidence from old fields in North America and Europe to suggest that succession should be studied as an emergent property of the interaction of many individual plant species, which remains the dominant paradigm today. McIntosh 1981 provides a useful review of theoretical studies on succession, and identifies some common misconceptions about earlier studies in the literature. Finally, Pickett et al. 1987 provides a review of both theoretical and empirical studies on succession, with a broad array of examples drawn from old fields.

Bazzaz, F. A. 1979. The physiological ecology of plant succession. Annual Review of Ecology and Systematics 10:351-371. Review article discussing physiological adaptations of plants to the varying conditions found during succession from an open field habitat to a closed canopy forest (e.g. changes in light availability, temperature, or soil moisture). Though the review is technically applicable to succession in general, the specific focus is well-suited for old fields in particular. Cramer VA, Hobbs RJ eds. 2007. Old fields: Dynamics and restoration of abandoned farmland. Washington, DC: Island Press. This book is currently the most extensive modern source on old fields. A broad review covering the history of the discipline, current theory and empirical work, and case studies from sites around the world. For several of these case studies, the work also includes novel analyses and results that have not been published elsewhere. Cramer V, Hobbs R, Standish R. 2008. What’s new about old fields? Land abandonment and ecosystem assembly. Trends in Ecology & Evolution 23:104-112.

A journal article summarizing and extending the discussion in Cramer and Hobbs 2007. Also presents a novel conceptual framework for categorizing and studying different types of old field succession, focusing on how the legacy of land use and environmental conditions at a site will influence recovery. McIntosh, R. P. 1981. “Succession and ecological theory.” In Forest Succession. Edited by D. C. West, H. H. Shugart, and D. B. Botkin, 10-23. Springer New York, NY. A general review of existing theory on succession at the time of publication, with a particular focus on Clements 1916, cited in *Historical Understanding of Succession*. Important both as a link between old field succession and modern ecosystem ecology, and as a careful reconsideration of early ideas about succession. Odum, E. P. 1960. Organic production and turnover in old field succession. Ecology 41:34-49. Study of succession in an old field in South Carolina, USA. Is of general importance because it represents one of the first published sources on old fields that included quantitative discussion of environmental characteristics, and biomass dynamics for individual species, total net primary productivity, and leaf litter. Pickett, S. T. A., S. L. Collins, and J. J. Armesto. 1987. Models, mechanisms and pathways of succession. The Botanical Review 53:335-371. Broad review of evidence and theory for succession, with primarily examples and theory drawn from old fields. Includes particular emphasis on limitations of existing theory at the time of publication. Similar to a longer and more recent review in Pickett and Cadenasso 2005, cited in *Historical Understanding of Succession*. *Rejmánek, M., and K. P. Van Katwyk. 2005. *Old-field succession: A bibliographic review (19011991)[http://botanika.bf.jcu.cz/suspa/pdf/BiblioOF.pdf]*. Extensive bibliography of references related to old field succession, organized by subject. Includes over 1500 references, with categories relating to geographic regions, species groups, environmental characteristics, types of disturbance, types of mechanisms, and much more. Whittaker, R. H. 1953. A consideration of climax theory: The climax as a population and pattern. Ecological Monographs 23:41-78. A synthesis paper reviewing evidence for American and European theories about succession. Is important as an early, strong criticism of theories that suggested succession ended in one or more stable “climax” states, instead arguing that succession arises from individual interactions among species and environments. This has strongly influenced modern concepts of the discipline. Journals Though many journals have published articles that include information about old fields, roughly half of all articles appear in about 50 journals. By far the largest number of articles appear in the journal **Ecology**, with **Plant Ecology** (previously published under the title Vegetatio) as a somewhat distant second, with roughly two thirds as many articles. Even in these journals, there are generally fewer than ten articles per year published on old fields. Since about 2005, the highest impact ecological journal, **Ecology Letters**, has also begun to publish several articles per year on old fields. Three journals, **The American Naturalist**, **Ecological Monographs**, and **The Journal of Ecology**, also regularly publish articles on old fields, but are of particular importance because they published several of the seminal papers in the field, primarily between ca. 1970 and 2000. Finally, two journals, **Ecological Engineering** and **Restoration Ecology**, are important as new sources for studies on how old field

successional dynamics might be used to restore degraded land, or provide other sorts of ecosystem functions and services. The American Naturalist General journal for ecology, evolution, and behaviour, started in 1867. Published several of the major influential articles on old field succession from ca. 1975-1995, with a particularly strong history for theoretical synthesis of species interaction mechanisms. http://www.journals.uchicago.edu/toc/an/current. Ecological Engineering An ecology journal with a focus on “designing, monitoring, or constructing ecosystems”. Includes several recent studies on restoration in old fields, particularly in Europe. https://www.journals.elsevier.com/ecological-engineering. Ecological Monographs Long-form journal published by the Ecological Society of America, meant to record “integrative and complete documentation” for empirical and theoretical studies. Includes several classical papers on old fields from the middle of the twentieth century, and also some newer articles. http://esajournals.onlinelibrary.wiley.com/hub/journal/10.1002/(ISSN)1557-7015/. Ecology Flagship journal of the Ecological Society of America, including both theoretical and empirical studies, but generally in a much shorter format than Ecological Monographs. Ecology has likely published more articles on old fields than any other journal, with several articles per year since the second half of the twentieth century, including at least a half dozen of the most influential papers in the field. http://esajournals.onlinelibrary.wiley.com/hub/journal/10.1002/(ISSN)1939-9170/. Ecology Letters One of the most influential journals in contemporary ecology by any metric, published by the French National Centre for Scientific Research. In general, papers are relatively short-form, and include both empirical data and theoretical justifications. The journal has published a substantial number of papers on old fields, particularly since 2004. http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1461-0248. The Journal of Ecology Published by the British Ecological Society, focusing on “the ecology of plants (including algae), in both aquatic and terrestrial ecosystems”. Has a particularly strong history from roughly 1970 to the present for publishing results from in-depth studies of old fields at specific sites, but also includes highly influential theoretical papers. http://besjournals.onlinelibrary.wiley.com/hub/journal/10.1111/(ISSN)1365-2745/. Restoration Ecology Published by the Society for Ecological Restoration, this is an interdisciplinary journal covering a broad set of goals related to restoration. The journal has published an increasingly large number of articles on old fields, particularly since 2010, many of which focus on how to promote ecological restoration through low-input management methods that take advantage of natural successional mechanisms. http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1526-100X. Plant Ecology/Vegetatio The journal focuses primarily on studies of vascular plants, with a strong emphasis on empirical work. Renamed from Vegetatio to Plant Ecology over 1996-1997, this journal has likely published more papers on

old fields than any journal other than Ecology, and has been particularly active in the field since 1980. https://link.springer.com/journal/11258. Historical Understanding of Succession While not the first academic paper on succession, Cowles 1899 was the first rigorously documented scientific study of succession in North America. The paper is also notable for being one of the first applications of chronosequences – an approach that compares mutliple sites in different stages of succession in order to try to reconstruct successional dynamics over time. Clements 1916 represented the next major publication on succession, and is generally regarded as the seminal work on successional dynamics. Though it is frequently interpreted to suggest that ecosystems undergo succession in a process analagous to organismal development, or the “organismal” view of succession (as opposed to “individualistic” views that focuses on the interactions of many individual species), the article focuses primarilly on detailed descriptions of commonly observed patterns in old field succession. Two subsequent papers, Gleason 1926 and Tansley 1935, are generally considered as reactionary responses against organismal theories of succession, though they are also important in their own right as they helped establish the modern individualistic view of succession that is still largely considered to be the dominant paradigm. Houston et al. 1987 is notable as one of the first attempt to describe succession as the result of explicitely modeled interactions among species. Lortie et al. 2004 represents an important synthesis of ideas from both Clements and Gleason. Pickett and Cadenasso 2005 is a broad review of successional literature since Cowles, with a particular emphasis on old fields. Finally, Real and Brown 2012 is an annotated collection of classical papers, including many on succession, and is particularly valuable as a summary of how ideas from these papers are currently interpreted by ecologists.

Cowles, H. C. 1899. The ecological relations of the vegetation on the sand dunes of Lake Michigan. Botanical Gazette 27:95-117. First scientific publication on succession in North America. The work is particularly important because it championed the use of “chronosequences” (also called “chronoseries” or “space-for-time substitution”) by suggesting that the successional trajectory of sand dunes along Lake Michigan could be predicted by comparing spatially segregated regions that had been disturbed at different times in the past. Clements, F. E. 1916. Plant succession: An analysis of the development of vegetation. Washington, DC: Carnegie Institution of Washington. Frequently cited as the seminal work on old field succession. Posited a series of successional stages resulting from secondary disturbances (e.g. agricultural abandonment), ending in a predictable “climax” community. Subsequent authors associated this work with “organismal” views of succession, whereby community assembly follows a process akin to ontological development, though it has been more recently noted that this could constitute a misreading of the original source (e.g. McIntosh 1981, cited in *General Overviews*). Gleason, H. A. 1926. The individualistic concept of the plant association. Bulletin of the Torrey Botanical Club 53:7. Classic paper rejecting the “organismal” view of succession. Gleason suggests an “individualistic” view as an alternative, whereby interactions between individual plants and between plants and their environments determines successional outcomes (similar to the dominant paradigm today). Importantly, also discusses how this individual view implies that trajectories and outcomes in succession may be unpredictable. Huston, M., and T. Smith. 1987. Plant succession: Life history and competition. The American Naturalist 130:168-198.

Introduces a theoretical framework for studying succession, focusing on how species change their environment, and how these changes lead to interspecific competition. Includes discussion of “LotkaVolterra”, resource competition, and individual-based simulation models. Importantly, showed that simple models parametrized with species traits could explain many of the complex successional patterns observed in nature. Lortie, C. J., R. W. Brooker, P. Choler, Z. Kikvidze, R. Michalet, F. I. Pugnaire, and R. M. Callaway. 2004. Rethinking plant community theory. Oikos 107:433-438. A modern synthesis of aspects of both Clements 1916 and Gleason 1926. Suggests that community assembly (and therefore succession) depends on a mixture of stochastic outcomes (e.g. dispersal or chance disturbance events), and deterministic processes, such as species environmental tolerances, positive and negative interactions among species, and indirect interactions caused by changes in competing species’ abundances. Pickett, S. T. A., and M. L. Cadenasso. 2005. “Vegetation dynamics”. In Vegetation Ecology. Edited by Eddy van der Maarel, 172-198. Blackwell Science Ltd, Oxford, UK. Probably the best starting place for a modern review of succession. Uses term “vegetation dynamics” to distinguish contemporary models from older notions that assumed predictable successional trajectories and climaxes. Also includes a definition of the modern concept of succession, used by many other sources (e.g. Cramer and Hobbs 2007, cited in *General Overviews*). Real, Leslie A., and James H. Brown, eds. 2012. Foundations of ecology: Classic papers with commentaries. Chicago, USA: University of Chicago Press. A compendium of classic papers with synopses describing their content and historical relevance. Includes references from Cowles, Clements, Gleason, and Tansley. An invaluable and widely-used primer for beginning students. Note, however, that because there is often no consensus on the “proper” interpretation of many of these articles, reading the primary articles, in addition to the annotated commentary, is vital. Tansley, A. G. 1935. The use and abuse of cegetational concepts and terms. Ecology 16:284-307. A very useful review of the state of theory at the time, though also a self-described “blunt” (and often amusing) critique of “organismal” and “climax” views of succession. Introduces the concepts of “autogenic” vs. “allogenic” factors in succession (self-generated vs. external factors, respectively). Is also the reference in which Tansley coins the term “ecosystem”. Concepts of Old Fields By its narrowest definition, the term “old field” is generally used to describe the succession of naturally invading plant species found in cropland following agricultural abandonment. The first few pages of Cramer and Hobbs 2007 includes a somewhat broader discussion, and notes that many studies include abandoned pasture land as a form of old field as well. Importantly, the authors summarize their discussion in terms of disturbance: Regardless of the mechanisms, old fields always result from some kind of anthropogenic disturbance, and they are almost always transient. The “classical” conceptual model found in many textbooks generally describes a much more specific transition among vegetation types, proceeding from herbaceous and grassland species to forests. Egler 1954 includes an early conceptual figure describing this type of transition in detail, and the well-known Buell-Small Succession Study (see *Examples of Long-Term Studies* for more information on this site) described in Myster and Picket 1994 is a good example of a site that follows these dynamics. Because they also always result from some kind of anthropogenic land use, old fields can also be important to cultural and aesthetic views

of landscapes, as described in Etienne et al. 1998. Finally, Ratajczak et al. 2012 and Suding et al. 2004 are both important examples of contingency in succession, which is increasingly thought to be an important process in most old fields, and show how differences in starting conditions, environment, or other factors can dramatically alter successional landscapes. Cramer VA, Hobbs RJ eds. 2007. Old fields: Dynamics and restoration of abandoned farmland, pp. 1-2. Washington, DC: Island Press. Discussion of the types of definitions applied to old fields in the first chapter of a longer review book (see *General Overviews* for more details on the source). Lists broad array of potential disturbance types that could be associated with old fields, though notes that the primary focus is generally on agriculture or pasture. Egler, F. E. 1954. Vegetation science concepts I. Initial floristic composition, a factor in old-field vegetation development with 2 figs. Vegetatio Acta Geobotanica 4:412-417. Discussion of contemporary definitions and hypotheses relating to old field succession. One of the first papers to include a conceptual diagram of temporal dynamics in old fields. Suggests two primary factors: “Relay floristics”, describing temporal changes in community composition, and “initial floristic composition”, describing initial conditions. Argues that initial conditions may be much more important. Etienne, M., J. Aronson, and E. L. Floc’h. 1998. “Abandoned lands and land use conflicts in southern France”. In Landscape Disturbance and Biodiversity in Mediterranean-Type Ecosystems. Edited by P. W. Rundel, G. Montenegro, and F. M. Jaksic, 127-140. Springer Berlin Heidelberg, Berlin, Heidelberg, GE. Description of conflict in southern France between managing abandoned land as forests, or maintaining them as old fields. Reviews the “ecological, historical, and socio-economic” role of old fields, and, importantly, argues that old fields may merit conservation even if they are a uniquely anthropogenic habitat. Myster, R. W., and S. T. A. Pickett. 1994. A comparison of rate of succession over 18 years in 10 contrasting old fields. Ecology 75:387-392. Shows evidence for decline in rate of old field community change over time, consistent with expectations for succession towards a semi-equilibrium state. But, also shows that changes are not monotonic, and that they may depend on many different factors, such as the last agricultural crop grown, and the year, season, and method of abandonment. Ratajczak, Z., J. B. Nippert, and S. L. Collins. 2012. Woody encroachment decreases diversity across North American grasslands and savannas. Ecology 93:697-703. Shows that woody encroachment into grasslands and savannas leads to loss of diversity, with larger losses in wetter regions. Is an important counter-narrative to general notions that species richness increases over time as woody plants colonize grasslands. Note, however, that this article focuses on woody encroachment into natural grasslands and savannas after fire suppression, rather than forest recovery in abandoned old fields. Suding, K. N., K. L. Gross, and G. R. Houseman. 2004. Alternative states and positive feedbacks in restoration ecology. Trends in Ecology & Evolution 19:46-53. Discussion of alternative stable states in ecosystems, and how they might impact restoration efforts. Presents evidence and theory showing why some degraded habitats may not recover naturally, or may recover to different kinds of habitats than existed before disturbance. Alternative to classical hypotheses about unidirectional succession.

Origination Mechanisms The “classical” view of old fields is that they originate as abandoned cropland, as is the case for sites described in Bard 1952 and Thompson et al. 2013. Agricultural abandonment might be voluntarilly triggered by changes in food production systems and agricultural policy, as described in Burke et al. 1995, or involuntarily triggered by clamities such as a pest species, as described in Stevenson 1980. Abandonment of pasture land is also a commonly accepted origination mechanism for old fields, as described in Uhl 1988. Succession following abandonment of other forms of land use, such as old mining sites or landfills, are described in Rebele and Lehmann 2002 and Mudrák et al. 2016. These sorts of landscapes are occasionally classified as old fields, though abandoned industrial sites are typically categorized as “brownfields” instead. Finally, Staver et al. 2011 describes changes in the global distribution of grasslands following shifts in climate. While anthropogenic climate change may be responsible for these new environments, they are not typically referred to as old fields.

Bard, G.E., 1952. Secondary succession on the Piedmont of New Jersey. Ecological Monographs, 22:195-215. Early paper on old field succession in the New Jersey Piedmont, USA, which helped motivate subsequent work in the Buell-Small Succession Study. Demonstrates the classical notion of origination and development for old fields, from abandoned crop land to a “presumed end-point” as a deciduous forest. Burke, I. C., W. K. Lauenroth, and D. P. Coffin. 1995. Soil organic matter recovery in semiarid grasslands: Implications for the Conservation Reserve Program. Ecological Applications 5:793-801. Description of the Conservation Reserve Program in the USA, and its impacts on soil carbon stocks. Example of how policy and changes in agricultural practices can lead to agricultural abandonment though conservation efforts. Mudrák, O., J. Doležal, and J. Frouz. 2016. Initial species composition predicts the progress in the spontaneous succession on post-mining sites. Ecological Engineering 95:665-670. Study of vegetation succession in coal mining regions in the Czech Republic, abandoned between 19651995. Demonstrates dynamics very similar to those observed following agricultural abandonment. Example of alternative to agriculture and pasture use as secondary disturbances for initiating succession. Potentially classified as “brownfield” rather than “old field”. Rebele, F., and C. Lehmann. 2002. Restoration of a landfill site in Berlin, Germany by spontaneous and directed succession. Restoration Ecology 10:340-347. Study of vegetation succession in a landfill in Germany, which was abandoned and covered in 1995. As with Mudrák et al. 2016, shows dynamics very similar to those observed following agricultural abandonment. Another example of alternative to agriculture and pasture use as secondary disturbances, and also potentially an example of a “brownfield”. Staver, A. C., S. Archibald, and S. A. Levin. 2011. The global extent and determinants of savanna and forest as alternative biome states. Science 334:230-232. Global study showing how transitions between natural grassland, savanna, and forest biomes may be explained by changes in precipitation. Suggests that climate change is likely to shift these boundaries, leading to changes in frequency and distribution of these biomes. Example of climate driven succession between grasslands and more forested states.

Stevenson, I. 1980. The diffusion of disaster: The phylloxera outbreak in the département of the Hérault, 1862-1880. Journal of Historical Geography 6:47-63. Description of the outbreak of grape phylloxera in France in the late nineteenth century, which devastated many wine growing regions and resulted in the largescale abandonment of vineyards. Example of how pest outbreaks and disease can contribute to land use change. Thompson, J. R., D. N. Carpenter, C. V. Cogbill, and D. R. Foster. 2013. Four centuries of change in northeastern United States forests. PLoS ONE 8:e72540. Shows changes in forest species composition in the north-eastern USA from pre-colonial times (ca. 1600) to the modern era. Important demonstration both of the large scale of deforestation, and the subsequent forest recovery following agricultural abandonment, in the region. Also reveals enormous changes in the relative abundance of species following recovery. Uhl, C., R. Buschbacher, and E. A. S. Serrao. 1988. Abandoned pastures in eastern Amazonia. I. Patterns of plant succession. The Journal of Ecology 76:663. Study of forest recovery in abandoned pastureland in the eastern Amazon. Land was abandoned between 2 and 8 years prior to the survey. Example of how intensity and duration of land use can influence successional trajectories. Also an important example of the surprisingly fast rate at which woody vegetation recolonizes many old fields. Long-Term Successional Trends A common long-term finding from old fields is that they accumulate biomass and species richness over time, examplified by Lohbeck et al 2015. However, there are many exceptions to this trend. For example, Bonet 2004 found a sharp increase in old field plant diversity in the first 10 years after agricultural abandonment, followed by a long, slow decline in diversity over subsequent years. Historically, it has been assumed that fields progress towards a common and predictable endstate over time. Foster and Tilman 2000 discusses a good example of a system for which this appears to be true. Nevertheless, an increasingly large number of studies find that dynamics over time can vary dramatically. For example, Fukami et al 2005 shows that replicate plant communities converged over time in terms of functional group but not species identity, and Li et al. 2015 shows an example of the common finding that old field plant communities become phylogenetically less similar to one another as they age. Finally, though a majority of studies about old fields focus on plant communities, long-term dynamics can differ substantially as well. For example, Schweiger et al. 2000 shows that mammal community dynamics appear to be independent of plant succession in young fields, but not in older fields. Alternatively Cline and Zak 2015 tracks fungal and bacterial community dynamics in old fields, and shows that these appear to be primarilly driven by changes in the plant community.

Bonet, A. 2004. Secondary succession of semi-arid Mediterranean old-fields in south-eastern Spain: Insights for conservation and restoration of degraded lands. Journal of Arid Environments 56:213-233. Study tracking old field succession in abandoned cropland along the Spanish Mediterranean coast. Particularly important because it shows a “hump-shaped” relationship between plant diversity and old field age: Diversity increases fivefold over the first 10 years after abandonment, and then slowly declines by about 50% over the next 50 years. Cline, L. C., and D. R. Zak. 2015. Soil microbial communities are shaped by plant-driven changes in resource availability during secondary succession. Ecology 96:3374-3385. Tracks bacterial and fungal communities over 16-86 years of old field succession on abandoned cropland at the Cedar Creek Ecosystem Science Reserve in Minnesota, USA. Shows that fungal communities

primarily responded to changes in soil organic matter caused by changes in plant community composition, while bacterial communities appear to be driven by changes in pH. Foster, B.L., and Tilman, D. 2000. Dynamic and static views of succession: Testing the descriptive power of the chronosequence approach. Plant Ecology 146:1-10. Compares results from across multiple fields at the Cedar Creek Ecosystem Science Reserve, Minnesota, USA to test whether different fields follow similar trajectories in plant community composition as a function of successional age. Shows a strong relationship between field age and both the rate of succession and abundance of common species, but no such relationship for species richness. Fukami, T., T. Martijn Bezemer, S. R. Mortimer, and W. H. Putten. 2005. Species divergence and trait convergence in experimental plant community assembly. Ecology Letters 8:1283-1290. Results from experimentally planted grassland communities on abandoned agricultural land in the Netherlands. Over nine years, mixtures planted with different plant communities diverged in terms of species composition, but converged in terms of plant traits. Suggests importance of functional roles, rather than species identity. Li, S., M. W. Cadotte, S. J. Meiners, Z. Hua, L. Jiang, and W. Shu. 2015. Species colonisation, not competitive exclusion, drives community overdispersion over long-term succession. Ecology Letters 18:964-973. Results from old fields in the Buell-Small Succession Study in New Jersey, USA. Finds that plant communities were colonized by species that were phylogenetically and functionally similar, followed by slow invasion of more distantly related and dissimilar species. Suggests that dispersal and colonization is more important than competition for structuring old field communities. Lohbeck, M., L. Poorter, M. Martínez-Ramos, and F. Bongers. 2015. Biomass is the main driver of changes in ecosystem process rates during tropical forest succession. Ecology 96:1242-1252. Study tracking 29 years of tropical forest recovery in abandoned maize fields in south-eastern Mexico. Finds strong increases in biomass, leaf litter production, and litter decomposition rates through time. Results suggest that change in biomass is the primary driver of these other changes. Schweiger, E. W., J. E. Diffendorfer, R. D. Holt, R. Pierotti, and M. S. Gaines. 2000. The interaction of habitat fragmentation, plant, and small mammal succession in an old field. Ecological Monographs 70:383-400. Study of mammal diversity in succession experiment where fragments of woodland of different sizes were allowed to recover after clearing. Find that over 12 years, mammal communities go from being structured by patch size to being structured by plant communities. Suggests a mixture of drivers for animal community assembly in old fields. Potential Successional End States Though the classical notion of a single, predictable “climax” at the end of old field succession has been largely abandoned (see *Historical Understanding of Succession*), there is evidence that old fields recover to something similar to their pre-disturbed state over reasonably short time scales. For example, Boecker et al. 2015 find few differences between old fields and native grasslands after 40 years of succession. More commonly, however, recoveries appear to be slow or incomplete. Sojneková and Chytrý 2015 find that even 50 years after abandonment, native plant diversity is not recovered at their site. Thompson et al. 2013 shows recolonization of almost all displaced tree species in the northeastern USA after agricultural abandonment, but the new relative abundance of species differes greatly from that

which existed before European settlement. Finally, depending on the species group, recovery success can vary greatly. For example, Hernández-Ordóñez et al. 2015 shows that amphibian and reptile diversity recovered in as little as 20-30 years, but the abundance of reptiles did not, nor did the original species composition of either group. Boecker, D., C. Centeri, G. Welp, and B. M. Möseler. 2015. Parallels of secondary grassland succession and soil regeneration in a chronosequence of central-Hungarian old fields. Folia Geobotanica 50:91-106. Study of abandoned agricultural fields in the Hungarian steps. Finds that after 40-50 years, fields are similar to 150-year-old remnant grasslands in terms of species richness and composition, soil organic matter, nitrogen mineralization, and phosphorus availability. Suggests the possibility for rapid, “autogenic” (i.e. self-sustaining) recovery. Sojneková, M., and M. Chytrý. 2015. From arable land to species-rich semi-natural grasslands: Succession in abandoned fields in a dry region of central Europe. Ecological Engineering 77:373-381. Study of agricultural fields in the south-eastern Czech Republic, abandoned in the mid-twentieth century. Fields appear to remain grasslands, rather than develop into forest. Shows that these grasslands were colonized by many native species, but were still accumulating species even after more than 50 years. Hernández-Ordóñez, O., N. Urbina-Cardona, and M. Martínez-Ramos. 2015. Recovery of amphibian and reptile assemblages during old-field succession of Tropical Rain Forests. Biotropica 47:377-388. Tracks 23 years of forest recovery from abandoned agricultural fields in south-eastern Mexico. Finds rapid recovery of amphibian and reptile richness. Amphibian richness, abundance, and diversity recover within 30 years. Richness and diversity for reptiles recovered in 20 years, but reptile abundance, and species composition for reptiles and amphibians, did not recover. Thompson, J. R., D. N. Carpenter, C. V. Cogbill, and D. R. Foster. 2013. Four centuries of change in northeastern United States forests. PLoS ONE 8:e72540. Long-term data on forest recovery from across New England (north-eastern USA). Despite intense logging, re-established forests have roughly the same species as the original forests, with the exception of American chestnut. However, there have been large shifts in relative abundance of tree species. Soil Formation and Dynamics Ploughing and agricultural use typically corresponds with sharp declines in total soil carbon and nitrogen stocks. This appears to be true even in sites that receive heavy rates of fertilizer application, as these tend to wash out of soils on relatively fast time scales. Soil carbon and nitrogen concentrations in old fields typically recover over the course of succession, but the rate of recovery is generally very slow, and it is not always clear that they will recover to their original concentrations. Post and Kwon 2000 review dynamics in soil carbon stocks across a variety of sites and environments, and confirm that concentrations typically drop dramatically during agricultural use, and then recover slowly over the course of succession. Burke et al. 1995 similarly shows very slow rates of soil carbon recovery in old fields after agricultural abandonment, suggesting that complete recovery may not occur. Knops and Tilman 2000 show that recovery from soil carbon and nitrogen loss from agricultural use in old fields could take as long as two centuries. Finally, as a point of comparison, Crocker and Major 1955 summarize results from primary succession following glacial retreat in Alaska, USA, and show that soil carbon and nitrogen recovery in these systems likewise appears to take at least two centuries.

Crocker, R.L. and Major, J. 1955. Soil development in relation to vegetation and surface age at Glacier Bay, Alaska. The Journal of Ecology 43:427-448.

Discussion of primary succession after glacier retreat in Glacier Bay, Alaska, USA. Shows that vegetation can rebuild soil from eroded bedrock substrates, but that recovery of soil carbon and nitrogen stocks can take upwards of 200 years. Boecker, D., C. Centeri, G. Welp, and B. M. Möseler. 2015. Parallels of secondary grassland succession and soil regeneration in a chronosequence of central-Hungarian old fields. Folia Geobotanica 50:91-106. Study of abandoned agricultural fields in the Hungarian steps. Finds that 40-50 years after abandonment, fields have recovered roughly the same soil organic matter content and nitrogen mineralization rates as are found in native grasslands in the region. Burke, I. C., W. K. Lauenroth, and D. P. Coffin. 1995. Soil organic matter recovery in semiarid grasslands: Implications for the Conservation Reserve Program. Ecological Applications 5:793-801. Study tracking the loss of soil organic matter in abandoned agricultural fields. Shows initial loss of soil carbon during agricultural use, followed by recovery after abandonment. However, shows that recovery is very slow, and may not return to the initial state. Knops, J. M., & Tilman, D. 2000. Dynamics of soil nitrogen and carbon accumulation for 61 years after agricultural abandonment. Ecology 81:88-98. Study of total soil carbon and nitrogen concentrations in old fields at the Cedar Creek Ecosystem Science Reserve. Suggests that agricultural use reduces soil carbon to roughly one fourth of the level found in undisturbed habitats, and soil nitrogen to roughly one third. Projections based on the observed rate of recovery show that full recovery of soil carbon and nitrogen would require roughly two centuries. Post, W. M., and K. C. Kwon. 2000. Soil carbon sequestration and land-use change: Processes and potential. Global Change Biology 6:317-327. Review of soil carbon dynamics following agricultural use and abandonment. General trend involves rapid loss during agricultural use, followed by a slow recovery after abandonment. However, these rates are generally slow, and it is not clear whether full recovery of lost carbon is possible. Global Extent of Old Fields The actual terrestrial area occupied by old fields is somewhat difficult to estimate, as land is frequently brought into and out of production. This is particularly true for pasture land, which requires little infrastructure and is difficult to identify from satellite imagery. Nonetheless, several sources have attempted to estimate the footprint of abandoned land. The main source for global abandoned cropland is currently Ramankutty and Foley 1999, with a useful graphical summary of this data in Cramer et al. 2008. Specific estimates for abandoned cropland in the USA are available from Zumkehr and Campbell 2013. Estimates for abandoned cropland in the European Union are available in Van Dijk et al. 2004. The extent of land abandonment in the Amazon is summarized in Houghton et al. 2000 and Lucas et al. 2002. Finally, the current global extent of pasture land is summarized in Asner et al. 2004, though this does not specifically include estimates of how much of that land is currently out of production.

Asner, G. P., A. J. Elmore, L. P. Olander, R. E. Martin, and A. T. Harris. 2004. Grazing systems, ecosystem responses, and global change. Annual Review of Environment and Resources 29:261-299. Review of the global extent of managed grazing lands, which the authors estimate as 25% at global land area. Includes current extent organized by region and habitat type, though not explicit information about abandoned rangeland.

Cramer V, Hobbs R, Standish R. 2008. What’s new about old fields? Land abandonment and ecosystem assembly. Trends in Ecology & Evolution. 23:104-112. General summary of old field succession, discussed in more detail in *General Overviews*. Includes a very helpful figure of global extent of abandoned cropland from 1700-1990, adapted from tables in Ramankutty and Foley 1999. Houghton, R.A., Skole, D.L., Nobre, C.A., Hackler, J.L., Lawrence, K.T., and Chomentowski, W.H. 2000. Annual fluxes of carbon from deforestation and regrowth in the Brazilian Amazon. Nature 403:301-304. Study showing positive net carbon flux attributable to the outcomes of deforestation and afforestation following land clearing and abandonment for agriculture in the Brazilian Amazon. Useful as a general citation for net effects of deforestation and land abandonment on global carbon cycling. Lucas, R. M., M. Honzák, I. D. Amaral, P. J. Curran, and G. M. Foody. 2002. Forest regeneration on abandoned clearances in central Amazonia. International Journal of Remote Sensing 23:965-988. Study of deforestation and land abandonment in the Amazon near Manaus, Brazil. Uses satellite data to show that up to 35.8% of cleared land has been recently abandoned. Shows that land clearing is often rapidly followed by abandonment. Ramankutty, N., and J. A. Foley. 1999. Estimating historical changes in global land cover: Croplands from 1700 to 1992. Global Biogeochemical Cycles 13:997-1027. Currently the definitive source on global historical trends on the extent of native vegetation, actively-used cropland, and of abandoned cropland. Includes detailed tables and figures organized by world region. Van Dijk, G., Zdanowicz, A., and Blokzijl, R. 2004. Land abandonment, biodiversity, and the CAP. DGL, Government Service for Land and Water Management, Utrecht, NL. In-depth discussion of the effects of the European Union’s Community Agriculture Policy on land use and biodiversity. Includes detailed estimates of the extent of agricultural land abandonment in central and eastern Europe, estimated around 10-20%. Zumkehr, A., and J. E. Campbell. 2013. Historical U.S. cropland areas and the potential for bioenergy production on abandoned croplands. Environmental Science & Technology 47:3840-3847. Analysis of bioenergy production potential of abandoned cropland in the USA. Estimates that roughly 10% of the terrestrial area of the continental USA has been cropped and abandoned since colonization by Europeans. Common Study Methods and Topics Because old fields have historically been a model system for many topics in ecology, approaches for studying old fields and their successional dynamics span a correspondingly broad array of methods. Nevertheless, some major unifying concepts are summarized below. *Chronosequences and Timeseries* describes two of the most common methods for measuring temporal dynamics in old fields. *Analysis of Community Dynamics* discusses major statistical and mathematical frameworks that have been used to summarize empirical data and test hypotheses in old fields. Though many of these methods and topic focus on plant communities, there is also a rich history of studying other groups of organisms, such as birds, insects, or soil microbes, which is discussed in *Non-Plant Communities*. Finally, *Theoretical Frameworks* summarizes some of the major conceptual and mathematical models that have been developed to study and characterize old fields. Chronosequences and Time-series

Empirical data on old field succession are available in two forms. In time-series based methods, fields are sampled repeatedly through time to directly record changes. More commonly, studies rely on “chronosequences” (also called “chronoseries” or “space-for-time substitutions”), in which data from multiple fields of different ages are collated to approximate dynamics through time. The general assumption behind this method is that fields that are spatially close to one another should follow similar successional trajectories. This assumption is generally supported by empirical data, exemplified in Li et al. 2016. Nevertheless, there is mixed support for the use of chronosequences based on comparisons to dynamics from actual time-series. Pickett 1989 provides a relatively comprehensive review of the topic, and suggests that chronosequences are best used for tracking qualitative dynamics, while time-series methods are required for quantitative comparisons. Foster and Tilman 2000 found that chronosequences successfully captured changes in the rate of succession and abundance of common species at their site, but did not capture species richness dynamics. Mora et al. 2015, on the other hand, found that chronosequences correctly captured dynamics in species richness and composition, but not physical characteristics of forest structure. Walker et al. 2010 likewise find mixed support, and suggest that slower, more predictable processes are best approximated by chronosequences. Despite these inconsistencies, chronosequences remain a much more commonly-used method than time-series, likely because timeseries analyses require many more years of data collection. Foster, B.L., and Tilman, D. 2000. Dynamic and static views of succession: Testing the descriptive power of the chronosequence approach. Plant Ecology 146:1-10. Compares results from chronosequences and repeated sampling in abandoned cropland at the Cedar Creek Ecosystem Science Reserve, Minnesota, USA. Found strong correspondence between the two for predictions about rate of succession and abundance of common species, but not for species richness. Li, S., M. W. Cadotte, S. J. Meiners, Z. Pu, T. Fukami, and L. Jiang. 2016. Convergence and divergence in a long-term old-field succession: The importance of spatial scale and species abundance. Ecology Letters 19:1101-1109. Tests for convergence in community composition at small and large spatial scales for abandoned cropland in the Buell-Small Succession Study in New Jersey, USA. Only detects community convergence for common species at large scales. However, results also suggest that there are no long-term differences imposed by starting conditions (e.g. year abandoned, last crop, type of ploughing). Mora, F., M. Martínez-Ramos, G. Ibarra-Manríquez, A. Pérez-Jiménez, J. Trilleras, and P. Balvanera. 2015. Testing chronosequences through dynamic approaches: Time and site effects on tropical dry forest succession. Biotropica 47:38-48. Chronosequence of forest succession in nine abandoned pastures compared to three old growth forests on the Pacific coast of Jalisco, Mexico. Finds that projections from the chronosequence over-estimate tree height and basal area, but correctly predict species richness and composition. Pickett, S. T. A. 1989. “Space-for-time substitution as an alternative to long-term studies”. In Long-Term Studies in Ecology. Edited by G. E. Likens, 110-135. Springer New York, NY. Likely the most widely cited discussion on the validity of chronosequences. Suggests that chronosequences are often valid for evaluating “general or qualitative trends”, but that repeated sampling is required for testing specific mechanisms or for analysing transient dynamics. Walker, L. R., Wardle, D. A., Bardgett, R. D. and Clarkson, B. D. 2010. “The use of chronosequences in studies of ecological succession and soil development”. Journal of Ecology 98:725–736.

Discussion of which kinds of factors in old fields can be appropriately studied through chronosequences. Identifies that long-term trends and slower processes (e.g. soil dynamics) tend to be well-approximated in chronosequences, while faster or less predictable processes (e.g. species composition or abundance) often cannot. Analysis of Community Dynamics Tracking changes in ecological communities is generally difficult, as it involves variables that are largely categorical (i.e. the presence or absence of each species) in systems that are often high dimensional and nonlinear. A common solution to this is to use some form of ordination, which collapses high dimensional systems into a lower dimensional system, while retaining as much information as possible. The approach was first popularized by Whittaker, with methods summarized in his 1982 book, which championed the use of oridination as a method for tracking plant community change along environmental gradients, including successional age. More modern ordination methods are described in Bonet 2004 and Li et al. 2016. Other methods include the use of functional traits, which can be used to generate quantitative estimates of community similarity. An early example of this approach is in Noble and Slatyer 1980, with a more modern approach that also includes information about phylogenetic relatedness in Purschke et al. 2013. Finally, examples of somewhat less commonly-used techniques are described in Bo et al. 2015, which applies a hierarchical clustering method to group communities based on similarity, and Vile et al. 2006, which uses structural equation modeling to account for complex interactions among functional traits.

Bo, Yaojun, Zhu, Q,, and Zhao, W. 2015. Old-field succession sequence in loess area in northern Shaanxi of China. The Open Biotechnology Journal 9:104-108. Study of abandoned grazing land in 29 old fields near Yan’an City, China. Uses a hierarchical clustering method based on an abundance index for each species to group fields by similarity. Bonet, A. 2004. Secondary succession of semi-arid Mediterranean old-fields in south-eastern Spain: Insights for conservation and restoration of degraded lands. Journal of Arid Environments 56:213-233. Study of abandoned cropland in south-east Spain. Uses correspondence analysis to track successional status of fields, and to test for relationships between community composition and various environmental variables. Li, S., M. W. Cadotte, S. J. Meiners, Z. Pu, T. Fukami, and L. Jiang. 2016. Convergence and divergence in a long-term old-field succession: The importance of spatial scale and species abundance. Ecology Letters 19:1101-1109. Analysis of data from abandoned cropland in the Buell-Small Succession Study in New Jersey, USA. Uses non-metric multidimensional data scaling to test for differences in community composition among fields as a function of age and various environmental variables and starting conditions. Noble, I. R., and R. O. Slatyer. 1980. The use of vital attributes to predict successional changes in plant communities subject to recurrent disturbances. Vegetatio 43:5-21. Early paper advocating the use of “vital attributes”, which correspond roughly to the modern concept of functional traits. Authors suggest criteria that should be used for identifying important attributes, develop a framework showing how to use these to analyse plant communities, and include several example applications of their technique. Purschke, O., et al. 2013. Contrasting changes in taxonomic, phylogenetic and functional diversity during a long-term succession: Insights into assembly processes. The Journal of Ecology 101:857-866.

Study of a very long-term chronosequence of abandoned cropland for the island of Öland, Sweden. Tests for changes in taxonomic, phylogenetic, and functional traits through time. Uses associations between species trait similarity, and the prevalence of particular traits, to argue that assembly is structured by deterministic processes and competition. Vile, D., B. Shipley, and E. Garnier. 2006. A structural equation model to integrate changes in functional strategies during old-field succession. Ecology 87:504-517. Uses structural equation modelling to test for trait changes among species through succession, using data from abandoned vineyards in southern France. Finds that time since abandonment, and species seed mass, can jointly explain changes in the prevalence of species functional traits through successional time. Similar methods used by Lohbeck et al. 2015, cited in *Long-Term Successional Trends*. Whittaker, R. H., editor. 1982 (Cited Edition from 2012). Ordination of plant communities. Handbook of Vegetation Science. Vol. 5-2. Springer Science & Business Media. The seminal work on the use of ordination for studying plant communities and ecological succession. Introduces many classical ordination methods, though most have been replaced by more modern methods. Importantly, includes discussions of how to use ordination to study of community change along environmental and temporal gradients. Non-Plant Communities Though much of the literature on old fields focuses on plant communities, other types of organisms clearly both affect and are affected by old field successional dynamics. Classically, it has often been assumed that consumer species’ community dynamics are predominantly structured by the underlying dynamics of plant succession. For example, Siemann et al. 1999 shows that arthropod abundance and species richness largely tracks trends in plant species richness during succession. Likewise, Blake and Loiselle 2001 suggests that many tropical bird species may depend on regular disturbance to maintain young secondary growth forest habitats. However, consumer communities do not always play a passive role in old field succession. Robinson et al. 1992 shows that successional dynamics in mammal and snake populations may be faster than those in plant communities. Wunderle 1997 reviews the role of animal dispersal of seeds in regulating the rate of plant succession after deforestation. Engelmann 1961 discusses the influences of soil arthropods on carbon, nutrient, and energy cycling. Lastly, Bever 1994 and van de Voorde et al. 2011 demonstrate the important of interactions between plants and soil microbial communities (i.e. plant soil feedbacks) in structuring plant succession in old fields.

Bever, J.D. 1994. Feedback between plants and their soil communities in an old field community. Ecology 75:1965-1977. An early paper testing the effects of plant soil feedbacks. Results for four perennial plant species common in old fields in the southeastern United States suggest strong negative feedbacks between plants and soil microbial communities, which could potentially limit the abundance of common plant species in old fields. Blake, J.G. and B.A. Loiselle. 2001. Bird assemblages in second-growth and old-growth forests, Costa Rica: Perspectives from mist nets and point counts. The Auk 118:304-326. Bird Assemblages in Second-Growth and Old-Growth Forests, Costa Rica: Perspectives from Mist Nets and Point Counts Study contrasting bird communities in old growth and secondary growth forests at La Selva Biological Station in Costa Rica. Results show that species richness, and the abundance of several threatened species, was higher in young secondary growth forests than in old growth forests. The authors suggest that maintaining younger forests may therefore be important for conserving bird species at their site.

Engelmann, M.D. 1961. The role of soil arthropods in the energetics of an old field community. Ecological monographs 31:221-238. A classic paper discussing the effects of soil arthropods (and in particular oribatid mites) on energy cycling in old fields in Michigan, USA. Suggests that multi-trophic interactions among soil arthropods can regulate the abundance of fungi and bacteria, thereby strongly influencing rates of energy and carbon cycling. Importantly, results suggest that the arthropod communities at this site assemble independently of plant community composition. Robinson, G.R., Holt, R.D., Gaines, M.S., Hamburg, S.P., Johnson, M.L., Fitch, H.S. and Martinko, E.A., 1992. Diverse and contrasting effects of habitat fragmentation. Science 257:524-526. Results from study of the effects of isolation and patch size on old field succession in Kansas, USA. Patch size had no detectable effects on plant species richness or abundance during the first six years of the experiment, whereas abundance of small mammals and snakes was significantly related to patch size.

Siemann, E., Haarstad, J. and D. Tilman. 1999. Dynamics of plant and arthropod diversity during old field succession. Ecography 22:406-414. Includes a survey of arthropod populations in old fields at the Cedar Creek Ecosystem Science Reserve in Minnesota, USA. Results suggest that arthropod species richness and abundance both increase over time, largely because of increases in plant species richness. However, community composition also shifts over the course of succession: Herbivores and parasites become more diverse, while other arthropod guilds do not. van de Voorde, T.F., van der Putten, W.H. and T. Martijn Bezemer. 2011. Intra‐and interspecific plant–soil interactions, soil legacies and priority effects during old‐field succession. Journal of Ecology, 99:945-953. Summarizes a greenhouse experiment and field observations testing plant soil feedbacks on a common old field species, Jacobaea vulgaris, in the Netherlands. Results show a positive correlation between old field age and feedback effects of J. vulgaris on other species in the community. The authors suggest that these feedbacks could lead to priority effects, and consequently alter successional dynamics. Wunderle, Joseph M. 1997. The role of animal seed dispersal in accelerating native forest regeneration on degraded tropical lands. Forest Ecology and Management 99:223-235. Review of the effect of animal vectors on restoration in deforested regions. Discusses how areas that are near potential seed sources, or that are attractive to potential seed vector species such as birds, are generally reforested much faster than other regions. Demonstrates the important role of animal communities in influencing successional dynamics. Theoretical Frameworks Early theoretical discussions of old field succession focused on a combination of dispersal limitation and interactions among organisms and their environment, summarized in Horn 1974. However, the first quanitative theoretical models were primarilly concerned with interactions among species. Connell and Slatyer 1977 is an early example of this, and posits that successional dynamics may result from a combination of positive, negative, and neutral interspecific interactions. Papers such as Tilman 1985 and Huston and Smith 1987 later integrated interspecific and species-environment interactions using a resource competition framework, which posited that species interactions could be modeled as the net outcome of species impacts on, and responses to, their environments. Finally, more modern frameworks,

such as that in Cramer et al. 2008, again include dispersal as well as species-environment interactions. A broad summary of contemporary theory and posited mechanisms is available in Hobbs and Walker 2007. Connell, J. H., and R. O. Slatyer. 1977. Mechanisms of succession in natural communities and their role in community stability and organization. The American Naturalist 111:1119-1144. Introduces three models based on positive, negative, and neutral interactions among species that could be used to explain successional transitions among types of plant communities without exogenous forces changing environmental conditions. Includes detailed discussion of potential mechanisms and evidence for each model, as well as how to test them. Hobbs, R. J., and Walker, L. R. 2007. “Old field succession: Development of concepts”. In Old fields: dynamics and restoration of abandoned farmland, 17-29. Edited by V. A. Cramer and R. J. Hobbs. Washington, DC: Island Press. Summary of potential mechanisms that could explain observed trends in old field succession. Includes a particularly useful table grouping these potential mechanisms with relevant citations. Cramer V, Hobbs R, Standish R. 2008. What’s new about old fields? Land abandonment and ecosystem assembly. Trends in Ecology & Evolution 23:104-112. Introduces a conceptual model for old field succession, based primarily on interactions between biotic and abiotic conditions that potentially allow for successful system recovery. Importantly, focuses on how to apply this model to guide restoration efforts. Horn, H. S. 1974. The ecology of secondary succession. Annual Review of Ecology and Systematics 5:25-37. Broad review of contemporary literature relating theoretical advances in population ecology to potential mechanisms in succession. Includes a very useful discussion of patterns in succession, including changes in community productivity and stability, and theoretical concepts that could be used to explain them. Huston, M., and T. Smith. 1987. Plant succession: Life history and competition. The American Naturalist 130:168-198. Discusses contemporary theory, and explains current limitations in describing successional dynamics. Posits that succession is driven by a combination of resource competition, trade-offs among species, and species impacts on their environments. Includes useful examples of several kinds of models, including examples of how to apply and interpret them. Tilman, D. 1985. The resource-ratio hypothesis of plant succession. American Naturalist 125:827-852. Discusses a model of competition for two limiting resources, and shows how changes in the ratio at which these resources are supplied could drive successional changes. In particular, suggests that species might change their environments over the course of succession in ways that favour their competitors, ultimately leading to their own competitive displacement, and enabling coexistence. Examples of Long-Term Studies Though there are many long-term studies of old fields from around the world, several sites, particularly in eastern and midwestern North America, have had a disproportionate impact in English language publications. In general, these sites appear to have been important largely because long-term records (>20 years) are available from them. The Park Grass Experiment, discussed in Silvertown et al. 2006, is notable as the oldest ecological experimental site on earth, and includes very long-term data on

grassland dynamics on abandoned pasture land. The Buell-Small Succession Study at the Hutcheson Memorial Forest, described in Pickett et al. 2001 and in Meiners et al. 2015, has probably been written about more than any other old field succession study, and is likely one of the origins of the “classical” model of old field succession transitioning from abandoned fields to climax forest. Finally, the Kellogg Biological Station and Cedar Creek Ecosystem Science Reserve Long Term Ecological Research sites, discussed in Gross and Emery 2007 and in Inouye et al. 1987, respectively, are important both as examples of sites where old field succession does not necessarily proceed to forest, and of sites where substantial additional experimental data on local grasslands and agricultural practices are available. Gross, K. L., and S. A. Emery. 2007. “Succession and restoration in Michigan old-field communities”. In Old fields: Dynamics and restoration of abandoned farmland. Edited by V. A. Cramer and R. J. Hobbs, 162-179. Washington, DC: Island Press. Example of a study from the old field experiment at the Kellogg Biological Station Long Term Ecological Research site in Michigan, USA. The site includes data from succession in agricultural fields abandoned in the early 1960’s. This study shows evidence for major inter-annual dynamics even after 40-60 years of succession, though non-native species cover does appear to reach an equilibrium. Inouye, R. S., N. J. Huntly, D. Tilman, J. R. Tester, M. Stillwell, and K. C. Zinnel. 1987. Old-field succession on a Minnesota sand plain. Ecology 68:12-26. Example of a study from the Cedar Creek Ecosystem Science Reserve Long Term Ecological Research site in Minnesota, USA. The site includes data from succession in agricultural fields abandoned as early as 1927. This study shows that old fields begin with similar species composition early in succession, but become less similar with age. Litter biomass, total soil nitrogen, and perennial species abundance also increase with age, while non-native species abundance decreases. Meiners, Scott J., Steward TA Pickett, and Mary L. Cadenasso. 2015. An integrative approach to successional dynamics. Cambridge, UK: Cambridge University Press. A broad overview of results from the Buell-Small Succession Study, and discussion of the implications of the study for other sites and systems. Also includes a comprehensive discussion of ecological succession. Particularly useful as a summary of important lessons learned from the study, as interpreted by the authors of many of the most influential papers resulting from work at the site. Pickett, S. T. A., M. L. Cadenasso, and S. Bartha. 2001. Implications from the Buell-Small Succession Study for vegetation restoration. Applied Vegetation Science 4:41-52. Example of a study from the Buell-Small Succession Study at the Hutcheson Memorial Forest in New Jersey, USA. The site includes data on succession in agricultural fields beginning as early as 1958. This study includes a general description of the site and its experimental goals. Silvertown, J., P. Poulton, E. Johnston, G. Edwards, M. Heard, and P. M. Biss. 2006. The Park Grass Experiment 1856-2006: Its contribution to ecology. The Journal of Ecology 94:801-814. Example of a study from the Park Grass experiment at the Rothamsted Experimental Station, UK. The site includes experiments in abandoned pasture and hayfields that have been monitored since 1856, and is the longest running ecological experiment in the world. This study summarizes findings from the experiment, and shows that even after long periods of time, dynamics for species abundance continues to vary through time. Drivers of Contingency

Though it has long been suspected that succession in old fields need not proceed predictably and identically across similar sites, this viewpoint has become particularly widespread since the end of the twentieth century. This has been largely driven by an increase in empirical evidence of diverging successional trajectories among seemingly similar fields, such as that from the Buell-Small Succession Study, summarized by Myster and Pickett 1992. A comprehensive review of potential mechanisms that are thought to alter old field successional trajectories is available in Sanderson et al. 2004. In general, hypothesized drivers of contingency are ascribed to one of two types of mechanisms. First, deterministic mechanisms lead to predictable divergences in successional trajectories due to specific differences in starting conditions or historical disturbances. For example, Carson and Barrett 1988 summarize a series of differences in plant charactaristics, nutrient availability, and proximity to invading woody plants that explain differences in successional trajectories at their site. Similarly, Laliberte et al. 2014 shows largescale effects of environmental filtering on successional dynamics. Second, stochastic mechanisms lead to divergence in ways that are, by definition, unpredictable in the short-term. For example, Bartha et al. 2003 suggests that chance disturbance events are an important determinant of the kinds of plant species that are able to establish in old fields. Even more broadly, Norden et al. 2015 show that old field succession in Neotropical forests appears to be much more strongly driven by chance events than by predictable ones. Note that this topic is closely related to that of alternate stable states and regime shifts (as an example, see Suding et al. 2004 in *Concepts of Old Fields*). Bartha, S., S. J. Meiners, S. T. A. Pickett, and M. L. Cadenasso. 2003. Plant colonization windows in a mesic old field succession. Applied Vegetation Science 6:205-212. Results from the Buell-Small Succession Study in New Jersey, USA. Suggests that dense vegetation can prevent colonization by new species, and that chance disturbance events therefore appear to open potential colonization “windows”. Example of a potential mechanism that could lead to stochastic development patterns in succession. Carson, W. P., and G. W. Barrett. 1988. Succession in old-field plant communities: Effects of contrasting types of nutrient enrichment. Ecology 69:984-994. Study in 1-5 year-old experimentally abandoned cropland in Ohio, USA. Found that community and physical plant characteristics (“physiognomy”), nutrient addition, and woody encroachment all lead to changes in successional trajectories. Example of many potential drivers of contingency. Laliberte, E., G. Zemunik, and B. L. Turner. 2014. Environmental filtering explains variation in plant diversity along resource gradients. Science 345:1602-1605. Study of succession along a 2-million-year chronosequence of Australian dunes. Shows strong effects of soil pH on plant community composition. Not specifically an example from old fields, but shows that at large spatial and temporal scales, environmental filtering may overwhelm other mechanisms. Myster, R. W., and S. T. A. Pickett. 1992. Dynamics of associations between plants in 10 old fields during 31 years of succession. The Journal of Ecology 80:291-302. Results from the Buell-Small Succession Study in New Jersey, USA. Shows that the “proportion, number, and significance” of associations among plant species declined with field age. Suggests that community structuring caused by species interactions lessens over successional time. Norden, N., H. A. Angarita, F. Bongers, M. Martínez-Ramos, I. Granzow-de la Cerda, M. van Breugel, E. Lebrija-Trejos, J. A. Meave, J. Vandermeer, G. B. Williamson, B. Finegan, R. Mesquita, and R. L. Chazdon. 2015. Successional dynamics in Neotropical forests are as uncertain as they are predictable. Proceedings of the National Academy of Sciences 112:8013-8018.

Study of forest succession after clearcutting across the Neotropics. Finds that initial conditions are rarely a good indicator of future dynamics, and that stand age only explained about 20% of variability among sites. Suggests that succession may be strongly driven by site-specific or stochastic events. Sanderson, M. A., R. H. Skinner, D. J. Barker, G. R. Edwards, B. F. Tracy, and D. A. Wedin. 2004. Plant species diversity and management of temperate forage and grazing land ecosystems. Crop Science 44:1132-1144. Broad review of potential drivers of differences observed among pasture systems. Includes a very useful table summarizing relationships among human, environmental, and ecological drivers of potential ecosystem functions. Example of many mechanisms that can potentially alter successional landscapes, and how this might impact their economic value. Species Interactions and Invasion Species interactions are generally considered to be an important deterministic mechanism controlling successional dynamics in old fields. This is particularly true for invasions by non-native species, which often appear to suppress native species diversity. In a broad review of potential drivers of successional contingency in abandoned pastures, for example, Hobbs and Huenneke 1992 shows that supression of native species by non-native species is a dominant process. Similarly, Meiners et al. 2001 showed that two non-native species in the Buell-Small Succession Study were able to inhibit other native species and were associated with overall reductions in diversity. Stark and Norton 2015 show that an invading grass species is able to alter the environment in ways that increase its own survival and suppresses its competitors, suggesting that invaders could trigger alternate stable states in old fields. Interestingly, there is also evidence that species interactions might contribute to stochastic changes to successional trajectories. For example, Kuebbing et al. 2014 found that old field communities became less structured, and species interactions became less predictable, after invasion by non-native species. Nevertheless, in many cases, species interactions may also be a mechanism that resists contingency. Petermann et al. 2010 found that invading species were inhibited by resident species of similar functional groups, indicating that once particular functional roles are filled, it may be more difficult for invaders to establish.

Hobbs, R. J., and L. F. Huenneke. 1992. Disturbance, diversity, and invasion: Implications for conservation. Conservation Biology 6:324-337. Broad review of the effects of burning, grazing regimes, disturbance, and fertilization on pasture land. Strong focus on how these mechanisms influence invasion by new species. Includes important examples of invading species suppressing recovery by native species in plant communities. Kuebbing, S. E., L. Souza, and N. J. Sanders. 2014. Effects of co-occurring non-native invasive plant species on old-field succession. Forest Ecology and Management 324:196-204. Study of 17 fields near Oak Ridge, Tennessee, USA, which were abandoned from agricultural use around 1943. Finds that increased invasive species richness leads to more random communities with fewer predictable associations among species. However, could not predict which fields would be invaded most heavily. Meiners, S. J., S. T. A. Pickett, and M. L. Cadenasso. 2001. Effects of plant invasions on the species richness of abandoned agricultural land. Ecography 24:633-644. Results from abandoned cropland in the Buell-Small Succession Study in New Jersey, USA. Finds that two species, Lonicera japonica and Rosa multiflora, appear to inhibit succession by other species and lead to reduced diversity.

Petermann, J. S., A. J. F. Fergus, C. Roscher, L. A. Turnbull, A. Weigelt, and B. Schmid. 2010. Biology, chance, or history? The predictable reassembly of temperate grassland communities. Ecology 91:408421. Results from natural dispersal and experimental seed addition of invading plant species into experimentally-assembled grassland communities in Jena, Germany. Authors document that successful establishment by invading species into existing communities is inhibited by resident species in the same functional group. Stark, J. M., and J. M. Norton. 2015. The invasive annual cheatgrass increases nitrogen availability in 24year-old replicated field plots. Oecologia 177:799-809. Discussion of invasion by cheatgrass (Bromus tectorum) into experimentally ploughed and abandoned fields in north-western Colorado, USA. Results show that cheatgrass may positively reinforce its own abundance, and inhibit other species through changes in nitrogen cycling. Example of potential alternative successional trajectories. Grazing Grazing regimes are thought to have strong deterministic effects on old fields, both before and after abandonment. Noy-Meir 1975 is a classic theory paper showing how differences in grazing rates can lead to alternate stable states. Uhl et al. 1988 shows that differences in past grazing regimes can lead to differences in successional trajectories. Aide et al. 2000 is a good example of rapid recovery of systems after abandonment of pasture lands, but shows that the original species composition did not recover over the duration of their study. Lastly, Kimiti et al. 2016 presents evidence for alternate stable states in overgrazed systems, showing that addition of errosion barriors can help begin vegetation recovery even after sites have been dominanted by bare ground for long time periods.

Aide, T. M., et al. 2000. Forest regeneration in a chronosequence of tropical abandoned pastures: Implications for restoration ecology. Restoration Ecology 8:328-338. Study of succession in abandoned pastures in Puerto Rico. Shows relatively rapid recolonization by common woody species in a matter of a few decades. However, the original species composition and diversity does not recover over the same time period. Kimiti, D. W., C. Riginos, and J. Belnap. 2016. Low-cost grass restoration using erosion barriers in a degraded African rangeland. Restoration Ecology. [doi: 10.1111/rec.12426] Study of Kenyan rangeland “degraded” by overgrazing, plant species invasion, and erosion. While sites are initially dominated by bare ground, the authors find that simple erosion barriers allow for the reestablishment of native vegetation. Example of potentially inexpensive solutions to recovery after grazing. Noy-Meir, I. 1975. Stability of grazing systems: An application of predator-prey graphs. The Journal of Ecology 63:459-481. A classical theoretical model of alternative stable states in grazing lands. Includes simple models and very helpful conceptual figures suggesting that over-grazing might push systems into a stable lowproductivity state which is difficult to escape. Provides the theoretical basis for much subsequent work on rangeland restoration and alternate stable states in old fields. Uhl, C., R. Buschbacher, and E. A. S. Serrao. 1988. Abandoned pastures in eastern Amazonia. I. Patterns of plant succession. The Journal of Ecology 76:663.

Study of forest recovery in abandoned pastureland in the eastern Amazon. Shows that recovery in intensely used pasture achieved only half the annual biomass recovery rate of lightly used pasture, and accumulated fewer tree species. Suggests that intensity and duration of land use can influence successional trajectories. Fire and Litter Feedbacks Fire can have differential effects on different types of vegetation – for example, it typically decreased woody plant abundance and increases the abundance of herbaceous vegetation. Fire also reduces leaf litter depth, which alters environmental conditions. At large scales, more frequent fires appears to drive deterministic regime shifts towards grassland and savanna states (see Staver et al. 2011 in *Origination Mechanisms*). On smaller scales, effects can be more complex: If fires occur regularly, then they may contribute to deterministic changes in successional dynamics, whereas if they are rare, then fire might be considered as a stochastic mechanism. D’Antonio and Vitousek 1992 shows how invading species can increase fire frequency, driving out competing native species. On the other hand, in other locations fire and litter removal have been shown to increase native species abundance, as exemplified in Ruprecht et al. 2010. This is potentially because of the variable effects of leaf litter on native species survival, discussed in a general review in Facelli and Pickett 1991. Lastly, in some systems, there is little evidence that fires can shift successional trajectories. For example, Li et al. 2013 found little change in long-term successional patterns caused by different fire frequencies at the Cedar Creek Ecosystem Science Reserve.

D’Antonio, C. M., and P. M. Vitousek. 1992. Biological invasions by exotic grasses, the grass/fire cycle, and global change. Annual Review of Ecology and Systematics 23:63-87. Broad review of the effects of fire on invasion in grasslands. Discusses invading species in western North American grasslands which can increase fire temperature and prevalence, and which are disproportionately favoured by fire. Also discusses feedbacks between fire and grass abundance as a mechanism for maintaining grasslands. Facelli, J. M., and S. T. A. Pickett. 1991. Plant litter: Its dynamics and effects on plant community structure. The Botanical Review 57:1-32. Broad review of the effects of leaf litter on plant communities. Includes changes to chemical (e.g. nutrient cycling) and physical environment (e.g. shading, temperature buffering, water retention, “mechanical barrier”). Suggests that litter can be both an important factor for “nurse effects”, or for suppressing all but a few (usually rhizomatous) species. Li, W., X. Zuo, and J. M. H. Knops. 2013. Different fire frequency impacts over 27 years on vegetation succession in an infertile old-field grassland. Rangeland Ecology & Management 66:267-273. Case study on effects of fire frequency in old fields over 27 years at the Cedar Creek Ecosystem Science Reserve. Shows that some early successional species decreased in abundance with more frequent burning, as did species richness and litter. However, these changes did not seem to be large enough to change overall successional patterns. Ruprecht, E., M. Z. Enyedi, R. L. Eckstein, and T. W. Donath. 2010. Restorative removal of plant litter and vegetation 40 years after abandonment enhances re-emergence of steppe grassland vegetation. Biological Conservation 143:449-456. Study of abandoned pastures in the Transylvanian Lowland, Romania. Finds that removal of litter and standing biomass allows for reinvasion by target native species even 40 years after abandonment.

Dispersal Limitation Chance dispersal events or differences in the proximity of potential propagule sources is thought to be one of the primary drivers of stochastic differences in successional trajectories. Simply put, if different species arrive in different locations at different times, it stands to reason that this may lead to differences in successional dynamics. An example of this is described in Webb et al. 1972, which found that recolonization of forest species into Australian old fields was driven primarilly by chance disturbance and dispersal events after the first few years of succession. Similarly, Knapp et al. 2016 shows that species arrival times can vary greatly across sites, even if they use similar dispersal methods. More generally, Petermann et al. 2010 shows that even if community assembly is completely deterministic given a fixed species pool, dispersal can still lead to long-lasting differences among communities until all sites have been exposed to all potential propagule types. Nevertheless, there is some evidence that deterministic mechanisms may override the effects of stochastic dispersal events. Zanini et al. 2006 shows that local community composition is an important determinant of propagule success in tropical forest recovery, which could inhibit stochastic dispersal events. Li et al. 2015 shows that community composition at the Buell-Small Succession Study depends strongly on the order in which species colonize sites, but this order was similar across all fields in their study, suggesting that stochasticity may play a minor role in dispersal order.

Knapp, S., J. Stadler, A. Harpke, and S. Klotz. 2016. Dispersal traits as indicators of vegetation dynamics in long-term old-field succession. Ecological Indicators 65:44-54. Study of four abandoned agricultural fields in Germany, including roughly 20 years of post-abandonment data. Find a decrease in the diversity of species dispersal mechanisms over time, based on their functional traits. In particular, prevalence of animal dispersal peaks at intermediate field ages and then drops in some fields, but not in others. Li, S., M. W. Cadotte, S. J. Meiners, Z. Hua, L. Jiang, and W. Shu. 2015. Species colonisation, not competitive exclusion, drives community overdispersion over long-term succession. Ecology Letters 18:964-973. Results from the Buell-Small Succession Study in New Jersey, USA. Suggest that community structure is more strongly influenced by selective arrival of species driven by dispersal limitation and establishment than it is by competitive exclusion. Importantly, these fields are relatively close to one another, suggesting that they may be subject to similar levels of propagule pressure. Petermann, J. S., A. J. F. Fergus, C. Roscher, L. A. Turnbull, A. Weigelt, and B. Schmid. 2010. Biology, chance, or history? The predictable reassembly of temperate grassland communities. Ecology 91:408421. Results from natural dispersal and experimental seed addition of invading plant species into experimentally-assembled grassland communities in Jena, Germany. Shows that communities respond similarly to similar types of propagule pressure, suggesting that dispersal limitation is unlikely to have long-term effects. However, also showed that invasion was often successful at changing community composition. Webb, L. J., J. G. Tracey, and W. T. Williams. 1972. Regeneration and pattern in the subtropical rain forest. The Journal of Ecology 60:675. Study of subtropical forest regeneration in Queensland, Australia. Shows that after the “pioneering stage” of succession, subsequent community change is “not unidirectional and is probabilistic”, but potentially driven by a combination of random disturbance events and dispersal.

Zanini, L., G. Ganade, and I. Hübel. 2006. Facilitation and competition influence succession in a subtropical old field. Plant Ecology 185:179-190. Study of tropical forest recovery in clear-cut regions in Rio Grande do Sul, Brazil. Shows strong role of facilitation in the survival of tree seedlings. Suggests that the established plant community may be an important mediator of subsequent colonization success. Broader Implications Old field succession has played a profound role in structuring how researchers and managers think about ecological systems. Some of the earliest large-scale experiments in ecology, many of the first attempts to intentionally engineer particular types of ecosystems, and a surprising fraction of contemporary mathematical models have their origins in studies of old fields. *Ecological Understanding* summarizes some studies of old fields that have had particularly strong impacts in other fields in ecology. *Ecosystem Management* discusses lessons learned from attempts to manage and restore old fields, and notes some approaches that have proven to be particularly effective. Ecological Understanding Some of the most important theoretical and empirical work grounding contemporary ecological research originated in studies of old fields. Seminal papers such as those by Clements (1916) and Tansley (1935) largely determined how modern ecologists conceptualize ecological communities. Noy-Meir 1975 is notable as one of the first theoretical considerations of alernate stable states in an ecological system. The field methods used in Odum 1960 to quantify species abundances in old fields were relatively rare at the time, but have since become the standard practice for studies of terrestrial herbaceous plants. Similarly, the analytical ordination methods introduced in Whittaker 1982 have come to be the dominant data analysis technique in the field of community ecology. Lastly, the field experiment described in Tilman et al. 1997 is one of the first examples of a long-term large-scale ecological experiment that actively manipulated community composition, and has inspired similar methods in many other sites and systems.

Clements, F. E. 1916. Plant succession: An analysis of the development of vegetation. Washington, DC: Carnegie Institution of Washington. Discussion of successional stages resulting from secondary disturbances such as agricultural abandonment or deforestation. Clements largely championed the notion that succession usually follows predicable trajectories, and ends in a “climax” community. This article is often cited as the origin of contemporary concepts of predictable successional trajectories, as taught in many textbooks. Noy-Meir, I. 1975. Stability of grazing systems: An application of predator-prey graphs. The Journal of Ecology 63:459-481. A classical theoretical study of management strategies in grazing systems. Arguably the seminal paper suggesting that grazed systems may be subject to alternative stable states, and that over-grazing could cause a regime shift towards a lower-productivity plant community. The views and models from this paper are still widely used for studying alternative stable states today. Odum, E. P. 1960. Organic production and turnover in old field succession. Ecology 41:34-49. An early study of succession in an old field in South Carolina, USA. Though this was not the first study to quantify the flow of energy and nutrients through an ecosystem, it was one of the first to do so in a terrestrial system. The study includes quantitative analysis of how the biomass of individual herbaceous species changes over succession, and has greatly influenced modern field and analysis methods.

Tilman, D., Knops, J., Wedin, D., Reich, P., Ritchie, M. and E. Siemann. 1997. The influence of functional diversity and composition on ecosystem processes. Science 277:1300-1302. Results from the first large-scale experiment testing the effects of diversity on various ecosystem functions. Though communities were intentionally seeded with specific mixtures of grassland species, the resulting system is essentially a heavily managed old field. This study helped demonstrate that after controlling for environmental variability, there are strong, positive effects of increased species richness and functional group diversity on total community productivity and resource use efficiency, and inspired many subsequent large-scale experiments in ecology. Tansley, A. G. 1935. The use and abuse of vegetational concepts and terms. Ecology 16:284-307. A review of succession theory, based largely on earlier writings on old fields by Clements and Gleason (cited in *Historical Understanding of Succession*). Though primarilly remembered as the paper in which the word “ecoystem” was first used, this study is also important because it defined the concepts of “autogenic” vs. “allogenic” factors (self-generated vs. external factors, respectively). Tansley used these concepts to explore the kinds of processes that might influence successional trajectories, which has strongly influenced modern models of community assembly. Whittaker, R. H., editor. 1982 (Cited Edition from 2012). Ordination of plant communities. Handbook of Vegetation Science. Vol. 5-2. Springer Science & Business Media. Early summary of ordination methods (e.g. principal components analysis) for analysing ecological communities. Though initially developed for studying plant communties across successional and environmental gradients, these methods have expanded to be one of the primarily tools used by community ecologists. Ecosystem Management Because old field succession involves the recovery of landscapes after some kind of managed land use, there is substantial interest in applying these natural dynamics to restoration efforts. One of the earliest advocates for this is Clements (1935). More recently, a number of studies have focused on how succession can replace traditional management applications with low-intensity, low-cost strategies that meet the same goals. For example, Whisenant et al. 1995 shows how small changes in water mangament can lead to “autogenic” (i.e. self-sustaining) recovery in degraded pasture and agricultural land. Similarly, Gelfand et al. 2013 and Zumkehr and Cambell 2013 discuss the potential for biofuel energy production using biomass from old fields in the USA. However, it is important to note that succession in old fields does not always proceed along predictable trajectories as discussed in *Drivers of Contingency*, and generating desired outcomes from old fields could therefore be complex and costly. For example, Clark and Tilman 2010 shows that a combination of seed addition, litter removal, and nutrient reduction are all required in order to reverse the invasion by non-native species that occurred in fertilized old field grasslands at the Cedar Creek Ecosystem Science Reserve. In contrast, Kimiti et al. 2016 discusses rapid recovery of degraded rangeland in Kenya following from simple application of errosion barriors. Cramer et al. 2008 offers a framework that could potentially be used to predict which types of systems will be easy to restore, and which will require extensive management.

Clark, C. M., and D. Tilman. 2010. Recovery of plant diversity following N cessation: Effects of recruitment, litter, and elevated N cycling. Ecology 91:3620-3630. Study of restoration methods attempting to re-establish native species after heavy nitrogen addition in an invaded old field at the Cedar Creek Ecosystem Science Reserve. Shows that re-establishing native species requires simultaneous applications of seed addition, leaf litter removal, and reduction in soil nitrogen.

Clements, F. E. 1935. Experimental ecology in the public service. Ecology 16:342-363. Likely the earliest published record suggesting that natural successional dynamics could be used to provide particular desired services with minimal management input. Includes summary of existing evidence, and potential future services and experiments. Cramer V, Hobbs R, Standish R. 2008. What’s new about old fields? Land abandonment and ecosystem assembly. Trends in Ecology & Evolution. 23:104-112. A review of contemporary understanding of succession in old fields. Presents a conceptual framework that can be used to categorize different types of old field succession based on a site’s land use history and environmental conditions. The article argues that different categories of sites identified by this framework need to be managed differently to achieve particular types of restoration goals. Gelfand, I., R. Sahajpal, X. Zhang, R. C. Izaurralde, K. L. Gross, and G. P. Robertson. 2013. Sustainable bioenergy production from marginal lands in the US Midwest. Nature 493:514-517. Study of the energy production and soil carbon sequestration potential of successional vegetation grown on “marginal” lands in the Midwestern USA. Shows that 25% of the 2020 target for cellulosic ethanol production in the USA could be met on these lands, and that successional vegetation could provide energy output and net soil carbon sequestration rivalling that of commonly used agricultural crops. Kimiti, D. W., C. Riginos, and J. Belnap. 2016. Low-cost grass restoration using erosion barriers in a degraded African rangeland. Restoration Ecology. [doi: 10.1111/rec.12426] Study showing that erosion barriers can be a simple and cheap method for restoring native plant communities in highly invaded and eroded pastureland in Kenya. Offers a counter-argument to other studies which suggest that expensive, active management strategies may be needed to effectively remediate degraded old fields. Whisenant, S. G., T. L. Thurow, and S. J. Maranz. 1995. Initiating autogenic restoration on shallow semiarid sites. Restoration Ecology 3:61-67. Study of restoration efforts in Texas, USA for testing restoration methods in degraded pasture or cropland. Shows that planting seedlings of two potential keystone plant species in microcatchments increased water availability in the microcatchments, also increased seedling survival and establishment both in the microcatchment and in the surrounding area. Example of small changes that can utilize successional dynamics to make significant changes in ecosystems. Zumkehr, A., and J. E. Campbell. 2013. Historical U.S. cropland areas and the potential for bioenergy production on abandoned croplands. Environmental Science & Technology 47:3840-3847. Assessment of the extent of non-forested abandoned cropland in the continental USA. Suggests that “second generation” cellulosic bioenergy production on these lands could satisfy between 4 and 30% of the liquid fuel demand in the USA. Example of potential uses for abandoned cropland.