Annals of Botany 101: 709 –715, 2008 doi:10.1093/aob/mcn010, available online at www.aob.oxfordjournals.org

A Test of the Scale-dependence of the Species Abundance –People Correlation for Veteran Trees in Italy M A R CO PA U TAS S O 1, * and A L E S S A N D RO C H I A R U C C I 2 1

2

Division of Biology, Imperial College London, Wye Campus, High Street, Wye, Kent TN25 5AH, UK and Dipartimento di Scienze Ambientali ‘G. Sarfatti’, Universita` di Siena, Via P.A. Mattioli 4, Siena, 53100, Italy

Received: 8 November 2007 Returned for revision: 11 December 2007 Accepted: 20 December 2007 Published electronically: 4 February 2008

† Background and Aims The spatial correlation of the presence of people and species has been suggested to be scale-dependent. At local scales, large numbers of people often result in species impoverishment. At coarse scales, species-rich regions tend to be densely inhabited. Recently, broad-scale human presence has been shown to be correlated not only with numbers of species but also with their abundance, as predicted by the more-individuals hypothesis. However, it is not known whether the species abundance – human presence correlation could also be scale-dependent. † Methods This hypothesis was tested by use of a database of veteran trees in Italy. Veteran tree species richness and number of individuals were modelled as a function of human population size at two grains of analysis ( provinces and regions), controlling for variations in area, latitude and spatial autocorrelation. † Key Results A positive correlation was found between human presence and veteran tree species. As predicted, this correlation was stronger at a coarser resolution. However, only at the provincial but not regional level was there a positive correlation between human presence and veteran tree abundance when controlling for area and latitude. These results were confirmed for native and exotic trees. † Conclusions The present findings rule out the more-individuals hypothesis as an explanation of the scaledependence of the species – people correlation for veteran trees in Italy. Potential mechanisms behind the observed spatial coincidence of high numbers of people and veteran tree species are discussed and implications for conservation are highlighted. Key words: Ancient trees, biogeography, dendrology, historical parks, latitudinal gradients, macroecology, native and exotic flora, Quercus spp., spatial patterns, species– area relationship, tree inventories, urban ecosystems.

IN TROD UCT IO N The spatial co-occurrence of the presence of people and of biodiversity has been the focus of much recent attention. A positive correlation between human presence and species richness has now been reported from Africa (e.g. Balmford et al., 2001), North (e.g. McKinney, 2001) and South America (e.g. Fjeldsa˚, 2007), Asia (e.g. Hunter and Yonzon, 1993), Australia (Luck et al., 2004) and Europe (e.g. Arau´jo, 2003). The range of taxa involved in these studies ( plants, butterflies, amphibians, reptiles, birds and mammals) has been just as wide as the geographical extent of analysis (Luck, 2007). At first sight, this pattern appears to be counterintuitive. Species richness might be thought to be higher in wilderness areas, given the often reported negative impacts of humans on a wide range of species (Gaston, 2005). As a means of explanation, it has been suggested that the species – people correlation could be scale-dependent (e.g. Manne, 2003; Vazquez and Gaston, 2006). At large scales of analysis this correlation might tend to be positive, as people have settled preferentially and flourished in regions of mild climate and soil productivity, where species numbers were originally higher, or have been increased with introductions of exotic organisms and as a result of habitat modifications (e.g. Ku¨hn et al., 2004; Gaston, 2005; Williams et al., 2005; Luck, 2007). * For correspondence. E-mail [email protected]

Conversely, at a local scale, the impact of high numbers of people on biodiversity is often detrimental and species richness is generally lower where human population is higher (e.g. Hansen et al., 2005; Celesti-Grapow et al., 2006). There is some evidence that the species –people correlation might indeed be scale-dependent. In the Manchester region of the UK, a positive correlation of species richness with level of urbanization at a coarse resolution and a negative one at a fine grain of analysis were reported for butterflies (Hardy and Dennis, 1999). For birds in South Africa, the correlation of species richness with the presence of humans becomes weaker when narrowing the scale, or grain, of the study (Chown et al., 2003). Moreover, analyses of independent studies have shown that the correlation coefficient between human presence and species richness of plants and vertebrates is positively correlated with study grain and extent (Luck, 2007; Pautasso, 2007). However, it is not known whether this scale-dependence in relation to human presence might apply not only to species richness but also to species abundance. For European countries, both bird species richness and number of bird individuals are positively correlated with human population size (Gaston and Evans, 2004). This is expected if the more-individuals hypothesis applies as an explanation of the positive correlation between species richness and energy availability and if there is a positive

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Pautasso and Chiarucci — Veteran Trees and People in Italy

correlation between human numbers and energy availability. The more-individuals hypothesis states that high productivity allows species to persist with larger populations, thus reducing the chance of local extinctions and leading to higher species richness (e.g. Srivastava and Lawton, 1998; Mo¨nkko¨nen et al., 2006; Yee and Juliano, 2007). If (1) the more-individuals hypothesis explains the occurrence of high numbers of species in regions of high productivity, (2) people tend to be present at higher numbers in these regions and (3) the species – people correlation is scale-dependent, then the correlation between people and species abundances would be expected to be scale-dependent. Here, we test whether the correlation between human presence and species abundance is scale-dependent. Data on the abundance and species richness of veteran trees in Italy are used. Veteran trees have exceptional age, size, habit and history and have managed to withstand the threats posed by several generations of humans (e.g. White, 1997; Denzler, 1998, 1999; Arnould and Cieslak, 2004). In some cases this has been possible because of their active preservation by people in relation to a sacred or social meaning acquired by old-growth trees and patches of woodlands (e.g. Flasche, 1994; Salick et al., 2007). Ancient trees are thus a legacy of past ages and are of exceptional importance given their irreplaceable status and the biodiversity they harbour in their canopy (e.g. Grove, 2002; Caramiello and Grossoni, 2004; Lonsdale et al., 2008). Veteran trees are being inventoried in a number of countries to promote their preservation (e.g. Jim, 2005; Krebs et al., 2005; Orłowski and Nowak, 2007), but, to the best of our knowledge, data regarding the spatial location of ancient, heritage or veteran trees have not been yet analysed in relation to the species – people correlation. M AT E R I A L S A N D M E T H O D S Records of veteran tree species and abundance in Italy at the regional, provincial and municipal level were obtained from a national database (Alessandrini et al., 1990 – 1991). The data rely on a survey performed in 1982 by the Italian National Forest Service. There have been some regional and local updates since then (e.g. Lenna and Galasso, 2004; Russo et al., 2004), but the database used is still unmatched in terms of uniformity of coverage of the various regions. On the whole, the data set contains information about 1255 veteran trees of 140 species and is a selection of approx. 22 000 remarkable trees, which were initially considered. The five species with most veteran trees in the data set are Quercus pubescens (211), Fagus sylvatica (112), Cedrus libani (58), Castanea sativa (52) and Quercus ilex (52). Species of the genus Quercus comprise 35 % of the veteran trees reported. Together with species of the genus Fagus (9 %) and Cedrus (8 %), they make up the majority of the veteran trees analysed here. Fifty-one species have only one veteran tree individual in the data set. Roughly half of the veteran tree species and roughly one-quarter of the veteran tree individuals in the data set analysed are

exotic. The average height of the veteran trees analysed is approx. 22 m. The maximum height is 52 m (Liriodendron tulipifera, Lombardy). The average circumference is approx. 5 m, the maximum 22 m (Castanea sativa, ‘castagno dei cento cavalli’, Sicily). The number of veteran tree individuals (species) varied amongst Italy’s 20 regions between nine (five spp.) (Molise) and 192 (62 spp.) (Lombardy). The average was 63 (23 spp.), the median 44 (18 spp.) and the standard deviation 50 (15 spp.). For Italy’s 103 provinces, the number of veteran tree individuals (species) ranged between zero (14 provinces) and 79 (40 spp.) (Varese). The average was 13 (seven spp.), the median nine (five spp.) and the standard deviation 13 (six spp). For Italy’s 8101 municipalities, the number of veteran tree individuals and species was between zero (7395 municipalities) and 13 (Luino). Sixty-five per cent of the municipalities that had veteran trees (461) had just one veteran tree. Only 9 % of these municipalities had more than three veteran trees. Only five municipalities had ten veteran trees or more (Luino, Roma, Stresa, Pavullo nel Frignano, Varese). Human population size (2001) and area of Italian regions, provinces and municipalities (completely nested levels) were obtained from the Italian National Institute of Statistics (ISTAT). Human population size varied in Italy’s regions between approx. 120 000 (Valle d’Aosta) and approx. 9 million (Lombardy). Mean region population size was approx. 2.9 million, median approx. 1.8 million and the standard deviation approx. 2.3 million. For Italy’s provinces, human population size ranged between approx. 90 000 (Isernia) and approx. 3.7 million (Milano). Mean province population size was approx. 550 000, median approx. 370 000 and the standard deviation approx. 600 000. For Italy’s municipalities, human population size was between 33 (Morterone) and approx. 2550 000 (Rome). Mean municipality population size was approx. 7000, median approx. 2300 and the standard deviation approx. 39 000. The area of Italy’s regions varied between approx. 3300 (Valle d’Aosta) and approx. 25 700 km2 (Sicily). Mean region area was approx. 15 000, median approx. 14 300 and the standard deviation approx. 7400 km2. For Italy’s provinces, area ranged between 212 (Trieste) and 7560 km2 (Sassari). Mean province area was approx. 2900, median approx. 2600 and the standard deviation approx. 1700 km2. For Italy’s municipalities, maximum area was 1286 km2 (Rome) and minimum area was 0.1 km2 (Fiera di Primiero). Mean municipality area was 37, median 22 and the standard deviation was 50 km2. The correlation of human population size with veteran tree abundance and species richness was analysed controlling for variations in area and latitude amongst regions and provinces. This was achieved using multivariate models. Number of veteran tree species and individuals ( plus one at the provincial level, as 14 provinces had no veteran trees), human population size, region and province area were log-transformed to conform to the assumptions of statistical tests. At the municipal level, given that most municipalities with presence of veteran trees had one veteran tree only, it was not possible to analyse properly the number of veteran tree species or individuals as

Pautasso and Chiarucci — Veteran Trees and People in Italy a function of human population size, area and latitude. Therefore, on a presence/absence basis we investigated whether municipalities with veteran trees had more inhabitants than those without. Analyses were performed in SAS 9.1. Apart from ANOVAs, spatial autocorrelation was controlled using mixed models with exponential co-variance structure (as, for example, in Pautasso and Gaston, 2006). Results from non-spatial and spatial models are qualitatively consistent, but because spatial autocorrelation can lead to misleading parameter estimates due to non-independence of data points located close to each other (e.g. Dormann, 2007), only the results of the more conservative analyses involving spatial models are presented.

F I G . 1. The log– log relationship in the 20 Italian regions between (A) veteran tree individuals and veteran tree species, (B) human population size and veteran tree individuals, and (C) human population size and veteran tree species richness. The second relationship is not significant when controlling for variations in area and latitude (see Results).

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R E S U LT S At both the regional and the provincial level there were significant positive relationships between the number of veteran tree individuals and the number of veteran tree species (Figs 1A and 2A). Human population size was a significant factor in these relationships at the regional but not at the provincial level (Table 1). This result was confirmed when repeating the same analysis for native and exotic veteran tree species (Table 1). At the regional level there was no significant relationship between the number of veteran tree individuals and human population size (Table 2), although this relationship was significantly positive without controlling for area and latitude (Fig. 1B). However, at the same level there was a positive relationship between human population size and the number of veteran tree species (Table 2). These results

F I G . 2. The log– log relationship in the 103 Italian provinces between (A) veteran tree individuals (þ1) and veteran tree species (þ1), (B) human population size and veteran tree individuals (þ1), and (C) human population size and veteran tree species richness (þ1).

0.29 0.91 0.007 0.05 0.02 0.74 0.0001 0.0001 –0.75b (+0.24) –0.06b (+0.03) –1.75 0.17 Regional Provincial Exotic

0.92 0.95

þ0.85x (+0.12) þ0.83x (+0.02)

þ0.36a (+0.13) þ0.01a (+0.03)

–0.02c (+0.02) –0.001c (+0.003)

0.05 0.16 0.06 0.13 0.06 0.55 0.04 0.0001 þ0.28b (+0.14) þ0.07b (+0.05) –1.78 –0.79 Regional Provincial Native

0.89 0.90

þ0.17x (+0.08) þ0.68x (+0.03)

þ0.13a (+0.06) þ0.02a (+0.04)

þ0.015c (+0.007) þ0.011c (+0.008)

0.12 0.14 0.89 0.29 0.004 0.58 0.03 0.0001 þ0.025c (+0.016) þ0.05c (+0.008) –0.03b (+0.22) þ0.05b (+0.04) þ0.32a (+0.09) þ0.02a (+0.03) –2.24 –0.76 Regional Provincial Total

0.88 0.93

þ0.34x (+0.14) þ0.75x (+0.02)

Log area y Scale

r2

Log abundance

Log population

Latitude

Pab

Ppop

Parea

Plat

Pautasso and Chiarucci — Veteran Trees and People in Italy TA B L E 1. Results of models of total, native and exotic veteran tree species richness (y ¼ intercept) as a function of veteran tree number of individuals (x) controlling for (a) human population, (b) area and (c) latitude for Italy’s regions (n ¼ 20) and provinces (n ¼ 103). Parameter estimates are given together with their standard errors

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were confirmed repeating the same analyses for native and exotic veteran tree individuals and species (Table 2). The proportion of variance in veteran tree species explained by human population size alone was 68 % for all species (Fig. 1C), 72 % for native species and 41 % for exotic species. At the provincial level, there was also a positive relationship between human population size and the number of veteran tree individuals (Table 2). This relationship was significantly positive also when not controlling for area and latitude (Fig. 2B). At the same level, there was a positive relationship between human population size and the number of veteran tree species (Table 2). These results were confirmed repeating the same analyses for native and exotic veteran tree individuals and species (Table 2). The proportion of variance in veteran tree species explained by human population size alone was 15 % for all species (Fig. 2C), 10 % for native species and 12 % for exotic species. At the municipal level, the number of inhabitants was significantly higher (ANOVA, F1, 8099 ¼ 77, P , 0.0001) for the 706 municipalities with presence of veteran trees [mean population size (s.d.) ¼ 19 900 (119 300)] than for the 7395 municipalities without presence of veteran trees [5900 (20 200)]. However, area was also significantly larger (ANOVA, F1, 8099 ¼ 471, P , 0.0001) for municipalities with veteran trees [76.5 km2 (92.6)] than for municipalities without [33.7 km2 (42.4)], so that population density did not vary significantly (ANOVA, F1, 8099 ¼ 3.1, P ¼ 0.08) between municipalities with veteran trees [238 km22 (573)] and those without veteran trees [282 km22 (620)].

DISCUSSION Study of the distribution and ecological correlates of species richness, rarity and abundance is central to ecology. Biodiversity conservation through in-situ preservation requires information about the spatial location of priority sites and there is now a tradition in selecting areas of conservation importance. Increasingly, researchers have realized that over large scales there is a substantial co-occurrence of species-rich sites and areas of high human population presence. This obviously makes conservation activities more difficult, given the detrimental impacts of humans. But just as the study of biodiversity has moved on from simple studies of species richness, there is a need for an enlargement of the view regarding the coexistence of people and biodiversity. The present study shows that not only species richness, but also the abundance of veteran trees correlates with human population size in Italy, although for abundance this result is present at the provincial but not at the regional level. Veteran trees are a neglected botanical resource, despite having become an unusual presence in the landscape of many countries, and deserve more study and protection as they are likely to harbour a wide variety of rare and endangered organisms. More research is needed for other taxa to extend the study of the correlation between people and biodiversity to factors such as species abundance, endemism and rarity. There is also a need for investigations of patterns

Pautasso and Chiarucci — Veteran Trees and People in Italy

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TA B L E 2. Results of models of total, native and exotic veteran tree individuals and of total, native and exotic veteran tree species richness (y ¼ intercept) as a function of (a) human population, (b) area and (c) latitude for Italy’s regions (n ¼ 20) and provinces (n ¼ 103). Parameter estimates are given together with their standard errors

Total individuals Native individuals Exotic individuals Total species Native species Exotic species

Scale

r2

y

Log population

Log area

Latitude

Ppop

Parea

Plat

Regional Provincial Regional Provincial Regional* Provincial Regional Provincial Regional Provincial Regional Provincial

0.77 0.36 0.62 0.31 0.79 0.30 0.85 0.41 0.85 0.38 0.70 0.29

–6.27 –8.06 –4.83 –6.78 – 10.22 –6.06 –4.06 –6.42 –2.49 –4.97 –6.85 –4.85

þ0.11a (+0.15) þ0.42a (+0.13) –0.03a (+0.14) þ0.24a (+0.12) þ0.45a (+0.23) þ0.48a (+0.12) þ0.36a (+0.10) þ0.33a (+0.10) þ0.15a (+0.07) þ0.19a (+0.09) þ0.76a (+0.24) þ0.39a (+0.10)

þ1.09b (+0.27) þ0.81b (+0.15) þ1.17b (+0.28) þ0.81b (+0.14) þ0.67b (+0.42) þ0.21b (+0.14) þ0.29b (+0.19) þ0.65b (+0.12) – 0.44b (+0.13) þ0.62b (+0.10) – 0.20b (+0.44) þ0.14b (+0.12)

þ0.06c (+0.02) þ0.09c (+0.03) þ0.04c (+0.04) þ0.08c (+0.03) þ0.13c (+0.02) þ0.07c (+0.02) þ0.04c (+0.01) þ0.07c (+0.02) þ0.02c (+0.007) þ0.06c (+0.02) þ0.08c (+0.03) þ0.06c (+0.01)

0.48 0.001 0.84 0.05 0.08 0.0001 0.003 0.002 0.06 0.04 0.006 0.0002

0.001 0.0001 0.0007 0.0001 0.14 0.12 0.14 0.0001 0.004 0.0001 0.66 0.23

0.02 0.001 0.30 0.02 0.0001 0.0001 0.001 0.0001 0.02 0.001 0.006 0.002

* Results from non-spatial model (mixed model did not converge).

of genetic, functional and phylogenetic diversity in urban vs. rural areas (Ku¨hn and Klotz, 2006). For both Italy’s regions and provinces, there were positive correlations between human population size and the number of veteran tree species. As predicted by the hypothesis that the spatial species– people correlation is scale-dependent, a stronger correlation between the number of inhabitants and number of veteran tree species was found for Italy’s regions ( proportion of variance explained ¼ 68 %) than for Italy’s provinces (15 %). This result is consistent with studies showing that the species – people correlation increases in strength when enlarging the grain of a study (e.g. Chown et al., 2003). At a more local grain, there was no evidence that Italy’s municipalities with veteran trees had a higher population density than those without reported veteran trees. This finding is in agreement with studies at local to intermediate scales which failed to find a positive correlation between numbers of species and numbers of people (e.g. Roy et al., 1999). As for the more-individuals hypothesis, whilst there was a significant correlation of human population size with number of veteran tree individuals at the provincial level, we found no significant association of human population size with number of veteran tree individuals at the coarser grain, i.e. for Italy’s regions, when controlling for variations in area and latitude. This runs counter to expectations of a stronger correlation between people and species abundance at a larger scale of analysis, which would apply if the positive species – people correlation over large scales is explained by the more-individuals hypothesis. For veteran trees in Italy, there is a strong correlation at a regional grain between number of tree species and people, but not between number of tree individuals and people, and therefore an explanation other than the more-individuals hypothesis for the scale-dependence of the species – people correlation, at least for veteran trees in Italy, needs to be sought. This finding confirms the importance of scale in tests of ecological hypotheses. More work is needed to assess the influence of study grain and extent on the mechanisms behind broad-scale positive correlations between the presence of humans and biodiversity.

At both the regional and the provincial level there were significant increases in the number of veteran tree species and individuals with increasing latitude. This is an exception to the commonly reported negative latitudinal gradient of species richness and abundance (e.g. Willig et al., 2003; see also Pautasso and Parmentier, 2007). However, a reversed latitudinal gradient of veteran tree species richness and abundance cannot provide an explanation of the positive species – people correlation observed, as the number of people in both Italy’s regions and provinces does not vary significantly with latitude (our unpubl. res.). Another potential explanation of the observed species – people correlation could be that both species and people increase with area (e.g. Drakare et al., 2006; Pautasso and Weisberg, 2008). Although an increase of veteran tree species with area was observed (albeit for provinces only), and although the number of people and administrative area are positively correlated (our unpubl. res.), this cannot provide an explanation for the positive species – people correlation, as the latter was analysed controlling for variations in area. Italy is located at the heart of the Mediterranean hotspot of plant biodiversity (e.g. Medail and Quezel, 1999). Optimal conditions for tree growth over the centuries are probably found in the region of the Northern lakes, given also the presence of several historical gardens founded at the time of the Grand Tour, where majestic trees contributed to making these places idyllic and were thus planned for and carefully maintained. Indeed, the highest presence of veteran trees in the data set analysed is located in areas around Lake Maggiore and Lake Como, and these also happen to be regions of high human population presence. The positive relationship between the presence of veteran tree species and individuals and latitude may also be the consequence of a more sustainable management of natural resources in the Northern regions. These factors can explain, at least in part, the spatial coincidence of large numbers of veteran trees and of people, but it is not clear whether a climatic and historical explanation could help explain why the correlation is stronger at a coarser resolution.

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A similar argument can be made regarding the idea that species –people correlations are the outcome of a sampling bias. This has been reported for the well-known productivity – diversity relationship (e.g. Chiarucci et al., 2004). According to this hypothesis, more populated regions have more species because they have been sampled more thoroughly. Although there is independent evidence that this need not be the case for birds in Britain and vascular plants in the USA (Evans et al., 2007; Pautasso and McKinney, 2007), it is not evident how a sampling argument can be invoked to justify the presence of a stronger species – people correlation at coarser resolutions. Given that the database of 1255 veteran trees analysed was screened starting from an initial set of approx. 22 000 trees, and given that the whole point of veteran trees is that they are remarkable, and thus easy to spot even by the uninitiated, it is unlikely that sampling bias can be a major issue here. The data analysed show that the patterns reported – (1) stronger correlation between human population size and veteran tree species richness at the regional vs. provincial level, (2) absence of a significant association of human population size with veteran tree abundance at the regional level, and (3) absence of significance of human population size at the provincial level in a model of veteran tree species richness as a function of veteran tree abundance – are consistent for native and exotic veteran tree species. This absence of differences in the results would suggest that, at least for Italy, native and exotic veteran trees do not seem to respond to, respectively, natural and cultural factors. In many cases people may have directly contributed to the presence of veteran trees with planting of both native and exotic trees in parks, streets, squares and botanical gardens, and climatic factors are also likely to operate in a similar way on both native and exotic tree species. More research is needed from other countries to assess the generality of the patterns reported here. For example, the UK has a remarkably large number of veteran trees, but it is not known whether their presence correlates positively with human settlements, at least over broad scales. In Bavaria, Germany, there is a widespread religious tradition of planting and preserving trees in the countryside, and this may well have produced a different association of veteran trees with human presence. Italy has a relatively high national human population density (nearly 200 inhabitants km22) and a long history of civilization and land-use alterations (e.g. Grapow and Blasi, 1998; Canova, 2006), even if its current proportions of protected (19 %) and forest (23 %) areas are relatively high. The evidence presented here shows that Italian people have managed to preserve a wide variety of veteran tree species across Italy’s regions and provinces. The finding that the numbers of veteran tree species and individuals significantly increase with human population size at different administrative scales suggests that the presence of people not only causes biotic destruction but can also be a force for biodiversity conservation and promotion. People might have actually felt a more pressing need to preserve trees of exceptional appearance in regions and provinces with higher human presence and landscape transformation.

ACK N OW L E D G E M E N T S Many thanks are due to the many people involved in the compilation of the database of veteran trees in Italy, to G. Aas, R. Caramiello, R. Davies, M. Dinetti, K. Evans, D. Fontaneto, R. Fuller, K. Gaston, O. Holdenrieder, S. Jackson, M. Jeger, M. McKinney, I. Parmentier, G. Powell, A. Rodrigues, B. Schlick-Steiner, M. Sieber, C. Steck, F. Steiner, L. Vazquez, P. Warren, P. Weisberg and M. Zotti for insightful discussions, and to O. Holdenrieder, T. Matoni, B. Shipley and two anonymous reviewers for helpful comments on a previous draft of the manuscript.

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