Hello volunteers! I’m very sorry for the lapse in my communication over the last couple of months. For personal reasons I am taking a break from my Honours year and therefore will not be completing my paper this semester. However, I have fully analysed the results of the project and am eager to share a summary of them with you! Project aims and questions Just to remind you, the aim of my project was to investigate whether the ranges of 6 common Banksia species in WA are contracting in response to climate change, particularly rainfall decline. My project consisted of two major questions: 1. Do current populations of banksia species show symptoms of increased water stress in response to climate change? 2. Has there been contraction of widespread banksia species in the Southwest over recent decades? The two questions are related: Question 2 seeks to find out whether ranges of banksia species are contracting, while Question 1 helps provide some understanding as to why. In carrying out the survey work, you were collecting the data I needed to answer Question 2! Results relating to Q1 To determine whether the banksia species I studied were exhibiting signs of water stress I measured the stable carbon isotope ratio (δ13C), and also some leaf attributes, such as thickness, weight and area, of banksia leaves from a range of populations along a climate gradient. Leaf δ13C provides an indication of how water-‐stressed a plant is. The more positive the δ13C value, the more water-‐limited the plant. Overall, we found that leaf δ13C values exhibited strong, linear relationships with climate variables e.g. mean annual temperature (MAT) and mean annual precipitation (MAP). Figure 1 displays the relationship between leaf δ13C and MAP. You can see that as precipitation decreases, δ13C becomes more positive. This suggests that banksia species at sites
receiving lower annual rainfall (i.e. northern sites around Kalbarri) are more water-‐stressed than from wetter southern sites.
This is me in the lab at UWA preparing my leaf samples for isotope analysis.
I measured leaf attributes of the banksia species, such as thickness, because changes in these characteristics can provide an indication of changes in environmental conditions, such as drying. In general, we found that leaf thickness exhibited a significant linear relationship with climate variables. Figure 2 illustrates the relationship between leaf thickness and MAP. As precipitation decreases, leaf thickness increases. The relationship is not as strong as for δ13C, however, these physical changes can be difficult to detect – we were excited to pick up a response! The results suggest that in drier and hotter environmental conditions, banksias divert more carbon into structural leaf tissue than at wetter sites. This might be an adaptation to reduce water loss from the leaf in water-‐limiting environments. Another possibility is that banksias are water-‐stressed at the northern ends of their range.
Some of my leaf samples ready for measurement. Weighing grandis leaves.
Overall, the isotope and leaf attribute results suggest that banksias at the most northern end of their distribution, particularly B. attenuata, B. prionotes and B. menziesii, are “doing it tough” – these populations are most vulnerable to the impacts of future climate change. These species are well adapted to arid conditions; however, their adaptations may not be sufficient to safeguard them against the effects of future climate change, if they have reached the edge of their tolerance. If rainfall decline continues, this could lead to large-‐scale contraction and extinction? See figures à
Figure 1: The relationship between leaf δ13C and MAP. As precipitation decreases, δ13C becomes more positive, indicating water stress.
Figure 2: The relationship between leaf thickness and MAP. As precipitation decreases, leaf thickness increases; an adaptation to decrease water loss form the leaf.
Results relating to Q2 Now for the results you are most interested in: the outcome of the range contraction surveys! We conducted statistical modeling to determine what factors (i.e. aridity, phytophthora or disturbance) were contributing most to the extinction rates of the banksia species. The take home message from the analysis is that there was NOT a strong signal that aridity was a major factor in extinction. Overall, disturbance (e.g. clearing and road realignment) came out as the most significant factor contributing to extinction rates. Phytophthora came out as a significant issue for southern populations. These results highlight an important message: that currently the biggest threat to banksia populations is not future climate change, but the immediate impacts of human activity! Although aridity was not the main factor affecting banksia extinction, it still seems to be playing a role! Aridity came out as, not a strong, but a significant influencing factor in explaining local extinction in the northern populations of B. attenuata, B. menziesii, B. prionotes and B. grandis. Aridity NEVER came up as an influencing factor in the southern populations. These are really interesting results! The above four banksias are the most widespread species in my study, and are not found near water sources and damp areas like B. seminuda and B. littoralis. Therefore, they need to be adaptable to a wide range of environmental conditions. This is where these results now tie-‐in with my isotope and morphology results, which demonstrate that these four banksia species are adapted to their local environments. Plants from dry northern populations exhibit adaptations to arid conditions. However, what the range contraction results suggest is that these adaptations will not be enough to buffer them against the onset of aridity under climate change. We may be seeing the very beginning of increased aridity starting to exert a small influence on banksia extinctions in the northern end of their range, an influence that is very likely to grow. The good news is that we didn’t find any catastrophic collapse; the bad news is that we may have found the first hint of the beginning of an expected range contraction in these species. Thank you! I would like say thanks just once more to all the volunteers for their time, effort and support with my banksia surveys! I’d like to remind you
that together we surveyed 95% of my field sites, so it is thanks to your generosity that we have these results! I am disappointed that I am unable to finish my paper just now, however, I want to reassure all of you that these important results will be published within the next 12 months! This project proves what an incredible scientific resource the community can be! Many thanks, Sarah
