Physiological tolerance to frost and drought explains range limits of 35 European tree species

Models are key to evaluating how climate change threatens European forests
and tree species distributions. However, current species distribution
models struggle to integrate ecophysiological processes. Mechanistic
models are complex and have high parameter requirements. Some correlative
species distribution models have tried to include traits but so far have
failed to directly connect to ecophysiological processes. Here, we propose
an intermediate strategy where species distributions are based on their
safety margins which represent their proximity to their physiological
thresholds. We derived frost and drought safety margins for 38 European
tree species as the difference between physiological tolerance traits and
local maximum stress. We used LT50 and P50 as tolerance traits for frost
and drought, respectively, and local minimum temperature and minimum soil
water potential as maximum stress. We integrated these safety margins into
a species distribution model, which tests if the probability of species
presence declines rapidly when the safety margin reaches zero, when
physiological stress exceeds the species' tolerance traits. Our
results showed that 35 of the 38 studied species had their distribution
explained by one or both safety margins. We demonstrated that
safety-margins-based model can be efficiently transferred to species for
which occurrence data are not available. The probability of presence
dropped dramatically when the frost safety margin reached zero, whereas it
was less sensitive to the drought safety margin. This differential
sensitivity may be due to the more complex regulation of drought stress,
especially as water is a shared resource, whereas frost is not. However,
we did not find a clear effect of local competition hierarchy on the
response to safety margin, but competition needs further analysis. Our
analysis provides a new approach to linking species distributions to their
physiological limits and shows that, in Europe, frost and drought safety
margins are important determinants of species distributions.