Uncertainty in the modelled mortality of two tree species (Fraxinus) under novel climatic regimes

Aim: Based upon species distribution models (SDMs), many studies have predicted that climate change will cause regional extinctions of tree species within the next 50–100 years. SDM-based predictions have been challenged on procedural and theoretical grounds, but for tree species they are largely beyond the practical reach of direct experimental validation. Here we report the experimental consequences of moving seedlings from ~50 natural populations of each of two ash (Fraxinus) species to experimental sites spanning a range of 10°C colder to 10°C warmer (mean annual temperature) than home environments. Location: Eastern North America. Methods: We measured population-by-test-site survival percentages and mean trunk diameters at an average age of 35 years. We then used linear, mixed-effect models to develop transfer functions for each species and predict survival and mean annual growth as functions of fixed and random effects including, especially, the climatic distance (CD) between test site and home environment. Results: Survival and growth were highest at CD ≈ 0 and declined as populations were moved to warmer or colder environments, indicating that survival and growth were optimal when populations were in home-like climates. However, variance around the model fit was substantial, and we could not statistically detect, even at α =.50, elevated mortality following displacements into environments 3.5°C (white ash) and 4.1°C (green ash) warmer in mean annual temperature. Survival rates of 80%–100% were common even within populations subjected to warming conditions greater than those predicted to cause meso-scale extinctions in this century. We show that within-population genetic variance, phenotypic plasticity and idiosyncratic aspects of the non-climatic environment and its interaction with genotype each likely contributed to these unexpected responses to climatic displacement. Main conclusions: Results emphasize the uncertainty that underlies predictions of climate-induced extinctions of long-lived woody plants over time frames of 50 to perhaps 100 years into the future.