Leaf-level resistance to frost, drought and heat covaries across European temperate tree seedlings

Most trees die as seedlings, with harsh environmental conditions, such as early-spring frosts and summer heat waves, being important drivers of early mortality. However, it remains unclear whether tolerance to different environmental extremes (e.g. frost vs. heat) trades-off, or covaries synergistically and how stress tolerances relate to growth rates and life history strategies. Given the likely role of extreme environmental conditions as environmental filters, the ability to tolerate different stressors at the seedling stage could shape the occurrence and composition of present and future forests.

We explored the relationships between different leaf-level stress tolerances, functional traits and geographic distributions across 22 species of temperate European tree seedlings. We measured indicators of tolerance to frost, drought and heat and related these values to growth rates and to important functional traits (e.g., leaf mass per area, stem specific density). Finally, we explored the links between measured seedling stress tolerances and climatic niche limits inferred from adult trees' distributions.

We found that seedlings of most species were either moderately tolerant to all three stressors, or susceptible to all of them. Moreover, higher stress tolerances were associated with traits describing slower growth and lower competitive ability. However, seedling tolerances to climatic factors were unrelated to the environmental limits of their adult geographic distributions.

Synthesis. Our results suggest that temperate tree seedlings might not experience trade-offs when facing an increase in multiple extreme climate stressors, but may experience trade-offs related to growth rate and competitive ability in the establishment phase. The lack of correlation between leaf-level stress tolerances and the environmental limits of adult geographic distributions suggests that predicting species' current or future geographic distributions in Europe will require a more nuanced understanding of how climatic tolerances at juvenile and adult stages influence range limits. A better understanding of the interaction between survival in extreme climate, leaf-level stress tolerances of seedlings, and the factors driving species distributions is needed to understand future forest responses to climate change.