Weak global trade-off between frost and drought resistance in trees.

Drought and frost stresses play important roles in determining species distributions, especially at range margins. Understanding how stress resistance traits interact to determine vulnerability to climate change is critical. We developed a large global database of published and new measurements of drought resistance (xylem embolism resistance; P50) and frost resistance (electrolyte leakage; LT50), and investigated evolutionary trade-offs using Bayesian phylogenetic quantile regressions. Across all woody biomes, P50 ranged from -1 to -19 MPa, and LT50 from 0 to below -80°C with conifers generally more resistant than angiosperms. We found a weak trade-off between drought and frost resistance: Drought-resistant species tend to be less frost hardy, and vice versa. There are few species resistant to both stresses (e.g. junipers). Including the phylogeny reduced the strength of the relationship, reflecting the phylogenetic signal for these traits. We did not find any strong effects of LT50 on growth-related traits, but drought resistance is associated with denser wood, smaller conduits, shorter stature and lower specific leaf area. While we show a trade-off between frost and drought resistance, our study does not support the global fast-slow economics spectrum. Our results have implications for forests experiencing hotter, drier summers and potentially damaging late frosts.