Drivers of thermal tolerance breadth of plants across contrasting biomes: do mean or seasonality in climate indices matter more?

The Climate Variability Hypothesis (CVH) predicts that species from environments with more variable temperatures should have wide thermal tolerance breadth. We addressed this question in plants and asked which climate predictors (e.g., temperature, precipitation, variability/seasonality) best predict species thermal limits. Measures of low (Tcrit-cold) and high (Tcrit-hot) photosystem II thermal thresholds were used to determine thermal tolerance breadth (TTB), along with ice nucleation temperature (NT) of 69 plant species in alpine, desert and temperate biomes. Tolerance metrics Tcrit-cold, NT and Tcrit-hot all differed among biomes. Notably, desert species had the most cold and heat tolerant leaves with the widest TTB. Alpine and temperate biomes had similar TTB and each biome TTB exceeded their climate thermal ranges. Overall, climate drivers explained the most variation (~50%) in TTB and NT, with species a second strong predictor. The climate variables best explaining tolerance differed for cold and heat: species from more variable (higher temperature seasonality) environments (alpine and desert) were more cold tolerant, whereas mean annual temperature (MAT) was the best predictor of Tcrit-hot with species in higher MAT environments having higher heat tolerances. TTB and NT were explained by both seasonality and MAT. Unexplained variation could be due to microclimate-driven plasticity, leaf traits or thermoregulatory mechanisms. Our results provide some support for the CVH. Depending on the thermal limits examined, climate means and seasonality remain important predictors of plant thermal tolerance. ### Competing Interest Statement The authors have declared no competing interest.