Thermal Safety Margins and Peak Leaf Temperatures Predict Vulnerability of Diverse Plant Species to an Experimental Heatwave

Extreme heat can push plants beyond their thermal safety margin (TSM) if maximum leaf temperature (T_{leaf_max}) exceeds leaf critical temperature (T_crit). The TSM is potentially useful for assessing heat vulnerability across species but needs further validation, so we exposed 50 tree/shrub species in controlled glasshouses to a 6-day heatwave (peak air temperature = 41°C). Many species increased their mean T_crit during the heatwave (42%), with ΔT_crit ranging from +1°C to 4°C, but other species did not acclimate or were impaired by heat stress (58%). Species T_{leaf_max} explained ~55% of the variation in species T_crit and was a key correlate of the plasticity of T_crit among species. Species with high ΔT_crit also had higher ΔT_{leaf_max}, with leaves being 7°‒12°C hotter during the heatwave than under baseline conditions. Both T_{leaf_max} and TSMs were correlated with heatwave damage across diverse species from contrasting climate zones. Species differences in TSMs were stable across measurement temperatures, correctly identified the most vulnerable species, and were strongly associated with T_{leaf_max}. Our results suggest that (1) T_{leaf_max} alone is more informative than T_crit for ranking species heat tolerance, and (2) species vulnerability to heatwaves is most reliably assessed by using TSMs that integrate T_{leaf_max} with T_crit across species.