To help predict the effects of global warming on plants, previous studies have investigated how the distributions and physiological performances of plants relate to environmental temperatures. This approach implicitly assumes that leaf temperatures are tightly linked to regional air temperatures. However, the thermoregulatory behaviors and physical properties of leaves can differ greatly between species, leading to different plants having different leaf temperatures even when they occur under similar conditions. It is important to understand this variation in leaf thermoregulatory traits and temperatures in order to predict how individual species will be impacted by global warming. We measured the thermal properties of leaves from >50 tropical and subtropical tree species grown at the Gifford Arboretum (Florida, USA). For each species, we measured maximum leaf temperature and rate of leaf warming using a new standardized protocol to control for both environmental variation and select plant thermoregulatory behaviors. We tested the relationships between the two thermal variables and several leaf functional traits. Even under laboratory-controlled conditions, leaf temperatures varied by over 8.5°C among species. Maximum leaf temperature was positively correlated with leaf area, and rate of leaf warming was negatively correlated with water content per leaf area. This suggests that some species may be able to offset rising air temperatures through acclimation or adaptation of these leaf properties. Next, we tested the relationships between the species' leaf thermal properties and their climatic niches/distributions, but we found no significant associations.These results call into question the use of regional air temperatures to model plant physiological and demographic performance.

The heat tolerance of photosystem II (PSII) may promote carbon assimilation at higher temperatures and help explain plant responses to climate change. Higher PSII heat tolerance could lead to (a) increases in the high‐temperature compensation point (Tmax); (b) increases in the thermal breadth of photosynthesis (i.e. the photosynthetic parameter Ω) to promote a thermal generalist strategy of carbon assimilation; (c) increases in the optimum rate of carbon assimilation Popt and faster carbon assimilation and/or (d) increases in the optimum temperature for photosynthesis (Topt). To address these hypotheses, we tested if the Tcrit, T50 and T95 PSII heat tolerances were correlated with carbon assimilation parameters for 21 plant species. Our results did not support Hypothesis 1, but we observed that T50 may be used to estimate the upper thermal limit for Tmax at the species level, and that community mean Tcrit may be useful for approximating Tmax. The T50 and T95 heat tolerance metrics were positively correlated with Ω in support of Hypothesis 2. We found no support for Hypotheses 3 or 4. Our study shows that high PSII heat tolerance is unlikely to improve carbon assimilation at higher temperatures but may characterize thermal generalists with slow resource acquisition strategies.

The heat tolerance of photosystem II (PSII) may promote carbon assimilation at higher temperatures and may help explain plant responses to climate change. PSII heat tolerance could lead to 1) increases in the high temperature compensation point (Tmax); 2) increases in the thermal breadth of photosynthesis (i.e. the photosynthetic Ω parameter) to promote a thermal generalist strategy of carbon assimilation; 3) increases in the optimum rate of carbon assimilation Popt and promote faster carbon assimilation; and/or 4) increases in the optimum temperature for photosynthesis (Topt). To address these hypotheses, we tested if the Tcrit, T50 and T95 metrics of PSII heat tolerance were correlated with each carbon assimilation parameter for 21 species. Hypothesis 1 was not supported, but we observed that T50 may estimate the upper thermal limit for Tmax at the species-level, and that community mean Tcrit may be useful for approximating Tmax. The T50 and T95 heat tolerance metrics were positively correlated with Ω in support of hypothesis 2. We found no support for hypotheses 3 or 4. Our study shows that high PSII heat tolerance is unlikely to improve carbon assimilation at higher temperatures, but may characterize thermal generalists with slow resource acquisition strategies.