Photosynthetic heat tolerances do not explain the decelerating growth rates of tropical trees

Background/Question/Methods: The leading hypotheses to explain the widely observed decelerating tree growth rates in tropical forests are rising air temperatures and/or drier conditions that cause higher respiration rates that deplete carbon reserves, and ultimately reduce tree growth. Recent research, however, has shown that tree respiration can acclimate to higher temperatures. This suggests that additional explanations for the observed declines in forest growth rates are needed. Photosynthetic Heat Tolerances (PHT) are traits that represent the highest temperatures that a plant’s photosynthetic machinery can experience before suffering permanent damage, and are unexplored for understanding how tropical forests respond to climate change. Since leaves of different species possess different PHTs and different physical properties that influence leaf temperature (e.g. leaf size, shortwave absorptivity, and stomatal conductances), even co-occurring species will experience different amounts of thermal stress. Consequently, it is important to know the temporal trends in leaf temperature, and the occurrence of thermal damage in order to examine the potential effect of thermal stress on changes in tree growth rates. To test the hypothesis that thermal stress contributes to the decelerating growth rates of tropical trees, we measured photosynthetic heat tolerances and thermoregulatory leaf traits for 8 mature canopy tree species growing in a lowland tropical rainforest for which 8 years of growth rate data were available. We coupled Leaf thermoregulatory data and high-resolution (30-minute interval) climate data to parameterize a leaf energy balance model, and estimate day-time leaf temperatures for the 8 years associated with tree growth data.

Results/Conclusions: Predicted leaf temperatures ranged from 15.0 to 63.4˚C, and species’ mean annual leaf temperatures ranged from 28.2 (+/-3.0) to 33.2 (+/-6.3) ˚C - temperatures close to thermal optima for photosynthesis. Predicted leaf temperatures rarely exceeded species’ PHTs. Differences between maximum daily leaf temperatures and PHTs were used a metric of thermal stress and were compared to the annual growth rates. Thermal stress was a significant predictor of growth in only one of the study species (p<0.05, slope=8.3^-4, r²=0.55), and predicted less than 20% of the growth variation in the other species. The inability of thermal stress to predict growth rates may be due to the relatively short duration of the study. Our results suggest that PHTs may not be as important for influencing plant fitness as initially hypothesized, and questions their use for understanding species’ responses to climate change. Empirical evidence for the physiological mechanism(s) explaining the decelerating growth of tropical trees remains elusive.