Weak phylogenetic and climatic signals in plant heat tolerance

Aim: High heat tolerance is a potential way for plants to maintain performance under high temperatures that can be acted upon by environmental filters to influence community assembly. Plant heat tolerances are phenotypically plastic and thus common garden experiments are needed to test if species from hotter environments have consistently higher heat tolerance than species from colder environments. Past studies that have measured heat tolerance from species grown in common gardens have found conflicting relationships between species' climatic origins and their heat tolerance, possibly due to phylogenetic non-independence of study species. In this study, we test the hypothesis that phylogenetic structure can help to explain variation in heat tolerance in order to resolve the confliciting relationships between climate and plant heat tolerance. Location: Fairchild Tropical Botanic Garden and the Gifford Arboretum, Miami, FL, USA. Taxon: Pteridophytes, Gymnosperms and Angiosperms. Methods: We tested for phylogenetic signal in the photosynthetic heat tolerance of 123 species of ferns, gymnosperms, magnoliids, monocots and eudicots by calculating Blomberg's K. Phylogenetic independent contrasts of heat tolerance and climatic distributions for >100 species were used to test the hypothesis that climate can predict variation in heat tolerance. Results: Species' heat tolerances were not phylogenetically conserved according to Blomberg's K, but we found significant differences in the heat tolerance of ferns, gymnosperms, magnoliids, monocots and eudicots. When controlling for phylogenetic non-independence, we found a significant, but weak relationship between the mean maximum temperature of the warmest month of species' climatic distributions and their photosynthetic heat tolerance. Main conclusions: We conclude that phylogeny and climate are weak predictors of photosynthetic heat tolerance. However, differences among the groups we studied suggest that the variation in heat tolerance may be better explained by differences in microenvironment, thermoregulatory traits and leaf temperatures.