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Abstract 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.
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Abstract Photosynthetic heat tolerances (PHTs) have several potential applications including predicting which species will be most vulnerable to climate change. Given that plants exhibit unique thermoregulatory traits that influence leaf temperatures and decouple them from ambient air temperatures, we hypothesized that PHTs should be correlated with extreme leaf temperatures as opposed to air temperatures.
We measured leaf thermoregulatory traits, maximum leaf temperatures (
T MO) and two metrics of PHT (T critandT 50) quantified using the quantum yield of photosystem II for 19 plant species growing in Fairchild Tropical Botanic Garden (Coral Gables, FL, USA). Thermoregulatory traits measured at the Garden and microenvironmental variables were used to parameterize a leaf energy balance model that estimated maximum in situ leaf temperatures (T MIS) across the geographic distributions of 13 species.T MOandT MISwere positively correlated withT 50but were not correlated withT crit. The breadth of species' thermal safety margins (the difference betweenT 50andT MO) was negatively correlated withT 50.Our results provide observational and theoretical support based on a first principles approach for the hypothesis that PHTs may be adaptations to extreme leaf temperature, but refute the assumption that species with higher PHTs are less susceptible to thermal damage. Our study also introduces a novel method for studying plant ecophysiology by incorporating biophysical and species distribution models, and highlights how the use of air temperature versus leaf temperature can lead to conflicting conclusions about species vulnerability to thermal damage.
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Abstract 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 Poptand faster carbon assimilation and/or (d) increases in the optimum temperature for photosynthesis (Topt). To address these hypotheses, we tested if the Tcrit, T50and T95PSII heat tolerances were correlated with carbon assimilation parameters for 21 plant species. Our results did not support Hypothesis 1, but we observed that T50may be used to estimate the upper thermal limit for Tmaxat the species level, and that community mean Tcritmay be useful for approximating Tmax. The T50and T95heat 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.
