Functional traits and drought strategy predict leaf thermal tolerance

Heat stress imposes an important physiological constraint on native plant
species – one that will only worsen with human-caused climate change.
Indeed, rising temperatures have already contributed to large-scale plant
mortality events across the globe. These impacts may be especially severe
in cities, where the urban heat island effect amplifies climate warming.
Understanding how plant species will respond physiologically to rising
temperatures and how these responses differ among plant functional groups
is critical for predicting future biodiversity scenarios and making
informed land management decisions. In this study, we evaluated the
effects of elevated temperatures on a functionally and taxonomically
diverse group of woody native plant species in a restored urban nature
preserve in southern California using measurements of chlorophyll
fluorescence as an indicator of leaf thermotolerance. Our aim was to
determine if species’ traits and drought strategies could serve as useful
predictors of thermotolerance. We found that leaf thermotolerance differed
among species with contrasting drought strategies, and several leaf-level
functional traits were significant predictors of thermotolerance
thresholds. Drought deciduous species with high specific leaf area, high
rates of transpiration, and low water-use efficiency were the most
susceptible to heat damage, while evergreen species with sclerophyllous
leaves, high relative water content, and high water-use efficiency
maintained photosynthetic function at higher temperatures. While these
native shrubs and trees are physiologically equipped to withstand
relatively high temperatures in this Mediterranean-type climate, hotter
conditions imposed by climate change and urbanization may exceed the
tolerance thresholds of many species. We show that leaf functional traits
and plant drought strategies may serve as useful indicators of species’
vulnerabilities to climate change, and this information can be used to
guide restoration and conservation in a warmer world.