Leaf tolerance to heat is independent of leaf phenology in neotropical savanna trees

Key Message

Although evergreens and deciduous are considered different functional groups due to their dissimilar strategy in canopy maintenance, they exhibit similar leaf heat tolerance to elevated temperatures in Cerrado savannas of Brazil.

Abstract

The escalating temperatures resultant from recent climate shifts have begun to influence the structure and function of tropical ecosystems, according to the Intergovernmental Panel on Climate Change (IPCC). The rise in average temperatures within the tropics implies significant challenges for plant persistence, pushing them beyond the optimal threshold for critical physiological functions, notably photosynthetic process, which temperatures effects can be inferred by measuring leaves chlorophyll fluorescence. Consequently, species lacking leaf traits associated with thermotolerance to high temperatures face heightened risks of permanent damage. The Cerrado is an extremely diverse tropical savanna ecosystem, with an abundance of species exhibiting different foliar habits that are being threatened by such temperature rises. The present study aimed to understand leaf heat tolerance in tree species of different leaf phenologies (evergreens vs. deciduous) exposed to high temperatures (35–65 °C) and to discern any effects and correlations between morphological traits (such as specific leaf area (SLA) and leaf thickness) and heat tolerance capacity measured by the T₅₀. It was hypothesized that evergreen species, with their persistent foliage, might exhibit greater heat tolerance to high temperatures than deciduous species, and that SLA will be correlated with leaf heat tolerance independent of leaf habit. Contrary to the initial hypothesis, the results indicate similar levels of leaf heat tolerance, with deciduous leaves showing a T₅₀ value around 49 °C and evergreens leaves around 50 °C. We did not find differences in the studied morphological traits between the groups, with SLA and leaf thickness not differing between groups, and neither relationships with heat leaf tolerance measured by the T₅₀ values were found. The importance of additional research considering water scarcity is highlighted, as water potential can influence leaf traits and canopy phenology, thus affecting thermotolerance. In summary, our study suggests that, within the context of global warming, leaf phenology alone might not wield substantial influence over the thermotolerance capabilities of these species, at least for the group of Cerrado species studied here.