Leaf temperature regulation in tree species of the eastern United States: the influence of leaf traits and resource availability

Abstract

Temperature has a direct effect on plant function (e.g., photosynthesis, respiration). However, the temperature that plants experience at the leaf level (Tleaf) is often decoupled from air temperature (Tair). Theoretical and experimental work has indicated that leaf morphology and physiology can influence Tleaf regulation, defined as β (β = ΔTleaf – ΔTair). Yet, our understanding of which traits most strongly influence Tleaf regulation is limited. Moreover, it is not clear whether variation in leaf traits, across species or resource environments, influences Tleaf regulation. In this study, seedlings of 10 angiosperm tree species native to the eastern U.S. were grown outdoors for 10 months under a factorial combination of water and nutrient availability. Each month of the peak growing season (May and September), diurnal measurements of canopy Tleaf and stomatal conductance were made on seedlings of each species growing in each resource environment and temporally matched with Tair measurements. Leaves were collected to measure leaf dimensions, leaf dry matter content, leaf mass per area, leaf chlorophyl content, and stomatal density. We addressed the following questions: 1) Does Tleaf regulation (β) differ among tree species of the eastern U.S. and between different water and nutrient environments? and 2) Do species leaf traits (structural, anatomical, gas-exchange) explain variation in Tleaf regulation (β)? Averaged across timepoints (month), species, and treatments, we found that seedlings were largely poikilothermic (β = 1). However, month, species, and water availability each contributed to significant variation in β. High water availability dampened β, although differences between water treatments decreased over time. Among species, four species were found to have β < 1, and six species had β > 1. Nutrient availability had no direct effect on β. Leaf traits exhibited variability across months, species, treatments, or a combination thereof. Across months, species, and resource environments, we found that β tended to be reduced when stomatal conductance, leaf size, and leaf dry matter content were higher, but leaf nitrogen was lower. These results provide new insight into the traits or trait combinations that influence Tleaf regulation across species and resource environments.