Disentangling the Role of Forest Structure and Functional Traits in the Thermal Balance of the Mediterranean–Temperate Ecotone

The thermal balance of forests is the result of complex land–atmosphere interactions. Different climate regimes and plant functional types can have contrasting energy budgets, but little is known about the influence of forest structure and functional traits. Here, we combined spaceborne measurements of surface temperature from ECOSTRESS with ground-based meteorological data to estimate the thermal balance at the surface (∆Tcan−air) during four summers (2018–2021), at the Mediterranean–temperate ecotone in the NE Iberian Peninsula. We analyzed the spatiotemporal drivers of ∆Tcan−air by quantifying the effects of meteorology, forest structure (stand density, tree height) and ecophysiology (hydraulic traits), during normal days and hot spells. Canopy temperatures (Tcan) fluctuated according to changes in air temperature (Tair) but were on average 4.2 K warmer. During hot spells, ∆Tcan−air was smaller than during normal periods. We attribute this decrease to the advection of hot and dry air masses from the Saharan region resulting in a sudden increase in Tair relative to Tcan. Vapor pressure deficit (VPD) was negatively correlated with ∆Tcan−air, since the highest VPD values coincided with peaks in heat advection. Nonetheless, Tcan increased with VPD due to decreased transpiration (following stomatal closure), even though sufficient soil water availability enabled some degree of evaporative cooling. Our findings demonstrate that plot-scale forest structural and hydraulic traits are key determinants for the forest thermal balance. The integration of functional traits and forest structure over relevant spatial scales would improve our ability to understand and model land–atmosphere feedbacks in forested regions. © 2023. The Authors.