Observation-based evidence reveals increased control of tree traits rather than hydroclimate on tree transpirational...

Trees are the ultimate solution for local climate mitigation; however, their direct transpirational cooling (TC) effects vary across global regions, primarily affected by hydroclimatic conditions and tree traits. Although vegetation biophysical effects are examined by modelling and satellite remote sensing in some regions, yet, the global-scale synthesis of trees' TC dynamics remains underexplored by ground observations. We used the SAPFLUXNET database and evapotranspiration (from GLEAM) to compare trees' TC effects across global regions in response to hydroclimatic and biotic variables. We found that trees provide significant cooling, with air temperature reduction (ΔT) of 3.25 °C m^-2 d^-1 in warm-wet regions of Central and South America (C/SAM), while the lowest cooling, with ΔT of 0.46 and 0.26 °C m^-2 d^-1 in hot-dry regions of Africa (AF) and Central and East Asia (C/EA), respectively. This can be explained by an increase in leaf area index (LAI) and ample soil water content (SWC) in warm-wet regions, whereas trees in hot-dry regions remain less effective in cooling due to low LAI and aridity. This study uniquely characterizes the nonlinear connection between ΔT and hydroclimatic and biotic factors, highlighting that biotic factors have more impact on the dynamics of daily mean ΔT, followed by hydroclimatic factors. Additionally, region-specific changes in TC are attributable to ΔT's increased sensitivity to air temperature and vapor pressure deficit in warm-wet regions, and precipitation and SWC in hot-dry regions, primarily controlled by stomatal regulation. These findings offer critical insights into the importance of considering trees' biophysical effects when designing local climate mitigation and adaptation strategies.