Atmospheric Boundary Layer Control on Forest Thermal Properties

Forest canopy, air temperatures and air humidity ( $T_{\mathrm{can}}$ , $T_{\mathrm{air}}$ , and $q_{\mathrm{air}}$ ) play a central rol in regulating energy and gas exchange between vegetation and the atmosphere. Although often treated as independent drivers of canopy processes, $T_{\mathrm{air}}$ and $q_{\mathrm{air}}$ are dynamically coupled to $T_{\mathrm{can}}$ via surface energy fluxes and atmospheric boundary layer (ABL) development. We investigated how plant physiology mediates this coupling. Using data from a tropical ecosystem, we studied a process-based forest model dynamically coupled with an ABL growth model to simulate diurnal interactions between the canopy and the atmosphere. We systematically varied plant traits related to water use and thermal regulation to assess their effects on $T_{\mathrm{can}}~T_{\mathrm{air}}$ coupling and feedback. We focused on three metrics: the slope of the $T_{\mathrm{can}}~T_{\mathrm{air}}$ relationship, the peak of $T_{\mathrm{can}}$ reached during the day and the lag between the maximum $T_{\mathrm{air}}$ and $T_{\mathrm{can}}$ , indicating hysteresis. Conservative water use, by reducing transpiration, leads to greater canopy warming, which intensifies sensible heat flux and accelerates ABL growth. This, in turn, raises near-surface air temperature and vapor pressure deficit (VPD), amplifying thermal and water stress. In contrast, greater water use enhances evaporative cooling and slows ABL development, thereby moderating these feedback. Surprisingly, the slope of the $T_{\mathrm{can}}~T_{\mathrm{air}}$ relationship is quite insensitive to plant water-use syndromes. This insight extends beyond modeling. Empirical studies often treat $T_{\mathrm{air}}$ and VPD as independent drivers of transpiration, photosynthesis, or stomatal conductance. Our results challenge this assumption, showing that these variables are influenced by plant function itself. $T_{\mathrm{can}}$ is not a passive outcome but an active mediator of energy, water, and carbon exchange, regulated by a feedback loop involving leaf physiology and atmospheric dynamics. Studies using $T_{\mathrm{can}}$ or the $T_{\mathrm{can}}~T_{\mathrm{air}}$ relationship-whether from remote sensing or field data-as a proxy for forest stress or function, must account for this coupling.