Absence of canopy temperature variation despite stomatal adjustment in Pinus sylvestris under multidecadal soil moisture manipulation.

Global warming and droughts push forests closer to their thermal limits, altering tree carbon uptake and growth. To prevent critical overheating, trees can adjust their thermotolerance (T crit ), temperature and photosynthetic optima (T opt and A opt ), and canopy temperature (T can ) to stay below damaging thresholds. However, we lack an understanding of how soil droughts affect photosynthetic thermal plasticity and T can regulation. In this study, we measured the effect of soil moisture on the seasonal and diurnal dynamics of net photosynthesis (A), stomatal conductance (g s ), and T can , as well as the thermal plasticity of photosynthesis (T crit , T opt , and A opt ), over the course of 1 yr using a long-term irrigation experiment in a drought-prone Pinus sylvestris forest in Switzerland. Irrigation resulted in higher needle-level A, g s , T opt , and A opt compared with naturally drought-exposed trees. No daily or seasonal differences in T can were observed between treatments. Trees operated below their thermal thresholds (T crit ), independently of soil moisture content. Despite strong T can and T air coupling, we provide evidence that drought reduces trees' temperature optimum due to a substantial reduction of g s during warm and dry periods of the year. These findings provide important insights regarding the effects of soil drought on the thermal tolerance of P. sylvestris.