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.