In situ evidence for a critical temperature threshold driving stomatal re-opening and widespread photosynthesis–conductance decoupling in tropical trees

Extreme heat is intensifying across the tropics, often coinciding with high atmospheric drought, forcing trees to balance evaporative cooling against the risk of hydraulic failure. A possible response that challenges major climate–vegetation stomatal model assumptions is the decoupling of photosynthesis and stomatal conductance (g_s), where g_s stays high while photosynthesis declines.

We aimed to quantify how widespread photosynthesis–g_s decoupling is across tropical tree species from contrasting climates and test whether it trades off with sensitivity to heat, drought, or vapour pressure deficit (VPD). We measured  in-situ temperature responses (22-49°C, VPD=2.5kPa) of photosynthesis, g_s, and g₁ in 80 mature individuals encompassing 16 species along an elevation gradient in Panama, as well as leaf-level turgor loss point and VPD sensitivity.

All individuals showed an exponential rise in g₁ with temperature, indicating widespread decoupling between photosynthesis and stomatal conductance. Although both photosynthesis and g_s declined above their thermal optima, stomatal re-opening at extreme temperatures (~45°C) occurred in 55% of curves. Notably, the temperature at which g_s increased again was higher in lowland than upland individuals, potentially indicating greater heat tolerance in trees from hotter environments. Contrary to expectations, there was no coordination between stomatal sensitivity to extreme heat, stomatal sensitivity to vapour pressure deficit, and turgor loss point, indicating that heat avoidance and hydraulic drought tolerance represent largely independent axes of variation in the tropical trees studied.

These results provide rare field-based evidence that tropical trees exhibit diverse temperature-dependent stomatal strategies that may shape forest resilience under future heatwaves. Future research must prioritise in situ measurements at extreme leaf temperatures (>45 °C), where tropical trees approach critical thermal thresholds and where the physiological mechanisms governing survival under heatwaves remain largely unresolved.