Decoupling of stomatal conductance from net assimilation at high temperature as a mechanism to increase transpiration

Photosynthetic assimilation (A net ) and stomatal conductance (g s ) are usually strongly coupled, but this relationship is decreased or even lost at high temperatures (T air ). The contributions of environmental drivers (T air , vapour pressure deficit (VPD), and soil moisture) in interaction with the physiological mechanisms behind this process are still unclear.
We exposed saplings of three temperate and tropical species to rising T air (20 to 40°C) at low (1.2 to 1.9 kPa) and increasing VPD (1.1 to 5.6 kPa), and at stable T air (35°C) to increasing VPD (1.4 to 4.3 kPa) under well-watered or chronic soil drought conditions (≤10 %). A net, g s , and transpiration (E) in the light and the dark and leaf thermoregulation were tracked throughout the experiment.
When VPD remained low, g s continued to increase while A net decreased at T air > 35°C, leading to stomatal decoupling. In contrast, under rising VPD, trees maintained the coupling between A net and g s at high T air.
While a decoupling of A net and g s only occurred when VPD was low, A net and E decoupled under both VPD regimes at high T air .
Our results indicate that, since g s and VPD collectively drive E, stomatal decoupling is needed to increase E when VPD is not sufficiently high.