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.