With projected increases in temperature in the future, the amount of water vapour that can be held at saturation – before it condenses into clouds, dew or water film – increases exponentially. This increased capacity by the atmosphere to hold water (or increased vapour pressure deficit), has important implications for plants and their interactions with the atmosphere. As this deficit increases plants tend to close their stomata, which reduces water fluxes into the boundary layer. Do models currently capture the observed leaf-level response to increasing vapour pressure deficit? What about at very high levels of this deficit?

In this work, CLEX researchers and colleagues tested a series of coupled photosynthesis-stomatal conductance models against leaf gas exchange measurements where the vapour pressure deficit regularly exceeded 2 kPa and reached as much as 8 kPA in summer. They found that commonly used models – including the one used inside the Australian land surface model, CABLE) – were unable to replicate observed decreases in photosynthesis and stomatal conductance.

The researchers tested an alternative hypothesis and found that models may need to incorporate a non-stomatal limitation, that is – a down-regulation in photosynthetic capacity, to match observations. Incorporating this alternative hypothesis in a stand-scale model led to a reduction in predicted transpiration of ~20% and improved the correspondence with sap flow measurements at a flux site in Western Sydney.

  • Paper: J Yang, R A Duursma, M G De Kauwe, D Kumarathunge, M Jiang, K Mahmud, T E Gimeno, K Y Crous, D S Ellsworth, J Peters, B Choat, D Eamus, B E Medlyn, Incorporating non-stomatal limitation improves the performance of leaf and canopy models at high vapour pressure deficit, Tree Physiology, tpz103, https://doi.org/10.1093/treephys/tpz103.