Joint dependence of leaf minimum conductance on vapour pressure deficit and temperature

Leaf minimum conductance to water vapour (gmin) is a key trait that determines plant water loss and desiccation under conditions of low water availability. While gmin exhibits sensitivity to temperature in many species, the response of gmin to changes in vapour pressure deficit (VPD) – the driving force for evaporative flux – is relatively unknown. Here, we disentangle the effects of temperature and VPD on gmin in two tree species. We measured gmin at 17 distinct combinations of temperature and VPD spanning 25–50 °C and 2.0–11.1 kPa. We found that gmin exhibited a consistently negative response to VPD which varied between species (mean decrease in gmin, -0.43 mmol m^-2 s^-1 per 1 kPa increase in VPD in Banksia integrifolia; -0.38 mmol m^-2 s^-1 kPa^-1 in Syncarpia glomulifera). VPD also affected the apparent temperature response of gmin: when VPD is held constant, a much stronger temperature response was observed compared to measurements in which VPD and temperature covary (12.0 vs. 5.4 mmol m^-2 s^-1 in B. integrifola, and 8.1 vs. 5.8 mmol m^-2 s^-1 in S. glomulifera at 50 °C). Estimates of breakpoint temperature, above which gmin increased rapidly, varied by 8 °C in B. integrifola depending on VPD control. If left unaccounted for, the VPD response of gmin may cause over- or underestimation of modelled transpiration rates. We recommend measuring the VPD response of gmin across a wider range of species, reporting VPD whenever measuring gmin, and incorporating the apparent VPD response of gmin into future modelling efforts.