Impacts of elevated temperature and vapour pressure deficit on leaf gas exchange and plant growth across six tropical rainforest tree species

Abstract

Elevated air temperature (T_air) and vapour pressure deficit (VPD_air) significantly influence plant functioning, yet their relative impacts are difficult to disentangle. We examined the effects of elevated T_air (+6°C) and VPD_air (+0.7 kPa) on the growth and physiology of six tropical tree species. Saplings were grown under well-watered conditions in climate-controlled glasshouses for 6 months under three treatments: (1) low T_air and low VPD_air, (2) high T_air and low VPD_air, and (3) high T_air and high VPD_air. To assess acclimation, physiological parameters were measured at a set temperature. Warm-grown plants grown under elevated VPD_air had significantly reduced stomatal conductance and increased instantaneous water use efficiency compared to plants grown under low VPD_air. Photosynthetic biochemistry and thermal tolerance (T_crit) were unaffected by VPD_air, but elevated T_air caused J_max25 to decrease and T_crit to increase. Sapling biomass accumulation for all species responded positively to an increase in T_air, but elevated VPD_air limited growth. This study shows that stomatal limitation caused by even moderate increases in VPD_air can decrease productivity and growth rates in tropical species independently from T_air and has important implications for modelling the impacts of climate change on tropical forests.

Keywords: acclimation; leaf temperature; photosynthesis; stomatal conductance; tropical forest; vapour pressure deficit; warming.