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

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.