Foggy days and dry nights determine crown‐level water balance in a seasonal tropical montane cloud forest

The ecophysiology of tropical montane cloud forest (TMCF) trees is influenced by crown‐level microclimate factors including regular mist/fog water inputs, and large variations in evaporative demand, which in turn can significantly impact water balance. We investigated the effect of such microclimatic factors on canopy ecophysiology and branch‐level water balance in the dry season of a seasonal TMCF in Veracruz, Mexico, by quantifying both water inputs (via foliar uptake, FU) and outputs (day‐ and night‐time transpiration, NT). Measurements of sap flow, stomatal conductance, leaf water potential and pressure–volume relations were obtained in Quercus lanceifolia, a canopy‐dominant tree species. Our results indicate that FU occurred 34% of the time and led to the recovery of 9% (24 ± 9.1 L) of all the dry‐season water transpired from individual branches. Capacity for FU was independently verified for seven additional common tree species. NT accounted for approximately 17% (46 L) of dry‐season water loss. There was a strong correlation between FU and the duration of leaf wetness events (fog and/or rain), as well as between NT and the night‐time vapour pressure deficit. Our results show the clear importance of fog and NT for the canopy water relations of Q. lanceifolia. We investigated the effect of microclimatic on canopy ecophysiology and branch‐level water balance in the dry‐season of a seasonal Tropical Montane Cloud Forest in Veracruz, Mexico by quantifying both water inputs (via foliar uptake, FU) and outputs (day‐ and night‐time transpiration, NT). Our results indicate that FU occurred on average 34% of the time and led to the recovery of 9 % (24 ± 9.1 L) of all the dry‐season water transpired from one branch. Night‐time transpiration accounted for approximately 17 % (46 L) of dry season transpiration from that same branch. Our results show the clear importance of fog for the canopy water relations of Q. lanceifolia, as well as for other TMCF plants.