Fog effects on carbon and water balance of tropical cloud forest trees
Fog occurrence is the single most important microclimatic feature affecting the distribution and functioning of Tropical montane cloud forests (TMCFs) plants. These forests are one of the most vulnerable ecosystems to climate change given their small geographic range, high endemism and dependence on a rare microclimatic envelope. The frequency of atmospheric water deficits for some TMCFs is likely to increase in the future, but the consequences for the integrity and functioning of these ecosystems are uncertain. Characterizing the interplay between abiotic drivers (microclimatic and soil water dynamics) and plant carbon and water relations is key to foster more realistic projections about climate change effects on TMCF functioning and distribution.
To investigate the impact of climatic variation (including periods of persistent fog and dry spells) on carbon and water balance of nine TMCF tree species, we conducted a suite of experiments in TMCFs of southeastern Brazil using a combination of sap flow, precision dendrometers, stable isotopes, and ecophysiological measurements under field and glasshouse conditions.
We found that foliar water uptake (FWU) is a common water acquisition mechanism for TMCF trees. Leaf-wetting events had a strong suppressive effect on tree transpiration (E). Foliar water uptake increased in magnitude with drier soil and during longer leaf-wetting events. Some species showed heightened stomatal sensitivity to soil drought and vapour pressure deficit during the night, which reduces night-time water loss. The difference between diurnal and nocturnal stomatal behaviour could be attributed to an optimization of carbon gain when leaves are dry, as well as minimization of nocturnal water loss. The leaf-wetting events on the other hand seem important to the water balance of TMCF trees, especially during soil droughts, both by suppressing tree transpiration (E) and as a small additional water supply through FWU. Our results suggest that decreases in leaf-wetting events in TMCF might increase water loss and decrease its water gains, which could compromise its ecophysiological performance and survival during dry periods.