The role of radiative cooling and leaf wetting in air-leaf water exchange during dew and radiation fog events in a temperate grassland

During prolonged dry periods, non-rainfall water (NRW) plays a vital role as water input into temperate grasslands, affecting the leaf surface water balance and plant water status. Previous chamber and laboratory experiments investigated air-leaf water exchange during dew deposition, but overlooked the importance of radiative cooling on air-leaf water exchange because the chamber is a heat trap, preventing radiative cooling. To complement these previous studies, we conducted a field study, in which we investigated the effect of radiatively-induced NRW inputs on leaf water isotope signals and air-leaf water exchange in a temperate grassland during the dry-hot summers of 2018 and 2019. We carried out field measurements of the isotope composition of atmospheric water vapor, NRW droplets on foliage, leaf water, xylem water of root crown, and soil water, combined with meteorological and plant physiological measurements. We combined radiation measurements with thermal imaging to estimate leaf temperatures using different methods, and computed the corresponding leaf conductance and air-leaf water exchange. Our results indicate that radiative cooling and leaf wetting induced a switch of direction in the net water vapor exchange from leaf-to-air to air-to-leaf. The leaf conductance and air-leaf water exchange varied by species due to the species-specific biophysical controls. Our results highlight the ecological relevance of radiative cooling and leaf wetting in natural temperate grasslands, a process which is expected to influence land surface water budgets and may impact plant survival in many regions in a drier climate.

Automated summary

During dry periods in temperate grasslands, non-rainfall water (NRW) has an important impact on leaf surface water balance and plant water status. Previous studies overlooked the role of radiative cooling in air-leaf water exchange, which prompted us to conduct a field study investigating the effect of radiatively-induced NRW inputs on leaf water isotope signals and air-leaf water exchange in a temperate grassland. Our results revealed the ecological relevance of radiative cooling and leaf wetting in temperate grasslands, which has implications for land surface water budgets and plant survival in a drier climate.