Wind increases "evaporative demand" but reduces plant water requirements

Abstract:

Transpiration is commonly conceptualised as a fraction of some potential rate, determined by stomatal or canopy resistance. Therefore, so-called "atmospheric evaporative demand" or “potential evaporation” is generally used alongside with precipitation and soil moisture to characterise the environmental conditions that affect plant water use. An increase in potential evaporation (e.g. due to climate change) is generally believed to cause increased transpiration and/or vegetation water stress, aggravating drought effects. In the present study, we investigated the question whether potential evaporation constitutes a meaningful reference for transpiration and compared sensitivity of potential evaporation and leaf transpiration to atmospheric forcing. Based on modelling results and supporting experimental evidence, we conclude that stomatal resistance cannot be parameterised as a factor relating transpiration to potential evaporation, as the ratio between transpiration and potential evaporation not only varies with stomatal resistance, but also with wind speed, air temperature, irradiance and relative humidity. Furthermore, the effect of wind speed in particular implies increase in potential evaporation, which is commonly interpreted as increased “water stress”, but at the same time can reduce leaf transpiration, implying a decrease in water demand at the leaf scale. In fact, in a range of field measurements, we found that water use efficiency (WUE, carbon uptake per water transpired) commonly increases with increasing wind speed, enabling plants to conserve water during photosynthesis. We estimate that the observed global decrease in terrestrial near-surface wind speeds could have reduced WUE at a magnitude similar to the increase in WUE attributed to global rise in atmospheric carbon dioxide concentrations. We conclude that trends in wind speed and atmospheric carbon dioxide concentrations have to be considered explicitly for the estimation of drought effects on plants.