Evapotranspiration responses to CO₂ and its driving mechanisms in four ecosystems based on CMIP6 simulations: Forest, shrub, farm and grass

Evapotranspiration (ET) is an essential process of the water cycle through which water is transferred from terrestrial ecosystems to atmosphere. However, in the climate context of increasing CO₂ concentration (also called as a CO₂-enriched climate), the variation of ET and its main drivers among different ecosystems remain unclear. This study analyzed the output data of the CMCC ESM2 model with a ridge regression method, and proposed the trends and drivers of ET in different ecosystems in a CO₂-enriched climate. In particular, the temporal - spatial characteristics of ET and its primary drivers for different periods and wetness levels were revealed. With the rising of CO₂ concentration, the atmospheric evapotranspiration demand increases, and the vegetation grows more luxuriantly. ET shows an overall upward trend, especially in the shrub ecosystems (7.41 mm decade^-1). Our results show that the thermal conditions are the main driving factors for humid forest and shrub ecosystems whereas relative humidity (RH) is the main driving factor for arid farm and grass ecosystems. In terms of the average contribution in all periods, surface solar radiation contributes 26% and 41% to ET variation in forest and shrub ecosystems, and RH contributes 49% and 32% to ET variation in farm and grass ecosystems, respectively. Notably, with the increase of wetness levels, the contribution of water conditions on ET becomes smaller, while that of thermal conditions becomes larger. Correlation analysis shows that LAI impacts on ET are regulated by environmental factors, which reflects the complexity of ET change mechanism. Overall, these findings further provide a reference for rational planning of ecosystems and efficient utilization of water resources.