Contrasting Evaporative Responses of Land Ecosystems to Heat Waves Traced to the Roles of Vapor Pressure Deficit and Canopy Resistance in the Surface Energy Balance

Understanding the response of land ecosystems to large-scale heat wave events is critical for managing and improving ecosystem services, especially under a changing climate.Using Ameriflux data, this study examines the energy exchange between four different land ecosystems (i.e., grassland, cropland, deciduous broadleaf forest, and evergreen needleleaf forest) and the atmosphere under heatwave and non-heatwave conditions.Results show that net radiation and sensible heat fluxes increase from non-heatwave to heatwave conditions across all four land ecosystems. Latent heat flux is also increased except over grassland. As a result, the reduction in evaporative fraction from non-heatwave to heatwave over grassland is larger than those over cropland, deciduous broadleaf forest, and evergreen needleleaf forest. An attribution analysis based on the newly developed two-resistance mechanism (TRM) model further demonstrates that changes in evaporative fraction are mainly caused by changes in vapor pressure deficit (positive contribution) and changes in canopy resistance (negative contribution). The imbalance between the positive and negative contributions varies across the four land ecosystems and is responsible for their contrasting responses to heat waves. When the temporal dynamics are considered, it is further found that the latent heat flux and evaporative fraction over deciduous broadleaf forest remain large for three consecutive heat wave days while those of grassland and evergreen needleleaf forest tend to decrease after thefirst heat wave day.