Disentangling the Effects of Vapor Pressure Deficit and Soil Water Availability on Canopy Conductance in a Seasonal Tropical Forest During the 2015 El Niño Drought.

Water deficit in the atmosphere and soil are two key interactive factors that constrain transpiration and vegetation productivity. It is not clear which of these two factors is more important for the water and carbon flux response to drought stress in ecosystems. In this study, field data and numerical modeling were used to isolate their impact on evapotranspiration (ET) and gross primary productivity (GPP) at a tropical forest site in Barro Colorado Island (BCI), Panama, focusing on their response to the drought induced by the El Niño event of 2015–2016. Numerical simulations were performed using a plant hydrodynamic scheme (HYDRO) and a heuristic approach that ignores stomatal sensitivity to leaf water potential in the Energy Exascale Earth System Model (E3SM) Land Model (ELM). The sensitivity of canopy conductance (Gs) to vapor pressure deficit (VPD) obtained from eddy‐covariance fluxes and measured sap flux shows that, at both ecosystem and plant scale, soil water stress is more important in limiting Gs than VPD at BCI during the El Niño event. The model simulations confirmed the importance of water stress limitation on Gs, but overestimated the VPD impact on Gs compared to that estimated from the observations. We also found that the predicted soil moisture is less sensitive to the diversity of plant hydraulic traits than ET and GPP. During the dry season at BCI, seasonal ET, especially soil evaporation at VPD > 0.42 kPa, simulated using HYDRO and ELM, were too strong and will require alternative parameterizations. Plain Language Summary: Plants close stomata to regulate water loss through transpiration when stressed by the dry soil or high atmospheric water demand (vapor pressure deficit, VPD) or both. Tropical forests have experienced periodic droughts in the past. Recent drought‐related plant mortality has been attributed to increasing VPD associated with climate change. To evaluate whether water stress from dry soil or VPD has more limiting effect on plant transpiration, we analyzed the results from statistical models fitted to two types of field observation data and a land surface model which can simulate water movement in the soil and the water transport within the plant at a tropical forest site in Panama. We found dry soil is more important in limiting plant water loss at the site during the drought of the El Niño event of 2015–2016 and identified future model improvement need. Key Points: Canopy conductance (Gs) sensitivity to vapor pressure deficit (VPD) was analyzed from observations using multiple methodsEstimated Gs indicates stronger limitation by soil water stress than VPD at Barro Colorado Island during the 2015 El Niño eventWith a plant hydrodynamics scheme in Energy Exascale Earth System Model (E3SM) Land Model, variation of plant hydraulic traits has greater effects on Gs limitation than on soil moisture [ABSTRACT FROM AUTHOR]