Sensitivity of gross primary production and evapotranspiration to heat and drought stress in a young temperate plantation in northern China

Assessing the sensitivities of ecosystem functions to climatic factors is essential to understanding the response of ecosystems to environmental change. Temperate plantation forests contribute to global greening and climate change mitigation, yet little is known as to the sensitivity of gross primary production (GPP) and evapotranspiration (ET) of these forests to heat and drought stress. Based on near-continuous, eddy-covariance and hydrometeorological data from a young temperate plantation forest in Beijing, China (2012–2019), we used a sliding-window-fitting technique to assess the seasonal and interannual variation in ecosystem sensitivity (i.e., calculated slopes, S_GPP-Ta, S_ET-Ta, S_GPP-EF, and S_ET-EF) in GPP and ET to anomalies in air temperature (T_a) and evaporative fraction (EF). The EF was used here as an indicator of drought. Seasonally, daily S_GPP-Ta, S_ET-Ta, and S_GPP-EF were greatest in summer, reaching maxima of 1.12 ​± ​0.56 ​g ​C·m⁻²·d⁻¹·°C⁻¹, 1.36 ​± ​0.56 ​g H₂O·m⁻²·d⁻¹·°C⁻¹, and 0.37 ​± ​0.35 ​g ​C·m⁻²·d⁻¹, respectively. Evapotranspiration was constrained by drought, especially during the spring-to-summer period, S_ET-EF reaching −0.51 ​± ​0.34 ​g H₂O·m⁻²·d⁻¹. Variables EF, T_a, soil water content (SWC), vapor pressure deficit (VPD), and precipitation (PPT) were the main controls of sensitivity, with S_GPP-Ta and S_ET-Ta increasing with T_a, VPD, and PPT (<50 ​mm·d⁻¹) during both spring and autumn. Increased drought stress during summer caused the positive response in GPP and ET to decrease with atmospheric warming. Variable S_ET-EF intensified (i.e., became more negative) with decreasing EF and increasing T_a. Interannually, annual S_GPP-Ta and S_ET-Ta were positive, S_GPP-EF near-neutral, and S_ET-EF negative. Interannual variability in S_GPP-Ta, S_ET-Ta, S_ET-EF, and S_GPP-EF was largely due to variations in bulk surface conductance. Our study suggests that the dynamics associated with the sensitivity of ecosystems to changes in climatic factors need to be considered in the management of plantation forests under future global climate change.