Plant water stress index as an indicator of water status and carbon-sink capacity in Chinese cork oak plantations

The Chinese cork oak ( Quercus variabilis ) plantations in northern China supply critical timber, but daily tracking of their water status and drought-induced carbon-sink limitation remains difficult. We hypothesized that the plant water stress index (PWSI dT ) calculated from the temperature difference between canopy and air could effectively indicate plantation water status and the impact of water availability on canopy conductance ( g c ) and gross primary productivity (GPP). For hypothesis testing, the canopy temperature, ecosystem fluxes, and meteorological elements in a Chinese cork oak plantation were measured in 2020 and 2021. Actual ecosystem water status was normalized by measured evapotranspiration against its potential rate (PWSI ET ). Compared with the minimum canopy resistance parameter, aerodynamic resistance parameters had a more substantial influence on PWSI dT performance, especially under the low vapor pressure deficit (high humidity) at our site. In aerodynamic resistance calculation, the influence of reference height should be considered. When the reference height was set high and the resistance difference between momentum and heat transfer was not considered, the PWSI dT obtained from the aerodynamic resistance parameters could accurately describe the water status of plantation ecosystems. The determination coefficient, agreement index, root mean square error, and mean error between PWSI dT and PWSI ET were 0.77, 0.93, 0.09, and −0.01±0.09, respectively. When the reference height was set close to the canopy height, the residual resistance must be incorporated into the aerodynamic resistance calculation. Moreover, drought events had serious negative impacts on the carbon sink capacity of plantation ecosystems. PWSI dT explained approximately 64% variation in g c and 70% variation in GPP of the plantation, which were larger than those explained by the soil water content index and vapor pressure deficit index. Therefore, PWSI dT provided a non-destructive, automated, and accurate way to monitor plantation water status and its consequences for ecosystem carbon sequestration.