Biophysical controls on canopy transpiration of Pinus tabulaeformis under different soil moisture conditions in the Loess Plateau of China

Abstract

Soil moisture is a critical variable controlling the processes of the Earth's energy-water-carbon cycles, balances and their feedback loops, which plays an important role in climate change prediction. However, soil moisture and meteorology are interrelated and interact through different ecological processes, resulting in variable and complex biophysical regulation of tree water use in water-scarce areas. This study selected Pinus tabulaeformis Carr. as the target tree species in the semiarid area of the Chinese Loess Plateau. Canopy transpiration (Ec), soil water content (SWC), and meteorological variables were continuously monitored during the growing seasons (May–September) in 2015–2020. Throughout the study period, precipitation and SWC were crucial factors affecting Ec and stomatal activity, and both were positively correlated with Ec and canopy conductance (Gc) on an annual scale. The threshold point of soil water stress for Pinus tabulaeformis was when the value of soil relative extractable water (REW) reached 0.36. Moreover, soil moisture can alter the response of Ec to meteorological variables. Under soil water stress (REW < 0.36), total solar radiation (Rs) and SWC contributed the most to Ec, and Ec trended to decrease under high vapor pressure deficit (VPD) and air temperature (Ta). At this point, VPD and Ta significantly suppressed Gc. In contrast, under non-water stress conditions (REW ≥ 0.36), Rs and VPD contributed more to Ec than other variables. As VPD, Rs, and Ta rose, Ec increased more quickly when REW ≥ 0.36, while VPD still exerted an inhibitory effect on Gc to avoid excessive water loss. Our results suggest that Pinus tabulaeformis exhibits a drought avoidance water use strategy by controlling transpiration with strict stomatal regulation under soil water stress. These findings will contribute to understanding how the changing soil moisture affects tree transpiration regulation under the background of climate change in semiarid areas.