Effects of environmental variables on canopy transpiration in two coniferous forests at different growing-season stages

Soil water content (SWC) and meteorological conditions, as key environmental variables influencing tree water use, vary highly within the growing season, hindering a better understanding of environmental control mechanisms on canopy transpiration (E_c). Disentangling the effects of these variables on E_c across growing-season stages is crucial for E_c estimation and forest management. In this study, 43-year-old Pinus tabuliformis Carr. and 31-year-old Platycladus orientalis (L.) Franco plantations in the semiarid Chinese Loess Plateau were monitored for E_c during the growing season of 2015–2020. The contributions of environmental factors to E_c were assessed using the boosted regression tree (BRT) model. Results showed that the contributions of SWC to E_c were greater at the early (May–June) and late (September) stages, while the contributions of vapor pressure deficit (VPD) and total solar radiation (R_s) to E_c increased at the middle (July–August) stage due to high soil water availability. Overall, E_c in both plantations was dominated by SWC (20.4% ​≤ ​contributions ≤ 48.8%) and R_s (22.7% ​≤ ​contributions ≤ 35.8%). Both species exhibited strong stomatal regulation of E_c. Specifically, stomatal opening was significantly inhibited by VPD at the early stage and strongly affected by SWC at the late stage. This study highlights that soil water conditions in artificial forests should be adjusted according to changes in influencing factors on E_c. Particularly during the early and late stages, measures (e.g., land preparation, thinning, and pruning) can be implemented to improve soil moisture in such dryland forests.