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.