Stomatal regulation alters environmental controls on Pinus tabuliformis transpiration in a semi-arid region of China

Stomatal regulation plays a critical role in tree water use in semi-arid regions, determining the amount of transpiration and its responses to environmental drivers. However, how dynamic stomatal behavior modulates the pathways and the magnitude of environmental control on tree transpiration remains inadequately elucidated. Based on growing-season observations in 2023–2025, this study applied boosted regression trees and partial least squares regression to disentangle environmental and stomatal controls on canopy transpiration ( E c ) across gradients of stomatal openness (canopy conductance, denoted as G c ) in Pinus tabuliformis plantations. Results showed that as G c increased, E c became more sensitive to vapor pressure deficit (VPD) and solar radiation, whereas its sensitivity to soil relative extractable water (REW) weakened. Stomatal control over E c was stronger at relatively low G c levels (i.e., the 0–20th percentile range). VPD consistently emerged as the dominant driver of E c as G c increased, its relative contribution rising from 45.3% to 70.1%, while that of REW declined from 22.8% to 8.1%. These changes were closely related to systematic adjustments in stomatal sensitivity in Pinus tabuliformis . Under low G c , stomatal behavior was primarily constrained by soil water limitation. In contrast, VPD dominated the regulation of stomatal aperture and E c under high G c . This study reveals a continuous transition from soil water-limited to atmospheric demand-limited conditions, accompanied by responsive changes in stomatal sensitivity across the G c gradient. These findings highlight the role of stomatal regulation in forest water-use strategies and provide a scientific basis for forest management under climate change. Future management should prioritize optimizing soil water availability through practices such as mulching and moderate thinning to maintain soil relative extractable water above the critical threshold of 0.4, thereby enhancing the drought resilience of Pinus tabuliformis .