Urban-rural differences in transpiration patterns of Pinus tabuliformis and environmental drivers

Tree transpiration plays a critical role in water cycling and energy balance, contributing to the regulation of urban microclimates. Rapid urbanization has created pronounced urban-rural environmental differences that can substantially alter tree transpiration and its regulatory mechanisms, yet these effects remain poorly understood.

We investigated the transpiration patterns of Pinus tabuliformis, a native tree species in Beijing, based on a 10-year dataset (2014-2023) of sap flow and environmental measurements collected from contrasting urban and rural sites. Linear models, piecewise structural equation modeling (piecewise SEM), and boundary line analysis were used to quantify transpiration responses to environmental drivers across contrasting urban and rural conditions, identify environmental thresholds, and assess stomatal regulation strategies under contrasting environmental conditions.

The results showed that transpiration rates were significantly higher at the urban site, with mean daily rates approximately threefold greater than those at the rural site, and peak transpiration occurring 1.3 h earlier. The responses of transpiration to environmental drivers differed between sites. For example, transpiration increased with vapor pressure deficit (VPD) and plateaued at approximately 1.76 kPa at the urban site, whereas it increased initially but declined beyond a threshold of 1.33 kPa at the rural site. Transpiration at the urban site was primarily driven by photosynthetically active radiation (PAR), indicating energy-limited conditions, whereas at the rural site it was mainly constrained by soil moisture, reflecting water-limited conditions. These differences were associated with contrasting stomatal regulation strategies, with rural trees exhibiting higher sensitivity to VPD (m/g_cref = 0.86) and stronger stomatal control under water-limited conditions.

Our results demonstrate that long-term urban-rural environmental differences reshape transpiration patterns, environmental responses, and stomatal regulation strategies in trees. These findings provide new insights into tree water-use strategies under urbanization and suggest that atmospheric drought and soil water availability jointly regulate transpiration responses across contrasting habitats. Our study also provides key parameters for improving ecohydrological models and urban forest management.