Seasonal Sap Flow Dynamics Under Variable Water Stress in a Himalayan Chir Pine (Pinus roxburghii) Forest

The Himalayan region is experiencing rapid hydroclimatic shifts, yet the physiological resilience of its dominant forest species remains poorly understood. Although Pinus roxburghii (Chir pine), one of the dominant species of the central Himalaya, covers 16% of the forest area, our understanding of its water-use strategies under compounding stress conditions such as low soil moisture (SM) and high vapor pressure deficit (VPD) is limited. Here, we investigated the hydro-physiological response of a Chir pine-dominated forest in the Kumaun Himalayas (Almora, India) using continuous Thermal Dissipation Probe (TDP) measurements over 304 days. By integrating sap-flux-derived transpiration with daily environmental data, we quantified tree water regulation across dormant and growing seasons. Efforts are also made to enhance our knowledge of the behavior of Chir-pine under water stress conditions, which was quantified by isolating 50th percentile thresholds (SM < 0.13 m³ m⁻³; VPD > 0.76 kPa) of the stress conditions. Our analysis reveals a significant seasonal variation in hydraulic sensitivity. During the growing season, mean sap flow (812.4 cm³ h⁻¹) was notably higher than during the dormant season (513.9 cm³ h⁻¹) driven by peak photosynthetic demand. We also found that SM emerged as the key determinant of Himalayan Chir-pine transpiration, while VPD did not have any such signatures. However, trees maintained high flux under isolated atmospheric drought (high VPD, high SM); the transition to combined stress triggered a sharp, non-linear decline in sap flow. This indicates an isohydric strategy of Chir-pine, where strong stomatal regulation prioritizes the prevention of xylem embolism over carbon gain during the environmental stress. This study provides the first mechanistic baseline for scaling tree-level hydraulics to forest-stand water balances in the Central Himalayas, offering critical insights for predicting regional forest water security under a changing climate.