Water Use Plasticity and Its Drivers in Picea asperata Forests Across Age Classes and Seasons on the Southeastern Tibetan Plateau

Climate warming poses dual challenges of rising temperatures and altered precipitation to alpine forests, yet how tree age and seasonal rhythms interact to shape water use strategies remains unclear. This study combined stable isotopes (δD, δ¹⁸O) and sap flow techniques to investigate seasonal water source dynamics of Picea asperata across age classes (young, middle-aged, mature) on the southeastern Tibetan Plateau.Over 70% of xylem water originated from the 0–80 cm soil layer, with rainwater as the primary source across all ages. During pre-sprouting (March), surface soil water (0–20 cm) uptake followed an age gradient (mature 9.1% > middle 8.5% > young 8.0%), while deeper soil (40–80 cm) showed the opposite trend (mature 8.3% < young 9.3%). Mature trees accessed shallow groundwater and glacial meltwater 30–60 days earlier and consumed 250 times more water monthly than young trees. Sap flow confirmed age-dependent hydraulic strategies: young trees exhibited high flow variability (-114.4 to 471.3 cm³·d⁻¹) with winter negative flow, whereas mature trees maintained stable flow (228.9–331.8 cm³·d⁻¹).Rainwater correlated positively with 20–40 cm soil water inputs ( p < 0.01) but negatively with 40–80 cm contributions ( p < 0.01). Air temperature suppressed glacial meltwater use ( p < 0.05), and mid-layer soil temperature inhibited rainwater use efficiency ( p < 0.05). This study reveals that age-specific hydrological plasticity fundamentally shapes alpine forest water use. Under warming, increased winter temperatures may exacerbate young tree water stress, elevating embolism risk and potentially driving population decline or downslope migration. Our findings provide critical insights for predicting treeline dynamics and informing adaptive forest management.