Abstract:

[Objective] In semi-humid to semi-arid transition zones, water availability is a primary constraint on forest ecosystem functioning. In these regions, stem water storage (W_s) plays a critical role in buffering mismatches between transpirational demand and root water uptake, particularly under fluctuating soil moisture conditions. However, the dynamic responses of W_s to variations in soil moisture at different depths remain poorly quantified. Most existing studies have focused on whole-profile soil moisture or atmospheric drivers while neglecting the layered structure of water supply within the root zone. To address this knowledge gap, this study investigated the response characteristics and regulatory mechanisms of W_s in relation to soil moisture at different depths in a Larix principis-rupprechtii plantation in the Xiangshui River area of the Liupan Mountains, Ningxia, China, a typical semi-humid to semi-arid transition zone. [Methods] The research was conducted during the 2024 growing season, from May 10 to October 10. Four sample trees with different stem diameters were selected within a permanent 30 m × 30 m plot. Stem sap flow was continuously measured at two heights—the crown base at 4.0 m and stem base at 0.35 m—using thermal dissipation probes (STDP30), with data recorded at 5-minute intervals. W_s was calculated from the difference in daily cumulative sap flow between the crown base and stem base. Soil volumetric water content was measured at three depths (0-20, 20-40, and 40-60 cm) using 5TE sensors, and relative soil water content (RSWC) was calculated from the measured values. Meteorological variables, including solar radiation, air temperature, relative humidity, and precipitation, were recorded using an automatic weather station. Potential evapotranspiration (E_p) was calculated using the Penman-Monteith equation. Leaf area index (LAI) was measured every 7-10 days using an LAI-2200c canopy analyzer. To model the response of W_s to multiple interacting factors, an upper envelope method was first used to derive single-factor response functions for E_p, LAI, and RSWC at each soil depth. These functions were subsequently integrated into a modified Jarvis-Stewart model to simulate daily W_s. Even-numbered days were used for model calibration (n=76), whereas odd-numbered days were used for model validation (n=78). To further investigate nonlinear and asymmetric responses to changes in soil moisture, gradient boosting regression trees (GBRT) combined with k-fold cross-validation were used to improve model stability. Sensitivity analyses were performed by perturbing RSWC in each soil layer by ±5% while holding E_p and LAI constant, thereby allowing the response magnitudes among soil layers to be compared. Finally, a factor-substitution method was used to quantify the relative contribution of each soil layer to W_s under dry ((RSWC_{0-60 cm} < 0.36) and wet (RSWC_{0-60 cm} ≥ 0.36) conditions. [Results] The diurnal dynamics of W_s exhibited a consistent cycle of morning recharge, daytime release, and evening recharge. Daily W_s ranged from 0.16 to 3.69 kg·d⁻¹, with a mean of 1.97 kg·d⁻¹. Single-factor response analysis showed a quadratic relationship between W_s and E_p, with W_s reaching its maximum at an E_p threshold of 3.68 mm and decreasing slightly beyond this threshold. LAI and RSWC at all three soil depths exhibited saturating exponential relationships with W_s. Notably, the RSWC threshold below which soil moisture limited W_s increased with soil depth, from 0.35 at 0-20 cm to 0.36 at 20-40 cm, and 0.37 at 40-60 cm. The modified Jarvis-Stewart model performed well, with R² = 0.82 and Nash-Sutcliffe efficiency (NSE) of 0.81 during calibration and R² = 0.75 and NSE = 0.74 during validation. The simulated cumulative W_s over the growing season was 296.79 mm, slightly lower than the observed value of 303.24 mm. The GBRT model revealed a clear asymmetric response: a 5% increase in RSWC produced a greater increase in W_s than the decrease in W_s caused by a 5% reduction in RSWC. Specifically, the mean increase in W_s following a 5% increase in RSWC was 0.31 kg·d⁻¹, whereas the mean decrease following a 5% reduction in RSWC was only 0.19 kg·d⁻¹. This asymmetry suggests a physiological priority for water recharge over excessive release under variable moisture conditions, thereby contributing to hydraulic safety and embolism avoidance. The sensitivity ranking was consistent across soil layers: 0-20 cm > 20-40 cm > 40-60 cm, indicating that shallow soil moisture exerted the strongest short-term control over W_s dynamics. Contribution analysis showed that under wet conditions, the 0-60 cm layer contributed more to W_s than under dry conditions. The relative contributions of the 0-20, 20-40, and 40-60 cm layers were 2.04%, 11.88%, and 0.34%, respectively, under dry conditions, and increased to 5.16%, 13.94%, and 1.27%, respectively, under wet conditions. The 20-40 cm layer consistently made the largest contribution under both moisture regimes, indicating that this layer represents a relatively stable source of water for Larix principis-rupprechtii. [Conclusion] First, the depth-dependent soil moisture thresholds indicate that water management should target specific root-zones layers rather than treating the entire soil profile uniformly. Information on these layer-specific thresholds can support precision irrigation and forest density regulation. Second, the asymmetric response to soil moisture perturbations implies that short-term wetting events may restore W_s more strongly than equivalent drying events deplete it, which has implications for rainfall-frequency effects and irrigation scheduling. Third, the dominant and stable contribution of the 20-40 cm soil layer highlights the importance of maintaining adequate soil moisture within this zone to support transpiration under both dry and wet conditions. In practical terms, regulating stand density and optimizing stand structure may reduce competition for water and enhance the stability of shallow and intermediate soil moisture, thereby improving forest resilience to climate variability. Nevertheless, this study has several limitations. The conclusions are based on observations from a single growing season characterized by relatively normal climatic conditions and the applicability of the model to years with extreme drought or prolonged wet conditions requires further validation. In addition, although the Jarvis-Stewart model captures the principal driving factors, its instantaneous-response structure and the buffering effects of stem capacitance may cause it to underestimate contributions from deep soil water under extreme conditions. Future studies should integrate multi-year observations and stable-isotope tracing to directly verify shifts in water sources among soil layers. Despite these limitations, the integrated modeling framework provides a robust quantitative approach for disentangling the effects of layered soil moisture on stem water storage and offers a scientific basis for water-oriented forest management in semi-humid to semi-arid transition zones.

Key words: Larix principis-rupprechtii forest, stem water storage, soil moisture contribution, Jarvis-Stewart model, GBRT model

摘要：

【目的】在半湿润－半干旱过渡区人工林生态系统中，树干储水对缓解蒸腾与根系吸水之间的不平衡至关重要，但其对不同土层土壤水分变化的动态响应机制仍缺乏定量认识。本研究旨在探究宁夏六盘山香水河小流域华北落叶松（Larix principis-rupprechtii）人工林树干储水对分层土壤水分变化的响应特征及调控机制。【方法】采用上外包线法提取单因子响应函数，构建修正的Jarvis-Stewart模型；引入梯度提升回归树模型进行敏感性分析；结合因子替代法量化各土层在不同水分条件下对树干储水的贡献率。【结果】随着土层加深，限制树干储水释放的土壤水分阈值升高，修正的Jarvis-Stewart模型能较好地模拟多因子耦合的响应。树干储水对土壤水分的扰动呈非对称性响应，土壤水分增加5%的促进作用强于减少5%的抑制作用，且不同土层的敏感性排序为：0－20 cm>20－40 cm>40－60 cm。20－40 cm土层在土壤干旱和湿润条件下贡献率均最高，是稳定的供水来源。【结论】树干储水对不同土层水分的响应存在分层特征和阈值效应，20－40 cm土层是维持树干储水稳定的关键土层。人工林经营中应将中层土壤作为水分调控重点，并强化浅层水分的动态监测，以提升应对干湿快速转变的水分调控能力。

关键词: 华北落叶松林, 树干储水, 土壤水分贡献, Jarvis-Stewart模型, GBRT模型