Data from: Mortality versus survival in drought‐affected Aleppo pine forest depends on the extent of rock cover and soil stoniness

Drought-related tree mortality had become a widespread phenomenon in forests around the globe. Recent drought years led to 5-10% mortality in the semi-arid pine forest of Yatir (Israel). The distribution of dead trees was, however, highly heterogeneous with parts of the forest showing >80% dead trees (D plots) and others with mostly live trees (L plots). At the tree level, visible stress was associated with low predawn leaf water potential at the dry season (-2.8 MPa, vs. -2.3 MPa in non-stressed trees), shorter needles (5.5 vs. 7.7 mm) and lower chlorophyll content (0.6 vs. 1 mg g-1 dw). Trends in tree ring widths reflected differences in stress intensity (30% narrower rings in stressed compared with unstressed trees), which could be identified 15-20 years prior to mortality. At the plot scale, no differences in topography, soil type, tree age, or stand density could explain the mortality difference between the D and L plots. It could only be explained by the higher surface rock cover and in stoniness across the soil profile in the L plots. Simple bucket model simulations using the site's long-term hydrological data supported the idea that these differences could result in higher soil water concentration (m3/m3) in the L plots and extend the time above wilting point by several months across the long dry season. Accounting for subsurface heterogeneity is therefore critical to assessing stand level response to drought and projecting tree survival, and can be used in management strategies in regions undergoing drying climate trends.

与干旱相关的树木死亡已成为全球森林中普遍存在的现象。近年的干旱事件导致以色列亚伊尔（Yatir）半干旱松林的树木死亡率达5%~10%。然而，枯树的空间分布具有高度异质性：部分林区的枯树占比超过80%（记为D样地（D plots）），其余区域则以活树为主（记为L样地（L plots））。在单株树木尺度上，可见的胁迫症状与旱季较低的黎明前叶水势相关：胁迫植株的叶水势为-2.8 MPa，而非胁迫植株为-2.3 MPa；其针叶更短（5.5 mm vs. 7.7 mm），叶绿素含量也更低（0.6 mg·g⁻¹干重 vs. 1 mg·g⁻¹干重）。年轮宽度的变化趋势反映了胁迫强度的差异：胁迫植株的年轮宽度较非胁迫植株窄30%，且这种差异可在树木死亡前15至20年被观测到。在样地尺度上，地形、土壤类型、树木年龄或林分密度的差异均无法解释D与L样地间的死亡率差异，仅L样地中更高的地表岩石覆盖率以及整个土壤剖面的更高石砾含量可解释这一现象。采用该站点长期水文数据开展的简单水桶模型（bucket model）模拟结果验证了这一假设：上述差异可使L样地的土壤含水量（单位：m³·m⁻³）更高，且在漫长的旱季中，土壤含水量维持在萎蔫点以上的时间可延长数月。因此，考虑地下异质性对于评估林分对干旱的响应以及预测树木存活率至关重要，且可应用于正经历气候变干趋势地区的森林管理策略中。