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的普遍现象。近年的干旱事件导致以色列亚提尔（Yatir）半干旱松林的树木死亡率达到5%-10%。然而，枯树的空间分布呈现高度异质性：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（predawn leaf water potential）相关：受胁迫树木的叶水势为-2.8 MPa，而非胁迫树木为-2.3 MPa；同时受胁迫树木的针叶更短（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）。树木年轮宽度的变化趋势能够反映胁迫强度的差异：受胁迫树木的年轮宽度比非胁迫树木窄30%，且这种差异可在树木死亡前15至20年被观测到。在样地（plot）尺度上，地形、土壤类型、树木树龄或林分密度（stand density）的差异均无法解释D、L两类样地间的死亡率差异。唯有L样地更高的地表岩石覆盖率与全剖面更高的石砾含量能够解释这一差异。利用该站点长期水文数据开展的简易水桶模型（bucket model）模拟验证了这一假说：上述差异可使L样地的土壤体积含水量（单位：m³/m³）更高，并在漫长的旱季中将土壤含水量维持在萎蔫点（wilting point）以上的时间延长数月。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，且该结论可应用于气候正趋于干旱化区域的森林管理策略制定中。