Future climate change will exacerbate drought stress in water-limited ecosystems. However, topography can alter the fine-scale climatic and hydrologic conditions that mediate plant response to meteorological drought. Here, with six new valley oak (Quercus lobata) tree-ring width chronologies, we assess how topography acts as a mediating factor on tree growth and drought sensitivity. Because valley oaks are known to be highly dependent on subsurface water, we predicted that trees growing in riparian sites would be less sensitive to precipitation variability due to greater access to groundwater. Trees were sampled in the Tehachapi Mountains of California across a landscape gradient of sites ranging from 375–1650 m elevation and across upland (55–69 m mean height above nearest drainage) and riparian (2–6 m mean height above the nearest drainage) hillslope positions. Interannual tree growth patterns and drought sensitivity varied substantially in association with hillslope position and elevation. Valley oak radial growth showed a consistently weaker response to precipitation at riparian sites. The influence of hillslope position on drought sensitivity varied with elevation, such that the riparian buffering effect was weakest at sites higher in the watershed and strengthened, progressively, toward the lower elevation sites with greater climatic water deficit. Upland tree growth exhibited a strong response to high-frequency interannual precipitation variability at the high elevation site, whereas trees at lower elevation upland sites responded more to low-frequency decadal trends in precipitation, possibly reflecting hydrogeological processes by which precipitation feeds groundwater lower in the watershed. Our results are consistent with groundwater-dependence of valley oak and indicate that riparian habitats are the most likely refugia for the species during prolonged drought.

中文翻译：

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景观位置调节加州山谷橡树（Quercus lobata）的干旱脆弱性

未来的气候变化将加剧水资源有限的生态系统的干旱压力。然而，地形可以改变调节植物对气象干旱反应的精细气候和水文条件。在这里，有六棵新的山谷橡树（栎树）树木年轮宽度年表，我们评估地形如何作为树木生长和干旱敏感性的中介因素。由于众所周知，山谷橡树高度依赖地下水，因此我们预测，由于更容易获取地下水，生长在河岸地区的树木对降水变化的敏感度较低。对加利福尼亚州特哈查皮山脉的树木进行了采样，采样点的景观梯度范围为 375-1650 m 海拔，横跨高地（距离最近排水系统平均高度 55-69 m）和河岸（距离最近排水系统平均高度 2-6 m）山坡位置。树木的年际生长模式和干旱敏感性与山坡位置和海拔有关，变化很大。山谷橡树径向生长对河岸地区降水的反应始终较弱。山坡位置对干旱敏感性的影响随海拔的变化而变化，流域内较高地点的河岸缓冲效应最弱，而气候缺水程度较低的地点，河岸缓冲效应逐渐增强。高海拔地区的高地树木生长对高频年际降水变化表现出强烈的响应，而海拔较低的高地地区的树木对降水的低频年代际趋势的反应更大，可能反映了降水向较低地区的地下水供给水文地质过程。分水岭。我们的结果与山谷橡树对地下水的依赖一致，并表明河岸栖息地是该物种在长期干旱期间最有可能的避难所。