Seasonal and Species‐Level Water‐Use Strategies and Groundwater Dependence in Dryland Riparian Woodlands During Extreme Drought

Jared Williams, John C. Stella, M. B. Singer, Adam M. Lambert, Steven L. Voelker, John E. Drake, Jonathan M. Friedman, L. Pelletier, Li Kui, Dar A. Roberts
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Abstract

Drought‐induced groundwater decline and warming associated with climate change are primary threats to dryland riparian woodlands. We used the extreme 2012–2019 drought in southern California as a natural experiment to assess how differences in water‐use strategies and groundwater dependence may influence the drought susceptibility of dryland riparian tree species with overlapping distributions. We analyzed tree‐ring stable carbon and oxygen isotopes collected from two cottonwood species (Populus trichocarpa and P. fremontii) along the semi‐arid Santa Clara River. We also modeled tree source water δ18O composition to compare with observed source water δ18O within the floodplain to infer patterns of groundwater reliance. Our results suggest that both species functioned as facultative phreatophytes that used shallow soil moisture when available but ultimately relied on groundwater to maintain physiological function during drought. We also observed apparent species differences in water‐use strategies and groundwater dependence related to their regional distributions. P. fremontii was constrained to more arid river segments and ostensibly used a greater proportion of groundwater to satisfy higher evaporative demand. P. fremontii maintained ∆13C at pre‐drought levels up until the peak of the drought, when trees experienced a precipitous decline in ∆13C. This response pattern suggests that trees prioritized maintaining photosynthetic processes over hydraulic safety, until a critical point. In contrast, P. trichocarpa showed a more gradual and sustained reduction in ∆13C, indicating that drought conditions induced stomatal closure and higher water use efficiency. This strategy may confer drought avoidance for P. trichocarpa while increasing its susceptibility to anticipated climate warming.
极端干旱期间旱地河岸林地的季节和物种水平用水策略及地下水依赖性
干旱导致的地下水减少和气候变化引起的气候变暖是旱地河岸林地面临的主要威胁。我们利用南加州 2012-2019 年的极端干旱作为自然实验,评估用水策略和地下水依赖性的差异如何影响分布重叠的旱地河岸树种对干旱的敏感性。我们分析了从半干旱的圣克拉拉河沿岸的两种木棉树(Populus trichocarpa 和 P. fremontii)采集的树环稳定碳和氧同位素。我们还建立了树木源水 δ18O 组成模型,将其与洪泛区内观测到的源水 δ18O 进行比较,以推断地下水依赖模式。我们的结果表明,这两个物种都是面生呼吸植物,在有浅层土壤水分的情况下利用浅层土壤水分,但在干旱期间最终依靠地下水来维持生理机能。我们还观察到物种在用水策略和地下水依赖性方面存在明显差异,这与它们的区域分布有关。P. fremontii 被限制在更干旱的河段,表面上使用了更大比例的地下水来满足更高的蒸发需求。P. fremontii 的 ∆13C 一直维持在干旱前的水平,直到干旱高峰时,树木的 ∆13C 才急剧下降。这种反应模式表明,在达到临界点之前,树木优先考虑的是维持光合作用过程而不是水力安全。相比之下,P. trichocarpa 的 ∆13C 下降更为渐进和持续,表明干旱条件会诱导气孔关闭并提高水分利用效率。这种策略可能会使三尖杉(P. trichocarpa)避免干旱,同时增加其对预期气候变暖的敏感性。
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