The water relations of riparian trees are often closely tied to stream hydrology and channel morphology, which can be altered by urban development. In regions with limited precipitation, urban water use can generate or augment dry-season stream flows, potentially providing a water subsidy to riparian trees. However, urbanization is also associated with high storm flows that cause stream channel incision, or downcutting, which could limit the effect of flow subsidies by lowering riparian water tables. We investigated the effects of urban dry-season flow subsidies and stream channel incision on the water status of streamside trees in Sacramento, California, which has a Mediterranean climate with a distinct and lengthy dry season. For two common facultative riparian species, Quercus lobata (valley oak) and Fraxinus latifolia (Oregon ash), we analyzed both midday stem water potential (Ψ_stem) measurements and leaf carbon isotope signatures (∂¹³C) to determine whether channel incision and dry-season flow had interacting effects on seasonal midday water stress and longer-term water use efficiency. We found that stream flow could substantially reduce dry-season water stress in both tree species, but only at low levels of channel incision. Leaf ∂¹³C signatures for Q. lobata suggested increased water use efficiency in trees growing along incised streams and those lacking dry-season flow. Urban management decisions that affect both dry-season base flows and channel incision can thus influence the growth and health of riparian trees, potentially leading to broader changes in riparian ecosystems.

中文翻译：

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城市河道切开和水流补贴对河岸树木的水状况有相反的影响

河岸树木与水的关系通常与溪流水文和河道形态密切相关，而河水形态和河道形态可因城市发展而改变。在降水量有限的地区，城市用水会产生或增加枯季河流流量，从而可能为河岸树木提供水补贴。但是，城市化还与高暴雨流量有关，暴风雨导致河道切开或下水，这可能会通过降低河岸水位来限制水流补贴的影响。我们调查了城市旱季流量补贴和河道切开对加利福尼亚州萨克拉曼多市河岸树木水状况的影响，该州拥有地中海气候，旱季明显而漫长。对于两种常见的兼性河岸种，栎栎（Quercus lobata）（谷栎）和Fraxinus latifolia（俄勒冈灰），我们分析了午间茎水势（Ψtem_茎）的测量值和叶片碳同位素特征（）¹³ C），以确定通道切口和枯季水流是否对季节性的午间水分胁迫和较长时间的水分交互作用。长期用水效率。我们发现，溪流可以显着降低两种树种的旱季水分胁迫，但仅在较低水平的河道切口处有效。叶∂ ^13个Ç签名问：葛藤建议增加沿切开的树木和缺乏枯季流量的树木的用水效率。因此，影响干季基础流量和河道切口的城市管理决策可能会影响河岸树木的生长和健康，并可能导致河岸生态系统发生更广泛的变化。