Dendrochronology reveals the response of a riparian forest to water management in an arid river basin.

Riparian corridors in arid climates sustain life in otherwise inhospitable environments, creating zones of ecological and cultural importance. However, rivers in arid climates are often managed to provide water for human populations at the expense of a river's freshwater biodiversity. In this study, ecosystem response to river flow management is assessed using mature cottonwood tree-ring growth and carbon isotope composition as bio-indicator proxies for river ecosystem health. We examine the ecological impacts of flow management on the Lower Truckee River in Nevada, USA, which runs through an arid-climate basin that has been subject to decades of heavy flow diversion and management. Particular attention is given to the effects of major lawsuits in 1973 and 1982 that restored spring and summer flows to the river following progressive dewatering since 1905. Most mature trees (>30 years old) downstream of diversions responded strongly to restored flows, with average annual tree-ring growth increases of 160%. Among tested streamflow metrics, average annual flow had the strongest positive influence on cottonwood growth, and aspects of the spring snowmelt recession were also influential. Precipitation was also linked with cottonwood growth, primarily during the period of management before 1973 when dry season flows were severely limited. Not all floodplain trees responded similarly to changes in flow metrics, suggesting that individual tree attributes and heterogeneity in floodplain soils are highly important to tree growth. Results offer promising evidence that flow restoration can lead to measurable improvement in riparian forest productivity, although site-specific considerations including channel form and location on the floodplain are important in determining response to changes in flow patterns.