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No evidence for cottonwood forest decline along a flow-augmented western U.S. river

May 23, 2023

In contrast to many other arid region rivers, streamflow in the South Platte River is heavily augmented by trans-basin water imports and irrigation return flows. Hydrological changes began in the 1880s, resulting in channel narrowing and the development of a continuous Populus-Salix forest by the mid-twentieth century. We assessed the composition, structure and regeneration status of the riparian forest and identified environmental variables affecting annual Populus deltoides tree growth. We sampled forest structure at four sites in 2015, and conducted dendroecological analysis at seven additional sites in 2019. The riparian forest was dominated by P. deltoides, which occurred at all sites, comprising 79% of total tree basal area and 62% of total tree density. Age structure data indicated ongoing though episodic recruitment of P. deltoides, at least over the past ~130 years. We tested 14 linear mixed effects models to describe the effect of climate and streamflow on individual tree growth (modeled as the log of BAI, n = 237 trees). The most parsimonious model selected with AICc explained 28.6% of BAI variability, and included hydrology and climate factors during the growing season (i.e., June–August streamflow, June–July PDSI), some aspects of off-season (i.e., previous November and March) streamflow, along with tree age and study site effects. The riparian forest developed in response to, and has been maintained by, current climate conditions and water management regimes. It may be negatively affected by future climate change and increased urban water demand in the basin.

Publication Year 2023
Title No evidence for cottonwood forest decline along a flow-augmented western U.S. river
DOI 10.1002/rra.4151
Authors Cetan Christensen, Gabrielle L. Katz, J. M. Friedman, Miranda D. Redmond, Andrew S. Norton
Publication Type Article
Publication Subtype Journal Article
Series Title River Research and Applications
Index ID 70243850
Record Source USGS Publications Warehouse
USGS Organization Fort Collins Science Center