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Cottonwood-willow stand structure on regulated and unregulated reaches of the Verde River, Arizona.
Beauchamp, Vanessa*,1, Stromberg, Juliet1, 1 Arizona State University, Tempe, AZ
ABSTRACT- Cottonwood (Populus sp.) and willow (Salix sp.), the dominant overstory species in western riparian forests, are disturbance-adapted species with narrow germination windows. Changes to flood cycles often lead to a decrease in recruitment success and survival of these species. This research investigates the effects of damming on the flow regime of a river managed for urban and agricultural water supply, and on the structure and composition of riparian cottonwood-willow forests downstream from the dam. Fifty-five years of stream gage data were used to compare flow regimes on unregulated and regulated reaches of the Verde River, in central Arizona. The species composition, stem density and basal area of cottonwood (P. fremontii) and willow (S. gooddingii and S. exigua) dominated stands were compared in above and below dam reaches. Dam operation has decreased peak flows and flow variability, shifted the timing of high flows, and increased summer base flows. However, regulated reaches along the Verde still experience spring floods in very wet years, allowing for periodic cottonwood and willow regeneration. Cottonwood and willow stem density was not different between above and below dam reaches in sapling (1-10 year) and mature (11-54 year) stands, but cottonwood stem density in old-growth (55+ year) stands was higher in unregulated reaches (P < 0.01). Flow regulation has altered other attributes of the riparian vegetation. For example, some measures of tree and shrub richness varied between reach types. Also, stem density of salt cedar (Tamarix ramosissima), a stress-tolerant, reproductively opportunistic species, was higher in regulated reaches, for sapling classes only (5.82 ± 2.15 stems/m2 vs. 0.03 ± 0.03 stems/m2, P < 0.001).
Key words: streamflow regulation, Populus, Tamarix, riparian vegetation