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Plant succession after hydrologic disturbance: Inferences from contemporary vegetation on a chronosequence of bars, Willamette River, Oregon, USA
Citation:
CLINE, S. P. AND L. MCALLISTER. Plant succession after hydrologic disturbance: Inferences from contemporary vegetation on a chronosequence of bars, Willamette River, Oregon, USA. River Research and Applications. John Wiley & Sons Incorporated, New York, NY, 28:1519-1539, (2012).
Impact/Purpose:
Historic unconstrained, unregulated streamflow along the upper mainstem of the Willamette River, Oregon, produced a floodplain of coalescent bars supporting a mosaic of vegetation patches.
Description:
Historic unconstrained, unregulated streamflow along the upper mainstem of the Willamette River, Oregon, produced a floodplain of coalescent bars supporting a mosaic of vegetation patches. We sampled the contemporary vegetation of 42 bars formed 3 to 64 + years ago in four, 1 km- to 4 km-long reaches and analyzed development in relation to historic hydrologic disturbance, streamflow, precipitation, and regional climate. Six aerial photo intervals between 1936 and 2006 bracketed the transition from relatively low to high regulation during dam and revetment construction. Pioneer trees rapidly colonized newly formed and disturbed bars, both historically and recently. The 70-year photo record captured a sequence of gravel bars with Populus-Salix (< 14 years), islands with maturing Populus (35-49 years), and islands coalesced to floodplain with Populus-Acer-Fraxinus (50-75 years). Acer-Populus (mean 97 years) dominated adjacent floodplain forests established prior to 1936. With time since last disturbance, overstory basal area, bar height, and silt/clay fraction increased, and overstory and understory wetland affinities decreased. Meanwhile, a distinct Salix lucida ssp. lasiandra community colonized off-channel areas formed during channel migration. Introduced species were > 60 % of total understory cover at all bar ages; Phalaris arundinaceae and Rubus armeniacus were most frequent and abundant. The historic bar disturbance rate oscillated with cool (warm) phases of the Pacific Decadal Oscillation (PDO), local precipitation, and bankfull discharge frequency. Hydrologic disturbances were below historic levels after full regulation, except during a recent wet period, and affected mostly new and existing bars colonized by pioneer tree seedlings. Results suggest that increased channel mobility is necessary to sustain the diverse suite of plant communities associated with the upper Willamette’s historic shifting habitat mosaic.
