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Keywords:

  • black cottonwood;
  • riparian plant communities;
  • introduced species;
  • alluvial river;
  • streamflow;
  • precipitation;
  • climate;
  • river regulation

ABSTRACT

Historic unconstrained, unregulated streamflow along the upper Willamette River, Oregon produced a floodplain of coalescent bars supporting a mosaic of vegetation patches. We sampled 42 bars and adjacent floodplain that formed recently to >64 years ago in four >1-km-long reaches to assess differences in vegetation and bar characteristics. Bar disturbance and environmental factors (streamflow, precipitation and regional climate indices) were estimated within six aerial photo intervals between 1936 and 2006. Black cottonwood (Populus balsamifera ssp. trichocarpa) and Salix species rapidly colonized new and disturbed bars, and independent estimates of bar and vegetation age were positively correlated across photo intervals (R2 = 0.95). Overstory communities varied with age: Populus–Salix spp. on gravel bars (trees aged 9–16 years [interquartile range]), Populus on islands (24–45 years), Populus/Acer–Fraxinus on islands coalesced to floodplain (49–72 years) and Acer–Populus on adjacent floodplain forested since 1936 (64–102 years). Meanwhile, bar height, silt/clay fraction, overstory basal area and community wetland prevalence index (WPI) increased. We identified a common but distinct Pacific willow (Salix lucida ssp. lasiandra) community (30–43 years) associated with back channels and island tails. Six understory communities varied by age and by WPI within age strata; in these communities 55–75% of total cover was introduced species, predominantly Phalaris arundinacea and Rubus armeniacus. Vegetation resets occurred in all photo intervals and affected 31% of sampled bars. Mean bar disturbance rate, Q1.5 frequency, precipitation and the Pacific Decadal Oscillation index followed cyclic trends across photo intervals whereas Qmax decreased linearly and relatively fewer, younger bars were disturbed. Results suggest that Populus patch initiation is ongoing but channel stability may be hindering the long-term self-sustainability of plant communities associated with the upper Willamette's historic shifting habitat mosaic. Published in 2011 by John Wiley & Sons, Ltd.