Maya Hayden and colleagues highlight the conservation importance of abandoned channels as refugia for sustaining pioneer riparian forest ecosystems (online in advance of print in the journal Ecosystems). Recognizing that organisms in disturbance-prone environments often persist in spatial refugia during stressful periods, they explore the importance of abandoned channels as potential refugia for a foundation tree species (Fremont cottonwood, Populus fremontii) within the riparian corridor of a large meandering river (Sacramento River in central California). They quantified the total proportion of cottonwood-dominated forest that initiated as a result of channel abandonment for a 160-km reach of the river via GIS analysis of historical air photos, and found that over 50% of the total extant cottonwood forest area was associated with channel abandonment. Tree-ring evidence showed that cottonwood stands commonly developed immediately following channel abandonment, and the recruitment window was less than 10 years at most sites (4-40 yrs). 
They connected patterns of tree establishment with floodplain accretion and sedimentation history, showing rates of floodplain rise and fine sediment accumulation to be higher in young sites and decreasing logarithmically over time. These results suggest that abandoned channels are an important refuge for cottonwood recruitment and that sedimentation processes influence the duration of the colonization window for cottonwood trees. They suggest that on rivers where tree recruitment along the active channel is severely limited by hydrologic regulation and/or land management, abandoned channel refugia may play an even more important role in sustaining an ecologically functional riparian corridor. Preserving bank erosion, active meander corridors, and forest regeneration zones created by cutoff events are therefore key conservation measures in shifting rivers.
Battles Lab
Forest Ecology & Ecosystem Dynamics at the University of California, Berkeley