Pioneer riparian forests dominated by cottonwood (Populus) and willow (Salix) are typical of alluvial rivers in semi-arid and temperate climates. These forests are essential to the structure and function of the riparian ecosystem. From our preliminary work along the middle reach of the Sacramento River, California, we have observed that the evolution of abandoned channels, which formed as a result of episodic channel cutoff, provide an alternate recruitment pathway for these pioneer species. Despite the prominence of abandoned channels on the landscape, the role of these features in maintaining pioneer riparian forests over the long-term, particularly under regulated river conditions, has received little study. We propose that at least one cohort of pioneers successfully establishes with channel abandonment, and that the rate at which sediment fills the abandoned channel determines the window of time available for additional recruitment. As the abandoned channel fills, sedimentation rate decreases over time. Texture of deposited sediment becomes finer, and is linked to improved water availability for plants. Over time vegetation succession occurs as the frequency and intensity of disturbance decreases. As a result, the abandoned channel moves from a period where abiotic factors are the primary drivers of forest dynamics, to conditions where biotic interactions, particularly competition for light and moisture, become the dominant factors. We hypothesize that a threshold occurs where woody pioneer species are excluded by competition with herbaceous vegetation, and that abandoned channels experiencing slower sedimentation reach this threshold more slowly, allowing for more recruitment opportunities.
Results/Conclusions
Based on vegetation mapping data, cottonwood-dominated forest associated with abandoned channels comprised 54% of the total mapped cottonwood forest area for the entire 100-mile Sacramento River study reach. We estimated the age structure of existing forest stands via tree-ring analysis at sites that ranged in age-since-cutoff and cumulative sediment depth. Preliminary results correlate well with estimates of time-since-cutoff based on time-series historical aerial photography, meaning the oldest cohort established at or soon after the cutoff event. The age structure of existing stands also reflected multiple recruitment events at some sites. Additional work is planned to empirically test the conceptual framework and compare it to well-studied recruitment processes on the active channel. In light of the documented evidence that recruitment on the active channel is severely limited by hydrologic regulation, abandoned channels may play an even more important role in terms of maintaining the riparian corridor of managed rivers into the future.
