Print PageWater development has greatly affected California’s rivers and has been implicated in recruitment decline in riparian tree species in the Salicaceae. Efforts to restore and manage these river systems must balance the need for water supply and flood control with providing flow regimes that maintain native species. In 2001, a portion of the Merced River and surrounding floodplain were restored and fitted with an engineered channel scaled to the modern flow regime. In 2010, we monitored seedling recruitment of three locally dominant species, Populus fremontii, Salix exigua, and Salix gooddingii. In addition, we monitored variables identified in the literature as being critically relevant, namely seed release timing, seed dispersal, depth to groundwater, and light competition. We used the results of a 2-D model of recent flows to investigate mortality due to hydrologic scour.
Results/Conclusions
Small but significant numbers of seedlings (individuals <1 m tall) persisted throughout the year-long study. There was little recruitment into this size class, compared to more natural rivers, balanced by relatively low mortality rates. Seedlings were limited to a narrow band immediately adjacent to the channel and below bankfull elevation, and were not more common on point bars. Spring dam releases corresponded with the leading edge of the seed release period, but seed dispersal was highly variable in space and was limiting for Populus fremontii. Flows were not high enough to significantly mobilize sediment and mortality was not noticeably correlated with flow events. Low recruitment and mortality rates seem to be caused by dispersal and drought. While previous studies of recruitment have emphasized the importance of safe sites and episodic recruitment dynamics, recruitment patterns under the modified flow regime at our site suggest the possibility of slow accumulation of individuals in a narrow near-channel zone.
