In Rocky Mountain National Park, Colorado, Gage and Cooper identified possible constraints on willow (Salix spp.) establishment in two heavily browsed riparian areas, in 2000 and 2001, by examining seed dispersal phenology, germinability, and the spatial patterns of aerial seed rain, quantifying the effects of soil surface relief, texture, and moisture on seed entrapment, and by examining natural patterns of seedling emergence in relation to seed source proximity. All species dispersed seeds following peak streamflow and exhibited high germination rates (85-99%). Total seed rain differed between years, although broad spatial patterns were similar. Seed entrapment rates varied significantly with soil moisture and surface relief, but not with texture, and were low (<30%) regardless of treatment. Their analyses indicate that willow seed rain patterns are strongly influenced by the distribution of seed-producing plants. Where browsing intensity is high, as it is in most of the study area, nearly all shoots from the previous year are consumed. This results in the production of few aments and severe reductions in willow seed rain density over broad areas. Without a reduction in elk (Cervus elaphus) population size, changes in their behavior, or intervention by managers to limit elk access to willow stands, the degradation of riparian willows will likely continue. Ongoing loss of willows because of heavy browsing, along with low establishment and recruitment rates, could result in the transformation of shrub-dominated riparian communities into herbaceous-dominated ones, resulting in the loss of important ecological functions.
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