Contraints on and Opportunities for willow establishment in Rocky Montain Natioanal Park

Cooper, David; Gage, Edward; Dickens, Joyce

From 1999 to 2000 we conducted field studies to investigate the factors constraining willow establishment in Moraine and Horseshoe Parks in Rocky Mountain National Park, and to conceptualize opportunities for future willow establishment in these areas. The documented decline of montane riparian willow communities in portions of Rocky Mountain National Park, Colorado (RMNP) is a significant concern for park managers. This study assessed factors that may constrain willow seedling establishment in Moraine and Horseshoe Parks, the primary winter range for elk in RMNP. We examined the dispersal phenology and seed germinability of the dominant willow species, the temporal and spatial patterns of aerial seed rain, assessed the role of hydrochory (i.e. dispersal by water) in the dispersal of sexual and asexual propagules, quantified the effects of micro-topographic relief, soil texture, and soil moisture on rates of willow seed entrapment, and related natural patterns of seedling establishment on point bars to seed source proximity. Seed rain densities declined exponentially with distance from seed source and many portions of the study areas received seed rain densities < 1 % of those observed in reference areas. Hydrochoric transport of propagules occurred, but at densities too low to compensate for the low levels of aerial seed rain observed in extensive portions of the study area. Rates of seed entrapment varied with soil moisture and surface relief, and were low regardless of treatment, indicating that only a portionof wind dispersed seeds are entrapped long enough for germination to occur. Observed seedling densities on point bars correlated negatively with distance to seed-producing willows.Small numbers of live willow stem fragments were deposited along study area point bars, but at rates too low to significantly affect willow population dynamics. Collectively, these results indicate that propagule availability may constrain current and future willow establishment in the elk winter range.

Contraints on and Opportunities for willow establishment in Rocky Montain Natioanal Park

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