PROCESSES GOVERNING HYDROCHORY ALONG RIVERS: HYDRAULICS, HYDROLOGY, AND DISPERSAL PHENOLOGY

Abstract

Rivers are important corridors for movement, migration, and dispersal of aquatic organisms as well as for dispersal of the seeds and vegetative propagules of riparian plants. In this investigation, the relationships between flow regime, channel morphology, dispersal phenology, and seed deposition patterns were evaluated using experimentation in a flume. A channel with geomorphic features common to a wide range of stream morphologies was constructed in a 1.8 × 20 m experimental flume through which three hydrologic regimes (one natural and two typical of dam releases) were routed in replicated trials. Relationships between dispersal phenology and hydrologic regime were examined using color-coded Betula fontinalis seeds released over each 10-min trial. Spatial patterns of seed deposition along stream margins were then compared to determine the individual and combined effects of flow regime, fluvial feature, and timing of seed release. Reynolds number, Froude number, Weber number, flow velocity, and a dimensionless recirculation index were then used to develop predictive models of patterns of seed dispersal. Fluvial feature, flow regime, and timing of seed release all described significant proportions of the variability in concentrations of seeds deposited. Fewer seeds were deposited during the ascending flow releases than under descending and stepped regimes, indicating that ascending flows during seed release may have a significant negative effect on hydrochoric seed deposition. Ascending flows entrain and transport rather than deposit seeds. Retention and recirculation of seeds in fluvial environments with low recirculation indices extend the period over which plants with buoyant seeds are able to disperse, possibly to the advantage of such species. Systematic delivery of seeds to specific types of fluvial environments in repeated trials and under varied regimes suggests that hydrochory may serve as a form of directed dispersal for some riparian plant species. Froude number explained a large proportion of the variation in seed deposition, followed by flow velocity, Reynolds number, and Weber number. The recirculation index, which quantifies the strength of recirculation in the fluvial environment, accounted for the greatest proportion of the variability in seed dispersal patterns. Poisson regression models of seed deposition as a function of dimensionless hydraulic parameters provide an empirical basis for incorporating spatial patterns of seed dispersal into riparian vegetation models. In addition, through incorporating dispersal phenology and information regarding the regeneration niche of species of special concern, specific hydrograph characteristics may be managed below dams to accommodate or inhibit species using hydrochory.