Document: MAR-3-69-35

Temporal variation in abundance and composition of pioneer seedling cohorts of riparian trees: Influence of river flow variation.

DIXON, M.D.* and M.G.TURNER

University of Wisconsin, Madison, WI 53706 USA ¹

Abstract:
Riparian vegetation dynamics are influenced by the interaction of species-specific demographic processes and fluvial geomorphic processes, both of which vary in space and time. We investigated the influence of river flow variation and timing on the species composition of first year seedling cohorts of pioneer riparian tree species, during 1997-1999, on sandbars in the Wisconsin River. Relative and absolute abundances of seedlings varied greatly among the years, which also differed in their growing season hydrographs. Betula nigra comprised 83% of the seedling cohort in 1997, a year in which the flows remained relatively low and stable following the recession of the spring peak, but was only a minor component in 1998 (13%) and 1999 (<1%), years characterized by mid-season flow pulses that may have caused high mortality to recent germinants. The 1998 pulse occurred in late June, near the end of the Betula dispersal interval, and the 1999 pulse occurred in late July, after all species had completed dispersal. Both the 1998 and 1999 cohorts had low overall seedling density. Salix species, which had a wide dispersal interval, made up about two-thirds of the seedlings in 1998 (Salix exigua 24%, Salix sp. 40%), but were nearly absent in 1999 (<1%). The 1999 cohort was dominated by Acer saccharinum (89%), the earliest dispersing species. These results suggest that variation in species composition of sandbar colonists among years is heavily influenced by the interaction between time of dispersal and the magnitude and timing of river flows. We conclude that interannual variation in river flow is an important control on species composition of pioneer seedling cohorts, which may influence successional trajectories in riparian forests.

Keywords: succession, riparian vegetation, fluvial geomorphology

This abstract is being presented at: 9:30 AM in session:
Oral Session #20: Riparian Ecology.