Mark D Dixon

Advisor: Monica G Turner

PhD Abstract:
Woody vegetation dynamics on Wisconsin River sandbars: spatial and temporal controls on seedling recruitment.

The dynamics of riparian communities are linked to both spatial and temporal variability in fluvial processes, making them sensitive to environmental changes that affect the flow regime. On the Wisconsin River (Wisconsin, USA) I investigated (1) the influence of broad and fine-scale physical characteristics of the river channel on the distribution of tree seedlings on river sandbars, (2) the influence of temporal variability in flow conditions and timing of seed dispersal on composition and density of seedling cohorts, and (3) the long-term effects of flow regulation and climatic variability on flow and simulated seedling recruitment. This study was conducted on a 16-km section of the Wisconsin River, near Portage, Wisconsin, from 1997-2000. Both fine and broad-scale variables influenced seedling distribution in 1998. The older seedling distributions were more strongly related to broad-scale variables (particularly channel width) than were new seedlings, and broad-scale context influenced the relationship between some fine-scale variables and seedling occurrence. Summer flow conditions varied strongly among the years of the study (1997-2000). Species composition and seedling density also varied strongly, with a 100-fold range in densities among years and with a different dominant species in each year. Temporary high flow events during 3 of the 4 summers strongly influenced both the density and species composition of new seedling cohorts, in ways that were consistent with differences among species in dispersal and germination phenology. Simulation modeling of seedling recruitment in relation to simulated natural, simulated regulated, and historic flows showed high variation in annual seedling recruitment over longer time scales. Colonication was limited by years with high June and July flows and high June precipitation, and overwinter seedling persistence was limited by high winter/spring flows and high winter precipitation. Flow regulation on the Wisconsin appears to have enhanced persistence of woody vegetation by reducing the size of the spring peak flow, thereby reducing overwinter mortality of seedlings. Consideration of the spatial, temporal, and human controls on vegetation establishment is important for understanding the dynamics of riparian vegetation communities, particularly in the face of potential environmental change.

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