Relationship among physical and hydrological stream and drainage basin characteristics and riparian vegetation structure

Abstract

The primary goal of this study was to examine the physical and biological structure of riparian zones along streams in the Boreal forest of Northwestern Ontario and to discover whether riparian structure was related to key hydrologic variables, including watershed area and surficial geology. The first objective was to verify the relationship between watershed area and physical and hydrological stream characteristics including discharge, temperature, and gradient. The second objective was to determine the variability in stream characteristics within a watershed scale based on differences in physical and hydrological drainage basin characteristics. The third objective was to describe the differences in the vegetation communities in riparian zones and upland communities. The fourth objective of the study examined riparian structure and composition along streams within watersheds that differ in physical and hydrological characteristics. The Mackenzie, Wolf, and Spruce River watersheds were used in a nested sampling design with watershed area class as a grouping variable. A total of 40 streams were sampled having approximately 1 (n = 12), 10 (n = 12), 40 (n = 8) and 100 (n = 8) km2 watersheds. Stream discharge, temperature and gradient, and riparian width and slope were measured while surficial geology was evaluated. Riparian and upland plots were sampled on both sides of the stream and vegetation species were recorded. The results showed clear relationships among physical and hydrological stream and drainage basin characteristics and riparian vegetation structure. As drainage area increased, stream discharge and temperature increased and stream gradient decreased. Stream temperature and gradient were lowest in streams with meadow marsh riparian zones. Riparian width decreased while riparian slope increased with increases in drainage area. Riparian width was greatest, and riparian slope least in meadow marsh riparian zones, while the width was narrowest and slope greatest in conifer swamp riparian zones. Streams with both 1 and 10 km2 watersheds had riparian zones which had lower vegetation species diversity than the surrounding upland timber zone. A discriminant function analysis revealed Speckled Alder, Aspen, and Beaked Hazel characterizing the riparian zones of smaller streams (1 km2 watershed), grasses and sedges characterizing the riparian zones of intermediate sized streams (10 km2 watershed), and Cedar, Honeysuckle and Currants characterizing riparian zones around large streams (100 km2 watersheds). There was a positive relationship between drainage area and the drainage basin’s surface roughness, width/depth ratio, and stream channel sinuosity, while there was a negative relationship between drainage area and the drainage basin’s stream channel and basin slopes. This study characterized riparian vegetation structure along a stream continuum to understand better the structure and function of riparian communities and will be used as part of a first step towards assessing the possible effects of timber harvesting on aquatic ecosystems to ensure the effective management of riparian forest ecosystems.