Influence of riparian vegetation and forest structure on the water quality of rural low-order streams in SE Brazil

Abstract

Land use changes have resulted in large deforestation of rural landscapes, thus influencing transport of water and materials along the watersheds. Riparian zones have strong effects on stream water quality, but most studies evaluated the effects of riparian vegetation (forested vs. deforested), although riparian forests may greatly differ in structure. Here we evaluated the effects of riparian vegetation characteristics (RV) and riparian forest structure (RFS) on stream water quality in a tropical rural landscape in SE Brazil. We sampled 15 low-order streams along a gradient in riparian degradation, from completely deforested streams to those with well-developed riparian forests. In each stream we established a 100m reach and evaluated RV (trees, grasses, vines, bamboo, canopy closure, and riparian forest width), RFS (tree density and height, vertical canopy structure, mean basal area and diameter at breast height), and stream habitat and water quality (mean water depth, fine sediment cover (FSC), electric conductivity (EC), dissolved oxygen (DO), ammonium, nitrate, total N, dissolved, particulate, and total P). We used Principal Components Analyses to reduce dimensionality of RV and RFS variables, and evaluated the separate effects of RV and RFS on water quality variables using conditional autoregressive models. We found effects of both RV and RFS on FSC, EC, DO and ammonium concentrations, and effects of only RFS on total and dissolved P concentrations. These results suggest that although RV variables are good predictors of the buffering role of riparian zones, the structure of the riparian forest can influence stream water quality variables. Thus, heterogeneity in riparian forest structure due to forest degradation or restoration should be considered when evaluating buffering effects of riparian zones.