Effects of Long-Term Disturbance on Riparian Vegetation and In-Stream Characteristics

Abstract

We assessed the influence of riparian disturbance on 26 stream variables in Linesville Creek and six tributaries, in northwestern Pennsylvania, USA. Redundancy analysis, a canonical ordination technique, was used in three separate analyses to test for significant relationships between riparian characteristics and the multivariate stream data (aspects of geomorphology, substrate composition, leaf litter processing and macroinvertebrate shredder species composition). Ordination (PCA) showed riparian vegetation to be strongly related to land use. Pasture sites, characterized by grasses and Eupatorium spp., had the most distinctive plant assemblages, while species composition was most variable among woodland sites. In-stream variables were significantly related to both land use and riparian vegetation. Generally, disturbed sites tended to have more silt and higher populations of Gammarus sp. Land use, in particular, had a great effect on detritus processing and storage. Stream reaches in pastures tended to have less particulate organic matter, higher detritus processing rates and lower detritus processing predictability than streams in woodland sites. Riparian vegetation was significantly related to channel substrate composition, and this relationship was independent of current land use. Mature woodland streams tended to be composed of more stable channel substrates (pebble, tree roots) while channels in pasture and immature woodland had more sand and silt. Additionally, the presence of the stonefly family Nemouridae was strongly related to the maturity of the riparian vegetation. Our primary conclusion was that anthropogenic disturbance destabilized stream ecosystem function. Specifically, agricultural land uses caused detrital processing rates to be less predictable than in forested streams. In general, we concluded that while riparian vegetation composition and land use had several similar effects on these streams, several independent effects were apparent as well.