Influence of forest cover on in-stream large wood in an agricultural landscape of southeastern Brazil: a multi-scale analysis

Large wood (LW) is critical to the structure and function of streams and forests are the main LW source to stream channels. To assess the influence of forest cover changes at different spatial scales on in-stream LW quantity, we selected eighteen catchments (2nd–4th order) in Southeastern Brazil with forests at different levels of alterations. In each catchment we quantified the pattern of forest cover (% cover and relative catchment position), the physical characteristics of catchments (elevation and slope), the characteristics of channels (wetted channel width and depth), the abundance and volume of in-stream LW, and the frequency of LW pools. We used simple and multiple linear regression to assess the response of LW variables to landscape and stream reach variables. Most of the LW was relatively small; 72 % had a diameter <20 cm, and 66 % had a length <5 m. Although percent forest cover at reach scale had substantial support to explain LW variables, the best predictors of LW variables were forest cover at broader scales (LW abundance and LW pool frequency were best predicted by forest at intermediate distance at the catchment scale and LW volume was best predicted by forest cover at the drainage network scale), suggesting that downstream transport is an important process in addition to local processes in our study area. These findings have important management implications because although low forested reaches receive less LW from local forests (or no LW in the case of deforested stream reaches), they are receiving LW from upstream forested reaches. However, the material is generally small, unstable and likely to be easily flushed. This suggests that not only should riparian forest conservation encompass the full drainage network, but forests should also be allowed to regenerate to later successional stages to provide larger, higher quality LW for natural structuring of streams.