Abrupt change of a stream ecosystem function along a sugarcane-forest transition: Integrating riparian and in-stream characteristics

Forest remnants can locally improve water quality of deforested streams in a reset effect, but few studies evaluated if leaf breakdown rates respond to forest remnants or, at a finer spatial scale, to riparian forest structure. We studied leaf breakdown rates along a deforested Neotropical stream as it flowed through a sugarcane/forest remnant transition; we adjusted a non-linear model to describe this relationship, and evaluated whether this model was further related to the effects of riparian forest structure, stream physical characteristics and shredder abundances. Modeled leaf breakdown rates rapidly increased as the stream entered the forest remnant, stabilizing in the forest interior after about 100 m. Observed leaf breakdown rates deviated from the model within the forest remnant. This unexplained residual variation was related to riparian forest structure, which was heterogeneous within the remnant. Leaf breakdown rates were not related to stream physical characteristics, but were significantly related to the abundance of shredders, especially with the dominant leaf-mining Chironomidae. Abundances of leaf-mining Chironomidae were strongly related to both distance along the forest remnant and riparian forest structure. Therefore, higher leaf breakdown rates as the stream flowed through the forest remnant were possibly due to increases in abundances of leaf-mining Chironomidae, which responded to variation at both spatial scales studied. These results suggest that forest remnants are important in rural landscapes not only by improving stream water quality but also by restoring ecosystem functions.