Leaf-litter breakdown in pasture and deciduous woodland streams: a comparison among three European regions

摘要

1. Human land-use has altered catchments on a large scale in most parts of the world, with one of the most profound changes relevant for streams and rivers being the widespread clearance of woody riparian vegetation to make way for livestock grazing pasture. Increasingly, environmental legislation, such as the EU Water Framework Directive (EU WFD), calls for bioassessment tools that can detect such anthropogenic impacts on ecosystem functioning. 2. We conducted a large-scale field experiment in 30 European streams to quantify leaf-litter breakdown, a key ecosystem process, in streams whose riparian zones and catchments had been cleared for pasture compared with those in native deciduous woodland. The study encompassed a west-east gradient, from Ireland to Switzerland to Romania, with each of the three countries representing a distinct region. We used coarse-mesh and fine-mesh litter bags (10 and 0.5 mm, respectively) to assess total, microbial and, by difference, macroinvertebrate-mediated breakdown. 3. Overall, total breakdown rates did not differ between land-use categories, but in some regions macroinvertebrate-mediated breakdown was higher in deciduous woodland streams, whereas microbial breakdown was higher in pasture streams. This result suggests that overall ecosystem functioning is maintained by compensatory increases in microbial activity in pasture streams. 4. We suggest that simple coefficients of breakdown rates on their own often might not be powerful enough as a bioassessment tool for detecting differences related to land-use such as riparian vegetation removal. However, shifts in the relative contributions to breakdown by microbial decomposers versus invertebrate detritivores, as revealed by the ratios of their associated breakdown rate coefficients, showed clear responses to land-use.