Stream biodiversity and function benefit from forest fragments in agricultural landscapes

Riparian forests buffer stream ecosystems from intensive land use. However, with ongoing land-use changes, riparian forests often remain as isolated fragments. Despite this, the relevance of isolated forest fragments in deforested landscapes remains unclear. We evaluated how downstream forest fragments can buffer the effects of upstream deforestation and whether upstream forest fragments can mitigate the impacts of downstream deforestation on abiotic conditions, biodiversity, and stream functioning. Additionally, we assessed how subsidized leaf litter of varied quality and diversity influences leaf-associated invertebrate assemblages and leaf litter breakdown rates. We conducted leaf breakdown experiments in eight streams within agricultural catchments-four flowing from an upstream forest fragment to a deforested reach, and four from a deforested area to a downstream forest fragment. Downstream forest fragments reduced the effects of upstream deforestation, as evidenced by higher shredder diversity and lower scraper abundance in these reaches than in the upstream deforested reaches. Upstream forest fragments also mitigated downstream deforestation by reducing sediment load, moderating water temperatures, and enhancing substrate diversity. The taxonomic similarity between the forest upstream and the deforested downstream reaches highlighted the role of forest fragments as sources of invertebrates and organic matter for deforested reaches. In forest upstream reaches, invertebrates increased the breakdown rates of C-labile fast-decomposing leaf species, whereas leaf diversity accelerated the processing of nitrogen-fixing slow-decomposing leaves. These results demonstrate that the presence of forest fragments in agricultural catchments benefits water quality, invertebrate communities, and leaf litter breakdown, thereby contributing to the resilience of stream ecosystems to land-use changes.