Authors: Marker, Jeffery; Eckstein, Lutz;Bergman, Eva;Lafage, Denis;

It is commonly assumed that forested riparian buffers protect and enhance aquatic systems through nutrient filtration, erosion reduction, biodiversity refuge, and habitat connection. However, the effectiveness of buffers around aquatic systems is dependent on many factors including size, condition, and upland pressures. Further, buffer effectiveness is rarely quantified through functional diversity measurements of animal and plant species within the system. We calculated functional richness, functional redundancy, and α-diversity of spiders and plants along 15 streams in southern Sweden using a variety of traits driven by environmental variables and forestry practice. We investigated the effects of buffer size, distance from streams, and abiotic variables regulated by each, against functional measures for spider and plant communities using structural equation models. SEMs showed buffers up to 40 m had higher spider α-diversity, driving spider functional richness. Plant functional richness was driven by α-diversity, buffer density, and canopy closure but had a negative effect on redundancy. To maintain high functional diversity in buffered riparian systems, forested buffers should be large enough to support high species diversity. However, fully forested systems showed lower overall species diversity and may benefit from selective thinning to regulate buffer density and canopy closure to enhance riparian function.