Testing the influence of riparian buffer design on stream biodiversity following drought

Abstract: Forestry is a major industry in Sweden and the most common method to harvest timber is to clear-cut large areas. Clear-cutting can alter multiple physical, chemical, and ecological characteristics of headwater streams. To minimize these effects, vegetated riparian ‘buffers zones’ are often spared along streams and lakes. Despite this, there are uncertainties regarding optimal width of buffers to safeguard streams from clearcutting effects. In this study, I ask how riparian buffer width influences stream macroinvertebrate communities, and how this influence may be altered by summer drought. I tested this in 24 headwater streams, half located in northern Sweden and the other half in southern Sweden. Streams in each region included four different buffer width categories (n = 3), including: “no buffer” (no trees left post-harvest), “thin buffer” (< 5 m), “moderate buffer” (> 5 m) and “reference” (no harvest). I analysed a suite of metrics that describe the abundance, richness, and composition of macroinvertebrates, and compared these across streams with different buffer properties. Regionally, southern streams had marginally greater taxonomic richness and relative abundance of sensitive taxa compared to northern counterparts, regardless of buffer conditions. Further, thin and absent buffers performed the best across several macroinvertebrate metrics, particularly for southern streams. Antecedent drought had no observable effects on macroinvertebrate communities, but taxonomic richness across region was positively correlated with stream pH. Overall, my findings, while tentative given low statistical power, suggest that retaining coniferous-dominated buffers may not lead to the desired ecological outcomes in boreal headwaters.