Differential responses by stream and riparian biodiversity to in-stream restoration of forestry-impacted streams.
Forestry can have detrimental impacts on stream ecosystems, particularly via excessive sedimentation. A key challenge to stream management is therefore to identify the best restoration practices to mitigate the harmful impacts of fine sediments on stream biodiversity and ecosystem functioning. We studied the effects of restoration of sediment-impacted headwater streams on the habitat structure, hydrologic retention, biodiversity (microbes, bryophytes, benthic macroinvertebrates, riparian plants) and ecosystem functions (periphyton accrual rate and leaf breakdown) by comparing four treatments: wood-restored, boulder-restored, impacted (by fine sediments) and near-natural streams. The restored streams were sampled 3-7 years post-restoration. Restoration by wooden or boulder structures aimed to reduce deposited sediments and increase channel heterogeneity and hydraulic retention. Wooden structures were ineffective in removing fine bed sediments and did not induce positive responses in aquatic biota. Boulder additions reduced substrate limitation and thereby proved beneficial for aquatic bryophytes. Benthic macroinvertebrates were clearly impaired by sedimentation but responded weakly to restoration. Leaf-decomposing microbes and ecosystem functions were unresponsive to restoration but neither did they differ between near-natural and impacted streams, suggesting that they were little harmed by sedimentation. Wood addition enhanced hydraulic retention, and riparian plant assemblages along wood-restored streams resembled those in near-natural streams, suggesting that increased retention re-established a more natural flood regime. By contrast, riparian plant assemblages in boulder-restored streams did not differ from those in impacted streams. Synthesis and applications. Restoration improved several aspects of stream and/or riparian biodiversity, but had limited effects on ecosystem functions. Different restoration measures resulted in differing biodiversity outcomes: boulder addition was more effective at restoring in-stream heterogeneity and aquatic biodiversity, whereas wooden structures helped restore channel hydrology and retentiveness, and, consequently, riparian vegetation. Therefore applying both measures in the restoration of forested headwater streams with naturally stony substrates enhances stream habitat variability at the watershed scale, providing the most promising scenario for biodiversity benefits in broad-scale restoration designs. In-stream restoration that increases hydraulic retention has impacts that extend beyond ecosystem boundaries, reinforcing the need to restore, manage and protect streams and their riparian forests in an integrated effort.