Long-term performance of in-stream restoration measures in boreal streams
Studies on the effectiveness of in-stream restoration have generally reported increased habitat heterogeneity, but biological responses have been more variable. One hypothesis states that the restored habitat structure does not persist through time, resulting in fading biological responses. We studied the durability of in-stream restoration in northern and central Finland by assessing short-term (0-1?years) and long-term (?10?years) changes in habitat structure after restoration. In 2010, we repeated the field surveys first conducted in the 1990s in 27 stream reaches. We also made similar habitat measurements in ten near-pristine sites. Restoration caused significant changes in the stream habitat that either remained unaltered or were reinforced through time, with several of the restored sites resembling closely the near-pristine reference sites 10-20?years post-restoration. Cover of aquatic mosses initially decreased sharply but recovered close to near-pristine level within about 15?years. However, substrate variability still remained somewhat lower in the restored than in near-pristine streams. Individual restoration structures had changed little over time. The most evident failures were the disappearance and entrenchment of gravel beds. Restoration of our study sites had shifted the sites to a trajectory towards more natural habitat conditions, and our results do not support the hypothesis that gradual destruction of the restored habitat might partly explain weak biological responses. From the perspective of channel evolution, the restorations were still fairly recent, and only long-term monitoring will tell if the sites will continue on the trajectory to hydromorphological and ecological recovery. Copyright © 2015 John Wiley & Sons, Ltd.