Changes in plant communities and nitrogen cycling in riparian zones along a chronosequence of restored streams in northern Sweden
Recent studies have reported a general lack of ecological responses in stream restoration projects and this is usually attributed to insufficient time for recovery. In an effort to examine this issue over longer timescales, the work presented here assessed the recovery time of plant species richness, understory productivity, and nitrogen (N) cycling in riparian zones along a chronosequence of restored streams within the Vindel River catchment in northern Sweden restored 2 to 25 years ago. The main channel and most tributary streams were physically modified before 1950 to accommodate timber floating, which ceased in 1976. Streams were channelized by moving instream boulders into riparian zones. Since the 1980s, streams have slowly been restored by removing boulders from the riparian zones and replacing them back into the stream. Restoration efforts and follow-up studies have primarily focused on instream measures targeted at fish, with little attention given to riparian zones. We inventoried the riparian zone for all vascular plant species, quantified cover and biomass in plots, and sampled soils, fine roots, and foliage of a perennial forb species occurring at all sites, Filipendula ulmaria, for changes in the stable isotope of N (δ15N).
We found that while species richness increased with time, understory biomass decreased. Forbs were the majority of the species added, while the biomass of graminoids decreased over time. The plant community composition of restored reaches differed from that of channelized references, with older restoration sites having converged on a unique climax community. Highly enriched foliage and root δ15N values in newly restored streams suggest that N cycling likely increased in the newly flooded riparian zone. As time progressed, δ15N values became more depleted and N isotopes more closely resembled the nearby N-limited boreal reference conditions. Foliar δ15N values were negatively correlated with plant species richness suggesting a potential relationship between niche portioning and N cycling. Given our results, timelines for achieving riparian plant species richness objectives in boreal zones should be extended to 25 years or longer. Furthermore, consideration should be given to actively restoring riparian zones in boreal systems to increase the speed of recovery of riparian plant communities as well as close the N cycle more quickly.