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Changes in nitrogen cycling in riparian zones along a chronosequence of restored streams in northern Sweden
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. (Landscape Ecology)
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Understanding how stream restoration affects nitrogen (N) cycling in riparian zones is crucial for setting realistic performance criteria for restored streams. Most streams in northern Sweden were channelized for timber floating, and many streams have now been restored. Channelization disconnected streams from the riparian zone, and reduced the flooding that creates anoxic conditions necessary for many N-cycling reactions. We used a space-for-time substitution consisting of stream reaches restored 2 to 25 years ago, unrestored channelized and natural reference reaches to determine how N-cycling in riparian zones changes with time after restoration. Using stable isotopes of N (δ15N), we found that restoration caused more enriched foliar and root δ15N in recently restored sites, suggesting more gaseous losses of N in younger sites. This enrichment in foliar and root δ15N decreased over the 25-year chronosequence suggesting that the N-cycle becomes tighter and loses less N as it ages. Although the [N] in foliage, roots, and soils did not change over time, understory biomass decreased over time, suggesting that more N was available to plants in younger compared to older sites. Changes in the mechanism of N acquisition (i.e., mycorrhizal colonization, as shown by Δδ15N), plant species richness, and cover of deciduous trees (carbon source), were the most important factors explaining variation in δ15N along with time after restoration. It is clear that the restoration of these streams causes a large and rapid change in nitrogen processing in the riparian zone and this alteration persists for at least 25 years.

Keyword [en]
boreal, chronosequence, nitrogen cycling, stream restoration, space-for-time substitution, stable isotopes, succession, time gradient
National Category
URN: urn:nbn:se:umu:diva-108075OAI: diva2:850988
Swedish Research Council Formas
Available from: 2015-09-03 Created: 2015-09-03 Last updated: 2015-09-03
In thesis
1. Gradients of time and complexity: understanding how riparian and instream ecosystems recover after stream restoration
Open this publication in new window or tab >>Gradients of time and complexity: understanding how riparian and instream ecosystems recover after stream restoration
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Why evaluations of the ecological outcomes of stream and river restoration have largely reported inconclusive or negative results has been the subject of much debate over the last decade or more. Understanding the reasons behind the lack of positive results is important for bettering future restoration efforts and setting realistic expectations for restoration outcomes. This thesis explores possible explanations for why researchers have failed to find clear and predictable biotic responses to stream restoration: recovery time has been too short, that restoration of habitat complexity is not clearly linked to instream biodiversity, that one monitored organism group is not representative of the entire community, that restoration effort was not intense enough to restore the potential habitat complexity of a system, and that reach-scale restoration done in the presence of catchment-scale degradation obscures restoration results. The overarching goal of this thesis is to study the holistic effect of reach-scale restoration of historic reach-scale simplification, due to timber floating in northern Swedish streams, thus avoiding the added pressure of catchment-scale degradation typically found at most restoration sites (e.g., non-point-source pollution and impervious cover). Using this model system, I was able to show that it took 25 years for riparian plant species richness at restored sites to increase above that of channelized sites. Furthermore, it was clear that restoration of these streams caused a large and rapid change in N-processing in the riparian zone and this alteration persists for at least 25 years. Additionally, multiple metrics of geomorphic complexity were needed to explain some of the more subtle responses of organism groups. Macroinvertebrates, diatoms, and macrophytes did not respond concordantly and cannot serve as surrogates or indicators for each other. I found that older best practice methods of restoration rarely restored the large-scale features needed to bring the sites up to their potential complexity because these elements were destroyed or removed from the system. Advanced restoration techniques used in more recent restorations added big boulders and instream wood and increased complexity to a level that elicited a biological response. By combining surveys of multiple metrics of structure, diversity of multiple organism groups, and process in this thesis I was able to get a holistic view of the effects of restoration of streams after timber floating. We now know that it takes at least 25 years for riparian plants and N-cycling to recover, we understand that multiple metrics of geomorphic complexity should be measured to be able to explain biotic responses, and that restored complexity should better match the potential complexity of the site in order to elicit a biological response. Finally, we know that multiple organism groups need to be assessed when evaluating the response of biodiversity to restoration.

Place, publisher, year, edition, pages
Umeå: Umeå University, 2015. 36 p.
bioassessment, biodiversity, boreal, bryophyte, chronosequence, complexity, diatom, geomorphology, habitat heterogeneity, hydromorphological, macroinvertebrate, macrophyte, nitrogen cycling, river restoration, riparian buffer, stable isotopes, succession, Sweden
National Category
Research subject
biology, Environmental Science
urn:nbn:se:umu:diva-108079 (URN)978-91-7601-302-1 (ISBN)
Public defence
2015-09-25, Älgsalen, Uminova Science Park, Umeå, 09:00 (English)
Available from: 2015-09-04 Created: 2015-09-03 Last updated: 2015-09-03Bibliographically approved

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