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Succession of riparian plants following dam removal in a boreal stream in central Sweden
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. (Landskapsekologi)
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. (Landskapsekologi)
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. (Landskapsekologi)
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Worldwide there are numerous aging and non-performing dams that may face removal, for economic or other reasons.  Dam removal initiates succession of plant and animal communities as a response to new morphological and hydrological conditions in the channel. We studied the succession of plant communities in riparian reaches upstream and downstream of a recently removed dam in central Sweden over 3 years. We monitored the vegetation development at each site and compared it with a reference site in an unimpacted upstream reach. The two reaches located in the former reservoir developed new riparian zones following dam removal. Plant colonisation in the new riparian zone was fast and species composition became increasingly similar to that of the reference reach. Dam removal largely restored species composition in the riparian zones that were formed in the previous reservoir, indicating that an appropriate species pool was available and that conditions for natural regeneration of riparian vegetation were sufficient.  However, a significant decline in species richness in the downstream reach following dam removal may imply that the upstream and downstream effects of removal may differ and that the removal itself may have disturbed the reach downstream of the former dam. Although remaining timber floating structures and four hydroelectric dams upstream may hamper a more complete vegetation recovery we foresee many years of riparian vegetation development before the successional processes slow down.

Keyword [en]
dam removal, plant colonisation, stream restoration, riparian vegetation, succession, Sweden
National Category
Biological Sciences
Research subject
biology
Identifiers
URN: urn:nbn:se:umu:diva-55566OAI: oai:DiVA.org:umu-55566DiVA: diva2:527467
Note
Submitted manuscriptAvailable from: 2012-05-22 Created: 2012-05-21 Last updated: 2012-05-22Bibliographically approved
In thesis
1. Ecosystem response to dam removal
Open this publication in new window or tab >>Ecosystem response to dam removal
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis aims to improve our understanding of how riverine ecosystems respond to dam removal. Riverine and particularly riparian ecosystems are among the most variable and important features of all landscapes. They connect landscape elements both longitudinally and laterally, and are governed by processes such as flooding, erosion and deposition that create dynamic, diverse and heterogeneous habitats. In fact, riparian zones are among the world’s most species-rich habitats. Worldwide there are millions of dams that fragment stream and river systems, regulate flows and degrade ecosystems. Dams impact freshwater, marine and terrestrial ecosystems and threaten biodiversity by disrupting organism movements and energy flows in the landscape. An important upstream effect of dams is inundation of habitats and development of new shorelines around impounded areas. Effects downstream of dams are mainly caused by changed hydrological regimes and retention of organic and inorganic materials in reservoirs, leading to reduced transport and dispersal of for example seeds to reaches downstream. The removal of dams create expectations that biota will eventually recover. We have studied a number of dam removal projects in Sweden. Our experimental results showed that following dam removal, newly exposed soils in former impoundments were rapidly colonized by pre-removal species. Their species richness increased slightly with time and their species composition indicated a slow change towards that in the reference site. In addition, the vegetation in formerly impounded areas showed a direction of change from lentic riparian plants (high proportion of aquatics) towards lotic ones, consisting of native perennials typical of free-flowing streams. We also found that the apprehensions that former impoundments would turn into pools of mud did not come true; in fact, a process towards more pristine channel morphology was observed. After removal there was erosion and downstream transport of sediment. We found only minor effects on macroinvertebrate communities. For example, a few species decreased over the years, suggesting that dam removal in itself might cause a temporary disturbance. This highlights the importance of long-term studies after dam removal, and also the importance of comparisons with pre-removal conditions and stretches unaffected by dams. Thorough documentation of executed dam removal projects and distribution of the results and experiences are tremendously important in the planning process of future decommissioning projects. Also, our experiences have taught us that in order to attain a successful dam removal it is important to involve stakeholders such as non-governmental organizations and local inhabitants in the process.

Place, publisher, year, edition, pages
Umeå: Umeå universitet, 2012. 26 p.
Keyword
dam removal, macroinvertebrates, plant species, plant succession, restoration, riparian zone, sediment deposition, species richness, Sweden
National Category
Natural Sciences
Research subject
biology; Ecological Botany; Hydrology
Identifiers
urn:nbn:se:umu:diva-55586 (URN)978-91-7459-367-9 (ISBN)
Public defence
2012-06-15, Uminova Science Park, Tvistevägen 48, Älgsalen, Umeå universitet, Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2012-05-25 Created: 2012-05-22 Last updated: 2012-05-22Bibliographically approved

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Lejon, AnnaMalm Renöfält, BirgittaNilsson, Christer
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