Results of the experimental work on riparian wetlands have been (and are being) disseminated to the broader scientific community via peer-reviewed publications and presentations at conferences and other (scientific) meetings. In addition, a summary report on the experimental results for a more general audience has been produced (Deliverable 4.4), with a special focus on putting the results in a perspective relevant to riparian wetland management, conservation and restoration.
To mitigate effects of future temperature and precipitation changes on stream riparian wetland hydrology, biology and functioning, or to mitigate any cumulating effects on entire catchments, it is crucial that measures are taken at the relevant sites and the appropriate scales in the landscape. This also applies to any measures aimed at restoration of wetland conditions. Stream riparian wetlands are, by definition, tightly connected hydrologically to streams, which again form a hydrologic network within catchments. This network transports water, nutrients, and possibly pollutants, uni-directionally downstream, but also provides a bi-directional longitudinal corridor for movement of riparian plant and animal species. In addition, riparian wetlands connect the surrounding uplands to the stream by passing runoff and (shallow) groundwater, including nutrients. But also, it connects the stream water body to the upland and as such provides a habitat, and a lateral corridor connecting habitats, for animals and plants. As such, it is inadequate to spatially limit any measures aimed at affecting the stream riparian wetland to only the extent of the wetland: also the upland and the stream, or even the entire catchment need to be considered.
As the stream riparian wetland and its surroundings are connected hydrologically, biogeochemically and biologically (through movements of animals and plants), all these connections need to be considered. This is generally a major challenge. To help do this effectively, the Operational Landscape Unit (OLU) concept has been developed. The aim of the OLU concept (Verhoeven et al. 2008) is to help identify key landscape connections for sound wetland management, in order to identify and apply measures at the right sites and the right spatial scales in the landscape.
Operational Landscape Units (OLUs) are defined as combinations of landscape patches with their hydrogeological and biotic connections, and used as a tool to facilitate wetland restoration in catchments with a high degree of fragmentation and strongly altered hydrology. Criteria for the delineation of OLUs in wetland restoration initiatives include the following key elements: 1) definition of the restoration objectives, 2) identification of spatial landscape mechanisms, and 3) information on historic and present land uses and hydrologic management. Discussions of proposed OLUs with experts and stakeholders (water authorities, nature conservation agencies, farmers) helps to gain shared insights, which lead to improvements of more traditional management plans and, if properly applied, will lead to more natural and more successful restoration. The OLU approach has been developed during a previous EU-funded project (Euro-limpacs). For more information, see Verhoeven et al. (2008).
Mitigation of climate change effects on wetland biogeochemistry, biodiversity and functioning
The results from the experimental manipulations show that riparian geochemistry, plant species composition, beetle species composition and functioning (greenhouse gas emissions, carbon sequestration/decomposition) all rapidly respond to both increased winter flooding and increased summer drought. This implies that changes in all these aspects of riparian wetland functioning can be expected to swiftly follow climate changes. For the management of these wetlands, aimed at the conservation of their biodiversity and functioning, this means that measures to improve ecosystem resilience and flexibility may be crucial.
Firstly, flooding appears to further enhance nutrient availability (especially regarding nitrogen) in the riparian zone and this leads to a risk of loss of plant species and lower species richness. Reduction of stream, soil and groundwater nutrient loading is therefore a first clear step to mitigate against negative impacts of climatic changes on riparian zone biodiversity.
Second, riparian zones with a wider range of habitat conditions and a greater area of each habitat, appear to be more resilient against climate changes, especially sudden shifts and extreme events. Wider and more varied riparian wetlands provide habitat for more species, and (perhaps even more importantly) a reservoir of nearby source populations to colonise newly available, disturbed sites. Heterogeneity within the riparian zone may provide ‘refuges’ for specific species to survive extreme flooding or drought events, and thus increase the system’s resilience. This is achieved by conservation, and where needed restoration, of wide, gradually sloping riparian wetlands bordering streams, with high internal heterogeneity (for example due to longitudinal variation in topography and meandering of the stream). Such riparian zones allow local and regional survival of source populations in refuges during extreme events, and clonal ‘migrations’ of plant species upslope and downslope, following more gradual climatic changes. For the preservation of waterbound beetles, such riparian zones provide maintenance of wet areas in the riparian zone, even during summer drought phases. Thirdly, riparian zones may not only allow upslope and downslope movements of riparian species, but also provide corridors for riparian species shifting ranges following climate change, and as such provide an important pathway for species conservation at larger spatial scales.
Finally, it is important that gradually sloping, wide and (topographically) heterogeneous riparian wetlands are not managed on too small a spatial scale: problems such as nutrient loading should be managed on a catchment (or OLU) scale, and the larger and more continuous the riparian zones, the better their connectivity. While measures taken at large scales are generally costly, they will be more cost-effective. Where climate-driven changes in precipitation are already forcing water boards to change their management of streams and water bodies in relation to safety and water supply, measures to protect riparian zone biodiversity and functioning may be even more cost-efficient.
Restoration of wetland biogeochemistry, biodiversity and functioning
Restoration of degraded local riparian zone hydrology and (biogeo)chemistry is essential to improve riparian wetland biodiversity and functioning, and increase resilience to ongoing global changes. To make these local restoration efforts a success, it is crucial that entire OLUs are managed (and where needed, restored) according to the goals set for the restoration project. Only in this way, stream and riparian habitats can be preserved that have the flexibility and resilience to adapt to, or deal with, climate change without major losses of characteristic species or functioning.
It is important to realize that where major restoration activities take place, these in many cases involve removing riparian zone topsoil and/or relandscaping. This is the case in measures aimed at stream re-meandering, stream channel or floodplain excavation, recreation of naturally sloping riparian zones, or simply the removal of nutrient-enriched topsoil to decrease the nutrient load of the system. In all those cases, restoration of riparian habitat is expected to result in the return of target riparian species. However, these species need to arrive and colonize the site. Earlier studies during a previous EU-funded project (Euro-limpacs) have already indicated that dispersal is a major factor limiting the success of restoration of stream and riparian habitats (Brederveld et al. 2011). Species expected to return to restored sites may not colonize, due to lack of nearby source populations or lack of long-distance dispersal abilities. In highly modified, agricultural landscapes, availability of and variation in source populations have often been strongly reduced. Hence, the sediment species pool of riparian zones in such regions is generally dominated by few, common species sharing very similar functional trait characteristics, with local differences mainly caused by differences in local species pools.
Thus, it is important that restoration projects are carried out near sites with source populations of target communities (again stressing the point of restoring an entire landscape, or OLU, with the populations of the typical species, instead of just isolated sites). Earlier studies (Brederveld et al. 2011) have indicated that restoration of sites directly adjacent to upstream source populations is most effective, as many species (plants and animals) disperse via water and largely passively (thus downstream). With increasing distances, even of a few hundred metres, arrival of seeds from source populations is already rapidly declining. Alternatively, arrival of target species into these sites needs to be assisted. Flooding and sediment deposition, being processes that are expected to be intensified in a future climate, may not suffice to regain diversity in currently species poor riparian areas along lowland streams situated in agricultural landscapes.
Mitigation of climate change effects on catchments
What applies to local riparian wetlands also applies to riparian zones across catchments: to preserve riparian wetland species diversity and functioning, nutrient loading should be reduced, and hydrology and riparian zone heterogeneity should be restored or preserved. Planning of management and restoration activities will only be efficient and effective when done at the OLU scale. While this scale implies a major effort, it is important to realize that measures taken at large scales may appear more costly, but will be more cost-effective.
Now that climate-driven changes in precipitation are already forcing water boards to modify streams, rivers and riparian zones, this opportunity could be used. Clever planning may incorporate measures to protect riparian zone biodiversity and functioning into such restructuring activities (primarily addressing water safety and quality issues), to help render them even more cost-efficient.