Salzau 2010 Abstracts February 2010
1: Schleswig-Holstein State Agency for Agriculture, Environment and rural Landscapes
2: Ecology-Centre, Christian-Albrecht’s-University, Kiel
3: Leibniz-Zentrum für Agrarlandschaftsforschung e.V., Müncheberg
4: Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Berlin
Wetlands fulfil several functions for society summarized with the term ecosystem services. In this paper, we discuss the questions if and how the concept of ecosystem services supports wetland restoration and management. Starting point for the discussion is the fact that wetlands are associated with several regulation functions including flood protection, water quality improvement, carbon accumulation, and production functions such as production of raw material and food. The services provided by wetlands were assessed and quantified in several studies ranging from local site scale to earth-wide. We know from these studies that the services provided by wetlands are largely determined by the wetland type and its hydrological setting and that the functions are severe modified by human activities such as land use and water management. However we know also that wetland functions change due to the natural succession of wetlands over longer time scales.
While there exists a wealth of studies about the restoration of wetlands there is still limited knowledge on the restorability of ecosystem functions. In fact it is more an exception than a rule that restoration of the natural functions is successful at least in human time perspectives. We will discuss this topic with four examples. We believe that using and improving ecosystem services requires first a definition which function or service we want to improve and second a sound understanding of the specific processes responsible for regulating the function.
In example 1 we consider using wetlands for water quality improvement. Nutrient retention in wetlands is determined by water and matter flow entering and exiting a wetland. Furthermore it is necessary to distinguish between the different nutrients like nitrogen or phosphorus because their turnover is controlled by different processes and factors.
Nitrogen retention is mainly an effect of the biogeochemical process denitrification and is controlled by nitrate and degradable carbon availability. Denitrification is considered as a process restorable within short time scales; what is needed is to create suitable physico-chemical conditions for the process (Nitrate inflow into an area with anaerobic condition where carbon is delivered by plants or from the soil). Nitrate retention can be further divided on the basis of the nitrate source in groundwater fed wetlands, surface flow wetlands and internal processes. Optimizing nitrate retention needs different approaches for these three cases.
Phosphorus retention especially in riparian wetlands is mainly the result of the physical process sedimentation. Sedimentation occurs when velocity of surface water decreases. Sedimentation is also considered as a process restorable within short time scales. Improving sedimentation in wetlands requires creating physical conditions where the surface area of the wetland is increased and connected to surface water inflow. Optimizing sedimentation rates is only possible in wetlands which are connected to the surface water system. Next to improving sedimentation rates we have to be aware of pollution swapping. The risk of phosphorus mobilization occurs frequently on drained wetland soils used intensively for agriculture. The phosphorus pool will adapt slowly to the newly established hydrochemical conditions in the soils and decrease over time. From lakes it is known, that the mobile phosphorus pool decreases over a time span of at least ten years. So the process of sedimentation is quickly restorable but can be counteracted by phosphorus mobilization depending on the chemical composition of particulate P.
In example 2 the carbon accumulation (sink) function of wetlands is optimized. Carbon accumulation is a quantitatively important process in undisturbed wetlands and especially in mires. A permanently high ground-water level, a low availability at nutrients, and the existence of a peat forming vegetation are prerequisite for it. It is well known that virgin mires are able to form an important carbon stock under such conditions in the long run. These C stocks are dismantled much faster again after drainage (strong C source and high global warming potential). However, there are only few studies which show that it is possible to induce again carbon accumulation at formerly drained peatland in short time spans. Of course one reaches a fast reduction of the CO2-C losses by re-increase in the groundwater level. But it needs apparently some time until the nutrient availability is reduced and a peat forming vegetation develops again. As a result, we may have to cope with an unchanged high global warming potential because of elevated methane emission and also with enhanced fluxes of dissolved organic carbon during the first phase of rewetting. At the moment we cannot predict when rewetted sites will act as a carbon sink and this environmental pollution disappear again. Assessing this function more accurately requires long-term monitoring of peatland restoration projects.
In example 3 the biodiversity value is optimized. Maintaining or improving biodiversity is seen as part of the information function. Peatland restoration is a well known technique in nature conservation management. However, the success of the restoration projects depends mainly on the extent of the peat soil degradation and the vegetation type present before restoration. From vegetation succession we know, that it is much easier to restore eutrophic vegetation types on previously drained and agricultural used sites than less disturbed mesotrophic ones. As just said in example 2, the effect on the reduction of global warming potential should be higher when drained rewetted sites are restored than when wet and mesotrophic sites are restored in the long run. But the later site types have better starting conditions for nature conservation projects aiming on maintaining biodiversity.
In example 4, flooding area should be increased. Wetlands connected to the surface river network may damp the flood wave. Restoring wetlands for flood protection seems to be quickly possible, because this again is also only a physical process. However, the effectiveness of this approach is determined by the size of the wetland area and the size of the upstream catchment. Spatial interactions between the upstream basin and the wetland itself must be considered.
These examples illustrate that wetlands fulfil a variety of ecosystem services; however the services are determined to a large extent by surface and subsurface water flow pattern, the present condition of the soil and vegetation type and the hydrological setting of the wetland. Some processes may have negative side effects. Additionally the conditions for optimizing the functions differ between ecosystem services and are in same cases contrary. A solution for this problem is to assess the services provided from each wetland individually and to take spatial interactions into account. On the wetland scale it should be checked first, if the service is relevant for the wetland type and secondly if and how the service can be improved. This step has to be solved on a mesoscale; because only then it is possible to use these results in spatial planning and to set up priorities for different wetlands based on their specific properties. This approach allows the formulation of realistic and achievable restoration objectives.
From a management perspective we know that wetland restoration in cultural landscape is only possible when land owner and water boards agree to the planned measures. Furthermore, restoration projects should be also communicated to the members of the local community. Thus it is necessary to cooperate with these stakeholders in the decision making process from the beginning. Due to the high costs needed for land purchase it is necessary to start only those projects where feasible environmental objectives have been identified on the site scale and are accepted by the local population.