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Forest development scenarios for the Hessian Ried
Project
Project code: keine Angabe
Contract period: 01.03.2008
- 31.12.2011
Purpose of research: Applied research
The aim of the joint project was to model the effects of the changing environmental conditions (climate change, groundwater condition) on the productivity and properties of forests as well as on the range of management options available to the forest enterprise in the Hessian Ried for a 30-year prediction period. With a compilation of different models, a spatial database for knowledge and decision making was developed to assist politicians, forest managers, and forest owners to re-assess the forest functions and community expectations of the forest in the area. Furthermore it would assist the development of avoidance or adaptation strategies and the introduction of specific measures for the rehabilitation of the areas of severely degraded forest.
The research area is bounded by the City of Frankfurt to the north, the highway B3 to the east and the state borders to the south and west. The forest area covers about 30 000 ha, comprising approximately 51 % state forest, 44 % communal forests, and about 5 % private and national forest. In the public forests, the predominant tree species are oak (26 %), European beech (31 %) and Scots pine (39 %). Much of area serves water catchment protection, nature conservation, climate protection and recreation functions. The Hessian Ried is characterised by forest sites with low productivity and the current climate is typically dry and warm.
The combined effects of the reduction in the water table in the mid-1960s, the occurrence of biotic and abiotic damage and atmospheric inputs have led to severe destabilisation of the forest ecosystems, imposing tight constraints on the silvicultural options available to the forest enterprises. The research approach adopted was very much a model-based one. A spatial, multi-scale area model was used. The model was supported by a GIS database which managed data collated on land use types, site characteristics, local climate, water management, stand data, the legal protection status (water, forest, and nature conservation protection areas), plant species surveys and forest condition. Under consideration of the protection regulations, forest management options were devised in a rules-based system. The current status of the Hessian Ried provided the basis for the forest development scenarios up until 2039. In the scenarios, the following assumptions were made.
Groundwater
- Status quo (GWO): the groundwater levels are set to the levels specified in the groundwater management plan for the Hessian Ried (RP DARMSTADT
1999).
- Further reduction in the groundwater table (GW-): the groundwater table falls to a level below that which can be reached by tree roots in all areas.
- Rise in the groundwater table (GW+): the groundwater levels are raised to the level prevalent before commencement of industrial groundwater extraction.
Climate
- Reference (Ref): unchanged climatic conditions based on the climatological standard normals between 1961 and 1990.
- Wet climate scenario (A1Bf): 20 of the wettest regional climate scenarios A1B run in the statistical model WETTREG.
- Dry climate scenario (A1Bt): 20 of the driest regional climate scenarios A1B run in the statistical model WETTREG.
By linking the GIS, the water budget model and the forest growth simulator and by incorporating expert knowledge, the module approach enabled both the rulesbased implementation of the different silvicultural development scenarios, their modification, flexible development, use and assessment as well as a combination of the variants in the area. WaldPlaner, the silvicultural decision support system developed by the Northwest German Forest Research Station, was the central prediction tool in this process. It made it possible to combine the following models, some of which are rule-based, into an entire simulation system:
- Groundwater levels
- Atmospheric nitrogen inputs
- Water budget
- Site productivity
- General mortality model
- Cockchafer mortality model
- Forest growth model (TreeGrOSS)
- Conservation regulations/conservation prescriptions
- Silvicultural rules/silvicultural strategies
In this way it was possible, not only to model the forest development from the mid-1960s up to the year 2007 and analyse current status, but also to simulate and quantify successfully the effects of various groundwater and climatic conditions up to the year 2039.
The results confirm that the Rhine-Main conurbation is one of the regions in Central Europe in which forest management faces major challenges. Land acquisition, forest fragmentation, deposition from the atmosphere, increasing demand for water and pressure for recreational areas destabilize the forests. The abiotic impacts, in particular drought stress and a marked lowering of the groundwater table since the beginning of the 1970s, have gradually weakened large areas of the forest ecosystems to the extent that massive biological damage, predominantly from cockchafer and Scots pine mistletoe infestations, have also affected the forests such that forest dieback is now evident. A regular forest management is no longer possible in many locations. The economic losses of enterprises in the last 40 years amount, for conservative estimates, to about 70 million Euros.
The ecological conditions will deteriorate further in the future due to climate change. The actual evaporation will increase, the groundwater discharge will decrease, and the infiltration needed to maintain the groundwater table at the same level will increase. The simulated groundwater scenarios (further drop [GW-], rise [GW+]) lead to a halving or doubling of the areas assured of access to groundwater respectively. Today, according to the groundwater management plan for the Hessian Ried, this area still amounts to 18%. (Reference [GW0]). Most of the sites without access to groundwater (> 60 % of the area) will be very sensitive to climate change. The water supply will worsen and drought stress will increase in the forest stands. As a result, the silvicultural development options will become increasingly limited. The silvicultural measures must focus on the stabilisation of existing stands, the reduction or distribution of risks and the conversion of the stands that neither today nor tomorrow are suitable for the site.
To stabilise stands and distribute the risks, graduated thinnings, the promotion of mixed species, increased number of future crop trees, variable target diameters in relation to vitality, quality and risk of devaluation and sound forest protection are particularly important measures. For site-specific forest conversion, which takes the given site conditions into consideration, the forest management now have access to site-specific information through the site data collated in the project and the newly developed matrices for forest development types (WET). This information will facilitate an assessment of current or future site suitability of the existing stands and the selection of tree species for regeneration that are appropriate for the site. The planting of Scots pine, Maple, Ash, and Douglas fir promise relatively stable economic yields even if the dry climate scenario eventuates.
The success of regeneration measures over much of the Hessian Ried area depends largely on the local cockchafer populations. In the infestation areas, an effective reduction in the larvae density is urgently needed from an ecological and economic perspective. In view of the serious ecological changes, static approaches for the forest rehabilitation in the Hessian Ried would not achieve the desired objectives. While forests will also exist there in the future, they will have a different composition and structure over much of the area, and will no longer be able to fulfil the same functions at the same quality. In particular, given the unfavourable outlook for oak and for beech forest communities, a review of the conservation situation of these forest communities is required. As part of this project, the identified hotspots of biodiversity will become the priority areas for the maintenance of oak forest.
The project provides politicians, forest managers and forest owners with a spatial knowledge and decision tool for assessing the forest functions and community expectations of forests in this area, for developing avoidance or adaptations strategies and for introducing specific measures for the rehabilitation of the already severely degraded forest areas.
For the documentation of vitality development and the provision of reliable planning data, a management category “Hessian Ried” for the forests managed or supervised by HESSEN-FORST should be created and the natural resources of these forests should be monitored via a permanent systematic sampling.
Conclusion:
In the Hessian Ried, the forests and, with them, sustainable, multipurpose forestry are in danger of failing. It is high time the multiple demands on forest enterprises were brought into better agreement with the owners interests. Here, the many objectives need to be reappraised, restructured into a hierarchical system, and assessed in terms of their spatial importance. To ensure the forest functions, decisions which have been postponed in the past must be made soon. The possible rehabilitation measures need to be evaluated in terms of their effectiveness and overall costs.
Keywords: Hessian Ried, groundwater, climate change, forest development scenarios, water budget, nutrient budget, silviculture, forest protection, nature conservation, economic assessment
Section overview
Subjects
- Forestry
