This study investigates the importance of species diversity for cool-temperate grasslands. Although experimentally limited to this type of vegetation, it is designed to identify general principles which are also relevant to other terrestrial ecosystems. The main objectives are:
1. Characterise the relationship between species diversity and elementary processes in ecosystems (carbon, nitrogen and water fluxes)
2. Estimate the effect of species diversity on the stability, resilience and resistance of vegetation exposed to perturbation.
3. Assess the impact of atmospheric and climate change on species diversity (long-term effects of elevated CO2 concentration and/or increased air temperature).
4. Develop instruments to support policy makers in questions of land use and land management (target fields: agriculture, nature conservation, planning and development).
With respect to objective (1), processes within the autotrophic component of ecosystems are tested for their sensitivity to diversity, and hypotheses on the nature of this relationship are tested. These linkages will be used (3) to interpret observed changes in species diversity in vegetation exposed to simulated climate change. To experimentally create a gradient of diversity, species mixtures of controlled composition are used, which allows to separate effects of species richness from effects of species identity. Different functional groups (perennial grasses, N-fixing legumes and non N-fixing dicots) are introduced into these ecosystems to discriminate between diversity within and between functional groups.
The experiments on stability (2) include representative types of disturbances for grassland (cutting to simulate mowing, heat and drought). Effects of CO2 and climate-warming are tested by exposing stands for entire growing seasons. Apart from classical exposure, a newly developed method is used to simulate climate change, called Free Air Temperature Increase (FATI), based on heating of the stand with an electronically controlled infrared irradiation system. FATI will also be used to apply heat and drought disturbance, using rain shields to prevent interference of natural rainfall. Data analysis is supported by the use and development of models on ecosystem functioning and population behaviour.
For policy-supporting purposes (4), different indices will be developed: a) indices which describe the ‘functional status' of vegetation, i.e. the degree of deviation from optimal functioning due to lack of diversity, and b) indices for the stability of vegetation exposed to disturbance. These indices will be used to assess the influence of management practices on species diversity, as a means to control functioning and stability. In conjunction with this, a long-term data set on species composition in grasslands (decade scale, from 1963 till present) will be analysed to examine the feasibility to manipulate diversity by mineral fertilisation and the mowing regime.