Raum-zeitliche Artenmuster entlang von Stressgradienten - ein Simulationsmodell in Anlehnung an die Pflanzensukzession auf den trockengefallenen Aralseeböden

Table of Contents: A typical phenomenon along environmental, i.e. abiotic, gradients is an abrupt change in species composition (zonation). At the Aral Sea, where plants are colonising the vast exposed seabeds, the abiotic conditions along a gradient that follows the receding sea water are characterised by an increase in salt and inundation stress. Motivated by this situation, the general spatio-temporal vegetation dynamics and species diversity patterns along environmental gradients have been investigated with a grid-based simulation model. The main attributes characterizing a plant type in the model are the maximum seed dispersal distance, stress tolerance, seed production and longevity. The study mainly focused on two questions: 1) which conditions favour coexistence of the different plant types, and 2) do the different plant types overlap in their spatial distribution or do they segregate along the stress gradient. The simulation experiments revealed that not only the interplay of dispersal and stress tolerance attributes of the plant types, together with the abiotic stress gradients, determine the successional dynamics. In addition, intra- and interspecific competition decisively govern the spatio-temporal vegetation dynamics. Furthermore the simulations showed that successions depend on stochastic establishment events and do not end in a distinct final state. This underlines the importance of the consideration of stochastic processes in the modelling of successions. Finally 13 main results are summarized: 1. The spatial pattern of species richness at the desiccated sea floor of the Aral Sea comprises two main zones. The first zone on the older areas has a higher species diversity than the second zone, the areas desiccated from the mid eighties onwards. This phenomenon motivated the theoretical investigations of spatio-temporal patterns along stress gradients. 2. A detailed literature review on community models revealed that spatially explicit, grid and rule based models are excellent tools to investigate spatio-temporal species diversity patterns. 3. With a Partial Differential Equation model after DeAngelis and Post (1991), the conditions for coexistence and zonation were investigated for a two species system along an environmental gradient. In this deterministic model, the two species only separated into zones along the gradient under strong interspecific competition. 4. A spatially explicit, rule based simulation model for up to 8 different plant types was developed. The central process of vegetation dynamics is the lottery competition of seedlings for space. 5. Simulations identified stress tolerance as the most important plant attribute for the successful establishment of an additional test type. 6. The number of coexisting plant types increases with the strength of intraspecific competition, whereas spatial segregation of different types only occurred with weak intraspecific competition. 7. The use of a global seed pool instead of spatially explicit seed dispersal reduced the number of coexisting types for all levels of intraspecific competition. The number of persisting species is increased by spatial aggregation or segregation of species along the gradient. 8. Compared to a linear stress gradient, both a random spatial distribution of stress intensity as well as the homogenous distribution of stress intensity reduced the species diversity. 9. A stepwise stress gradient leads to a zonation of different types. For strong intraspecific competition, the model results correspond to the empirical pattern substantially (spatial zonation in a species-rich and a species-poor area). 10. A trade-off between stress tolerance and competition strength increased diversity and stabilized the spatial pattern of zonation against an increase of intraspecific competition. Without such a trade-off, an increase in intraspecific competition resulted in an overlapping spatial distribution of the plant types. 11. When introducing additional types and investigating their potential to establish in this community, seed production for an annual test type and longevity for an perennial test type have been identified as the most important plant attributes for the successful establishment in the trade-off model. 12. An additional trade-off between fecundity and longevity, where the costs of reproduction limit the longevity of a plant type, increased the diversity only in combination with the trade-off between stress tolerance and competition strength. Otherwise the most stress tolerant annual type has the potential to become a dominant super species. 13. For replicates with the same parameterisation and initial conditions, different final plant type compositions are possible for the majority of all simulation experiments, i.e. successions do not inevitable end in a distinct final climax state.