Intra-specific density dependence is required to maintain species diversity in spatio-temporal forest simulations with reproduction

Including explicitly the link between parent trees and recruitment is indispensable for many forest model applications, e.g. for simulating tree species migrations. However this positive feedback can result in unrealistic dominance of single species. We examine in which way this feedback can be modelled to maintain species diversity and species composition similar to observed forests. A submodel of the spatio-temporal forest model TreeMig for simulating seed production, seed dispersal, seed bank dynamics and sapling development is presented. With the extended model, interactions of different factors that can influence species diversity in a forest are examined. In simulations under different bioclimatic conditions of Switzerland differential hypotheses for maintaining species diversity are tested. The simulated species diversity and dominant species are compared to data compiled from the Swiss National Forest Inventory. Intra-specific density regulation is necessary to produce in the simulations a species diversity and species composition similar to data. Without this intra-specific density dependence, strong seed producers, such as birch (Betula pendula) dominate the stand, in contrast to the data, and decrease species diversity. With intra-specific density regulation, species diversity is further increased by spatial or temporal variability. Temporal variability in environmental factors alone can enhance species diversity, if the factors fluctuate between the favourable and unfavourable range of the strong seed producers.