The effect of initial seed density on the structure of a desert annual plant community

1 Few experiments directly test the role of density dependence in natural plant communities. 2 We tested (i) whether different initial seed densities of the entire seed bank of an annual plant community affected performance (emergence, mean plant biomass and survival) and (ii) whether density-dependent processes were operating within the communities. We also tested whether life-stage, year and soil moisture (both on a topographic gradient and in an experimental manipulation) influenced the effects of seed density. 3 We considered two distinct phases: seed to emergent seedlings, and emergent seedlings to established plants. 4 The seed bank was collected from a semistabilized sand dune in the Negev Desert, Israel. This was added to sieved sand in plots at the same site at four different initial seed densities: 1/16x, 1/4x, 1x (natural seed density) and 2x. The experiment was repeated for three consecutive growing seasons. 5 Emergence of seedlings was significantly influenced by initial seed density in all 3 years, with higher initial seed densities having lower rates of emergence. 6 Mean final plant size was negatively density dependent and consistently unaffected by the initial seed density sown. 7 In general, there were no strong interactions of topographic position with initial seed density, processes within the vegetation were not density dependent and the experimental addition of water did not influence any of the performance measures tested. Density-dependent processes vary from year to year, while moisture effects do not. 8 We conclude that seedling emergence and some processes in the established plant community are density dependent, but the established plant community is also affected by other processes, such as resource limitation.