The dynamics of coexistence in annual plants

Abstract

This thesis is about coexistence mechanisms in a guild of seven species of annual plants that inhabit dry, open, compacted ground in Silwood Park, Berkshire. Annual plants were chosen for the study because of the importance of obtaining whole-generation estimates of the vital rates, and because short-lived plants help/hold out the prospect of being able to assess the importance of year-to-year variation in the rates of recruitment, growth, survival and fecundity. The field work was carried out at Pound Hill for the seed bank experiment, at Walled Garden for the seed dispersal experiment and at Ashurst for the main experiment in Silwood Park, Berkshire. The location was chosen because the area was already fenced and beds were laid out for a previous pilot study experiment in 1996. The intention was to have a range of contrasting ecologies within our guild of annual plants. All species were capable of germination in autumn followed by over-winter survival as rosettes, and then rapid growth and early seed set in spring. Some species were immediately precluded from consideration as a result of their thuggish behaviour in the pilot studies. The final choice consisted of a grass (Aira praecox), a legume (Ornithopus perpusillus), three contrasting herbs (Myosotis discolor (Boraginaceae), Cerastium glomeratum (Caryophyllaceae) and Veronica arvensis (Scrophulariaceae)), and two tiny crucifers (Erophila verna and Arabidopsis thaliana). All were locally frequent on gravel paths and other compacted open habitats within Silwood Park. The thesis describes 7 separate replicated experiments, each carried out over 5 years: monocultures of each of the seven species; saturation sowing; two-species mixtures (21 of them) and multispecies mixtures. I show clear pattern of year effects in both inter and intra-specific density dependence. The time series experiments are backed up by quantification of seed dispersal and seed bank dynamics.