Herbaceous plant strategies in disturbed habitats

A systematic theoretical evaluation has been made of three important plant life history traits: adult longevity, seed longevity and seed mass, where s eed mass is interpreted as being indicative of dispersal distance and seedl ing vigour. This model study examined the role of these three traits in rel ation to environmental disturbance. We chose temperate grasslands, widespre ad in north Western Europe and northern and eastern America, as our referen ce system for our simulations. Eight plant strategies were defined by allow ing two levels in each of the three and combining them in all eight possibl e ways. A simple, spatially explicit model was developed to simulate compet ition among individuals with these eight trait combinations at different le vels of disturbance. Simulation results were compared with the actual occurrence over a disturba nce gradient of species with similar plant trait combinations in a large da tabase from the Sheffield area (UK). This showed that with increasing distu rbance level, non-dormant perennials, dormant perennials, non-dormant annua ls and dormant annuals, respectively, became dominant but only if small-see ded, indicating the relative viability of these particular strategies with respect to disturbance. A new prediction from the model was that stable coexistence occurs between plant strategies with dormant and with non-dormant seeds over a range of le vels of disturbance. Plant strategies with large seeds were inferior to sma ll-seeded ones if competitive ability of seedlings is proportional to seed weight. This difference was highest at low seed densities and low germinati on probabilities, indicating that large-seeded species secure no advantage from being dormant (i.e. having a low germination probability). Finally, th e results indicated that dormancy is superior to dispersal as a method of c oping with disturbance.