Self-thinning and colonization success in ephemeral plant populations under varying resource availability

POGGIO, SANTIAGO L.; MAXWELL, BRUCE D.

Samsun

Congreso; 16th European Weed Research Society Symposium; 2013

European Weed Research Society

Ephemeral plants usually are pioneers occupying open gaps after disturbances, such as many annual weeds and invasive plants in frequently disturbed environments. Colonization success of ephemeral plants commonly results from the balance between initial seed density and resource availability in recently open patches. Thus, understanding self-thinning in short-lived plants may contribute to understand early stages of plant invasions in highly disturbed habitats. Here, we evaluate the impact of density-dependent mortality on the colonization success of ephemeral plant populations in response to varying initial density and resource supply. We used an individual based model that simulates individual plant growth at a one week time step for a short growth cycle (10 weeks) in a 1 m2 plot (100 by 100 cm grid=10,000 cells). Plant growth depended on resource supply and was a function of specific maximum growth rate and efficiency. Resources supply for a plant was influenced by its size, resources used by neighbor plants within a specified neighborhood radius, size and distance to neighbors, and two specific resource use rate parameters for the neighbors. Seeds were randomly distributed in the plot. We used two generic species differing in their potential growth rate. We explored a wide range of scenarios obtained by combining nine initial densities (between 4 and 800 seeds m2) and four resource levels. Weekly, total biomass per plot, mean individual mass, and survivor density were recorded, and mortality rates then calculated. At the tenth week, seed output per plot was obtained by multiplying total biomass of survivors by fecundity (20 seeds per gram for both species). Constant total biomass increased as initial density and resource availability increased. Maximum total biomass was lower and achieved earlier when resources were limiting. Self-thinning became important at high densities and was modified by resource availability. Mortality was lower and started later as resource availability increased. Mortality started earlier and was greater for the species with higher growth rate. In both species, all individuals died at high densities and lower resource supply. Mean individual mass was higher in the species with greater growth potential, albeit growth rate highly decreased at increasing densities due to size-asymmetric competition. When log mean individual mass was related to log survivor density, the thinning slope b was close to -0.5; the expected slope assuming a -3/2 rule. The highest seed output was observed at the highest resource supply. However, seed produced per plot was slightly higher in the species with the lower growth potential. There was an optimum initial density maximizing seed production, which was always higher in the species with the lower growth rate. Interestingly, for a given level of resource supply, a species with lower growth rate may produce more seeds than other with higher growth rate. Our results suggest that pre-emption of a greater share of resource early in the growing cycle may constraint seed production in ephemeral plants, which would reduce colonization success and impede self-perpetuation in highly disturbed environments.