Dispersal strategies, spatial heterogeneity and colonization success in fire-managed grasslands

Interactions between fire regime, dispersal strategies and patch structure were examined as key issues for the management of floristic composition of grasslands, through a model that simulates the population dynamics of two competing fire-cued and non-sprouting species. The model describes a heterogeneous environment composed by several patches of grassland, only related by seed dispersal. The last burn date at each patch determines the accumulation level of fuel-biomass provided by a third, dominant species, which in turn controls for the exclusion rate of both colonizer species. The population dynamics of both species was approached following density-dependent models and parameterized for two opposite dispersal strategies: low spatial and high temporal dispersion of seeds (type 1), high spatial and low temporal dispersion of seeds (type 2). Only under the most variable scenarios (when non-synchronous and irregular fire regimes were combined with a proportion of patches (p) with initially depleted seed banks) did the relative success of dispersal strategies vary with the length of the fire-free period. Irrespective of p, smaller interval lengths favored the postburn density of the strategy 1. Strategy 2 was favored over strategy 1 when the fire-free interval increased, such difference being maximum for intermediate p-values. These general tendencies agree with those observed from a reference system: the Flooding Pampa grasslands dominated by Paspalum quadrifarium where short no-fire intervals promote the postburn abundance of a type 1 species (Lotus tenuis) over two type 2 species (Carduus acanthoides and Cirsium vulgare) while for long fire-free intervals the opposite is true. This results suggest that frequency, time since last burn, and burning synchrony are useful components of a fire regime to take advantage of variation in dispersal strategies.