Disturbance contributes to niche-based community assembly by imposing an ecological filter that reduces population sizes or causes local extinctions of resident species, while simultaneously enhancing post-disturbance recruitment from the regional species pool. Disturbance-mediated community assembly may also depend on the history of resource availability in local communities, especially under resource-limited conditions that limit individual growth, population sizes, and species’ tolerances to disturbance. Here, we test the hypothesis that the effects of local fire disturbance on plant community assembly are mediated by pre-disturbance resource availability in a fire-frequented, species-rich longleaf pine savanna (30-40 species/m2). In a factorial field experiment, we manipulated soil moisture using rain shelters and irrigation (mimicking drought and high-rainfall conditions, respectively) and seed arrival by adding seeds of 30 species from the local species pool. After two growing seasons, we manipulated fuel loads to increase local fire intensity. We tested the prediction that high-intensity fire would have a stronger negative effect on species richness and abundances of seed-addition species in communities assembled under drought conditions relative to communities with higher soil moisture.
Results/Conclusions
Locally intense fire reduced abundances of seed-addition species and total species richness, suggesting an important ecological filter in community assembly. Moreover, the effect of high-intensity fire differed among communities assembled under contrasting soil moisture, but in a direction opposite to our predictions. Under high soil moisture, species richness was reduced by 50% following high-intensity fire relative to low-intensity fire. Under low soil moisture, in contrast, species richness was only reduced by 25%. At the community level, this resource-disturbance interaction may have reflected increased physiological stress and seed mortality of some species under saturated soil conditions. At the population level, low soil moisture reduced pre-fire abundances of the 5 most-common seed-addition species, suggesting that drought limited recruitment from the species pool. Post-fire abundances were positively correlated with pre-fire abundances following low-intensity fire, but uncorrelated with pre-fire abundances following high-intensity fire. This suggests that high-intensity fire may decouple effects of soil moisture on local abundances. Our results provide insights into how resource availability and disturbance interact to influence the dynamics of species-rich plant communities and have broader implications for biodiversity conservation in the face of changing climate (e.g., increased droughts and extreme rainfall) and disturbance (increased fire intensity) regimes.
