Ecologists have debated the potential for ecological communities to exhibit multiple alternative stable states. Degraded plant communities may exist as alternative community states stabilized by feedback mechanisms maintaining the degraded community. These degraded communities may exhibit other characteristics of alternative states such as hysteresis, where the history of change means that the community cannot be restored directly by a return along the same pathway that caused degradation, or by following inherent successional pathways that resulted in the original state. While some studies have identified factors that contribute to degradation of natural communities (e.g. novel disturbances or exotic species invasions), a precise understanding of the self-reinforcing feedbacks that maintain the community in the degraded alternative state is more difficult to pinpoint. Guánica dry forest, in Puerto Rico, contains two alternate states: forest and grass land communities dominated by invasive African grass species. Fire is occasionally present in the grass communities. Fire is a novel disturbance in the system because there are no natural ignition sources. Our objective is to document characteristics and mechanisms within each community that produce stability or lead to conversion from one state to the other. We analyzed species characteristics, vegetation structure, litter structure, and soil nutrient pools.
Results/Conclusions
In mature forest, litter structure does not support fire spread and combined with shade impedes invasion by exotic grass species. In contrast, grasses support wildfires ignited by humans. Fires eliminate native tree species. Only about 5% of stems in native forest have sufficiently thick bark to withstand the heat from low intensity grass fires. Almost no stems <5.0 cm DBH had thick enough bark to protect them from fire. Grass communities that frequently burn have lower levels of soil moisture, N, S, and C, and higher levels of Fe, Mg and sunlight at ground level than do adjacent forest. However, a single fire can convert mature dry forest to a stable grassland state. In 2 locations, ~30 years after a single fire only 2% of original stem density was found in the now grass-dominated community. Overcoming hysteresis may be mediated by using naturalized N-fixing tree species that can rebuild soil nutrients and shade out grasses. In 30 year old secondary forests dominated by Leucaenca leucocephala, native tree species comprised ~75% of the total importance value of the sapling community. Our results support the existence of alternate stable states in our system and indicate pathways that may allow restoration toward the original state.