Management of ecological thresholds to re-establish disturbance-maintained herbaceous wetlands of the south-eastern USA.
Abstract
The restoration of disturbance-maintained ecosystems may require management to overcome ecological thresholds and re-establish feedbacks that perpetuate an alternative community. We use hardwood-dominated depression wetlands (locally known as oak domes) embedded in the fire-maintained longleaf pine-wiregrass Pinus palustris-Aristida stricta ecosystem as an example where concepts developed from alternative state theory are applied to practical restoration. As extant communities were not available as reference sites, we based our restoration objectives on knowledge of vegetation dynamics, land-use history and historical data. We quantified a hardwood encroachment pattern beginning with the establishment of central nuclei during fire-free periods. Expansion of this core of hardwoods is positively reinforced by the accumulation of fuels that impede the spread of fire. In order to examine the feasibility of re-establishing herbaceous communities, we selected 10 depression wetlands in 2000 and randomly assigned a hardwood removal treatment to half of them. During the transition period of fine fuel accumulation, we adapted the management regime as necessary for control of hardwood re-sprouts and to promote the development of a fire-maintained community. After 5 years, hardwood removal communities had shifted toward herbaceous dominance, characterized by multi-layered, species-rich, native, wetland-specific ground flora. The rapid recovery of herbaceous species was probably possible because of initial seedling recruitment from a persistent wetland soil seed bank. This immediate recruitment of herbaceous vegetation produced fine fuels, allowing for the reintroduction of frequent prescribed fire and, thus, the re-establishment of the herbaceous community-fire feedback mechanism necessary to maintain the community state. Synthesis and applications. Our findings confirm that it is possible to re-establish a rare alternative community state in a fire-maintained ecosystem. Establishment of a desired transition trajectory required decoupling ecological feedbacks that inhibit reintroduction of fire while facilitating positive feedbacks to promote fire. Our approach incorporating ecological thresholds and biotic legacies, such as a persistent seed bank, can serve as a model to inform restoration strategies for other disturbance-maintained ecosystems.