Modeling silviculture after natural disturbance to sustain biodiversity in the longleaf pine (Pinus palustris) ecosystem: balancing complexity and implementation

摘要

Modeling silviculture after natural disturbance to maintain biodiversity is a popular concept, yet its application remains elusive. We discuss difficulties inherent to this idea, and suggest approaches to facilitate implementation, using longleaf pine (Pinus palustris) as an example. Natural disturbance regimes are spatially and temporally variable. Variability leads to a range of structural outcomes, or results in different pathways leading to similar structures. In longleaf pine, lightning, hurricanes, surface fires, and windthrow all lead to similar structures, but at different rates. Consequently, a manager can select among various natural disturbance patterns when searching for an appropriate silvicultural model. This facilitates management by providing flexibility to meet a range of objectives. The Outcomes of natural disturbances are inherently different from those of silviculture, for example, harvesting always removes boles. It is instructive to think of silvicultural disturbances along a gradient in structural outcomes, reflecting degree of disparity With natural disturbance. In longleaf pine this might involve managing for two-cohort structure, instead of multi-cohort structure characteristic of old growth stands. While two-cohort structure is a simplification over the old growth condition, it is an improvement over single-cohort management. Reducing structural disparity between managed and unmanaged forests is key to sustaining biodiversity because of linkages that exist between structural elements, forest biota, and ecosystem processes. Finally, interactions of frequency, severity, intensity, seasonality, and spatial pattern define a disturbance regime, These components may not have equal weight in affecting biodiversity. Some are easier to emulate with silviculture than are others. For instance, ecologists consider growing-season fire more reflective of the natural fire regime in longleaf pine and critical for maintenance of biodiversity. However, dormant season fire is easier to use and recent work with native plants suggests that seasonality of fire may be less critical to maintenance of species richness, as one component of biodiversity, than is generally believed. Science can advance the goal of modeling silviculture after natural disturbances by better illustrating cause and effect relationships among components of disturbance regimes and tile structure and function of ecosystems. Wide application requires approaches that are adaptable to different operational situations and landowner objectives. A key point for managers to remember is that strict adherence to a silvicultural regime that closely parallels a natural disturbance regime may not always be necessary to maintain biodiversity. We outline examples of silvicultural systems for longleaf pine that demonstrates these ideas.