Conceptual models for the role of natural disturbances in ecological restoration of southeastern pine savannas

The well-documented decline of the Pinus palustris ecosystem has resulted from several anthropogenic influences, such as forest clearing and urban development, both closely related to increases in human populations.  Other impacts arise from alterations in disturbance regimes maintaining the structure and function of these ecosystems.  Thus, restoration and management of degraded pine savanna ecosystems is critical for sustained use.  Identification of ecological processes determining the structure and function of the intact system is important because successful restoration requires sound scientific understanding.  We have developed a conceptual model relating natural disturbance to demography and physiognomic structure of P. palustris stands.  In contrast to other models of stand dynamics that emphasize close interactions among the vertically complex strata, we propose a model relating two different natural disturbances (fire, tropical storms) that affect different stages of the life cycle and the physiognomic structure of P. palustris stands.  Fires are periodic with high frequency and relatively low intensity and directly affect mostly early juvenile stages.  Tropical storms are episodic with lower frequency and sometimes high intensity affecting primarily adult (tree) stages.  Fire exerts direct effects on juvenile stages and indirect effects on the herb layer via fine fuel consumption and selective mortality of potential competitors of P. palustris juveniles. By contrast, hurricanes directly influence the overstory via wind-caused damage and mortality, and indirectly influence the herb layer by altering the spatial distribution of shading and litter accumulation.  Some global models suggest changes in these two disturbance regimes, altering frequency and severity in the future.  Such changes may greatly affect pine stands, and ultimately entire longleaf pine savanna ecosystems.  Using this model, we compare of overstory stand structure and groundcover composition two old-growth longleaf stands differing in disturbance regime:  the Wade Tract (frequent hurricanes/annual burning) and the Boyd Tract (infrequent hurricanes/long-term fire exclusion).