Abstract
We examined effects of natural disturbances on bottomland hardwood forest regeneration and focused on post-hurricane patterns of regeneration in the old-growth forests of the Congaree Swamp, disturbed by Hurricane Hugo in 1989. Predicting patterns of natural regeneration in floodplain forests is challenging because their environmental conditions are highly variable in space and time, and multiple environmental factors with potentially interactive effects are modified simultaneously by frequent disturbances. We present the main findings of a descriptive and an experimental study designed to investigate effects of windstorm disturbance and the resulting heterogeneity in light and hydrologic conditions on recruitment of bottomland hardwood species in the post-disturbance environment. Five years following the hurricane, sweetgum (Liquidambar styraciflua) remained the dominant tree species in most areas where the forest canopy was still intact. The abundance of this shade-intolerant species in the canopy pointed to the importance of past disturbances. Red maple (Acer rubrum), a shade-tolerant/flood-tolerant species, dominated the seedling layer 5 years following the storm, exhibiting peaks in abundance in highly disturbed plots. Over the subsequent 3 growing seasons, however, survival and growth of red maple seedlings were low compared with sweetgum and bottomland oaks. In the seedling layer, the proportion of shade-intolerant/very flood-tolerant species (e.g., bald cypress Taxodium distichum) increased slightly while very shade-tolerant/flood-intolerant species (e.g., pawpaw Asimina triloba) decreased along the disturbance gradient. Microsite data collected at the locations where individual tree seedlings became established revealed largely overlapping distributions of the 7 most abundant tree species along gradients of canopy openness and elevation, despite differences among the species in shade- and flood-tolerance rankings reported in the literature. Modal frequencies and shapes of abundance distributions, however, differed among species and generally corresponded with their respective tolerance ratings. A mesocosm experiment revealed that light, hydrology, and the interactions of these 2 factors can have complex effects on woody species’ regeneration patterns. Conflicts between life history stages (seed-seedling) also may develop in some species when microsites optimal for 1 stage of regeneration are sub-optimal for other stages. Natural disturbance in bottomlands produces a complex array of microsites that may result in differential establishment, survival, and growth that collectively cause shifts in species’ relative abundances.

Keywords: bottomland hardwoods, disturbance, environmental heterogeneity, flood tolerance, forest regeneration, hurricanes, Liquidambar styraciflua, microtopography, Quercus michauxii, seedlings, shade tolerance