Twenty-year survivorship of tree seedlings in wind-created gaps in an upland hardwood forest in the eastern US
Wind is a common canopy disturbance in the upland oak (Quercus)- and hickory (Carya)- dominated stands of the Central Hardwood Region of the eastern US. Canopy openings range from small gaps, resulting from windthrow of single dominant trees, to occasional large gaps of many hectares formed by straight-line winds associated with severe thunderstorms and rare subtropical hurricanes. Vegetation regeneration response in small gaps and larger artificially created (i.e., timber harvested) gaps has been extensively studied, but little has been reported on naturally formed large gaps (> 6 trees) where a partial canopy typically remains from residual overstory and midstory trees. We investigated advance regeneration survivorship on a landscape scale within and around 12 large wind-felled gaps created by Hurricane Opal (October 1995) in a Southern Appalachian watershed. We hypothesized that survivorship would (1) change along a linear distance gradient from the unaffected forest towards gap center (2) vary in relation to categorical gap locationswithin- gaps versus in the adjacent unaffected forest and (3) be affected by vegetation and environmental variables. In early 1996 we tagged tree seedlings in quadrats located along linear axes that extended from gap center into the unaffected forest and measured a variety of vegetation and environmental variables. Overall mean survivorship declined from 1997 (0.96) to 2016 (0.56). We found that survivorship was related to linear distance gradients for species rated intermediate in shade tolerance, such as red oaks, but not for shade intolerant, tolerant, and semi-tolerant species (e.g. white oaks). We found that survivorship increased on a linear distance gradient from gap center into the unaffected forest over the first 9 years of our investigation. By year 20 this relationship had reversed: survivorship increased from the unaffected forest to gap centers. Survivorship was also modeled as a function of categorical location of tagged seedlings within-gaps versus in the unaffected forest. Again, we found that survivorship of only intermediate shade tolerant species was related to categorical locations. Survivorship decreased in relation to time and cohort competition and increased with initial seedling height. Our 20-year investigation revealed changes in dynamics of regeneration that would not have been apparent in a short-term study. These results will enable resource managers to assess regeneration development in large gaps following windstorms in oak-dominated hardwood stands.