A Multi-scale Analysis of Disturbance Dynamics in Hardwood Forest Communities on the Cumberland Plateau, U.S.A.

The purpose of this research was to quantify forest disturbance processes and evaluate the influence of these processes on secondary hardwood forest communities on a section of the Cumberland Plateau in Tennessee. The specific objectives of this study were to: (1) reconstruct the disturbance history of a secondary hardwood forest using species composition, stand structure, tree-ring data, and soil charcoal analyses, (2) quantify canopy gap formation mechanisms, (3) document canopy gap characteristics, and (4) determine the forest response to small-scale disturbance events. This information is useful to understand the importance of localized disturbances on stand development and forest successional patterns. With the exception of one stand-wide disturbance in the early 1980s, the disturbance regime of the forest was characterized by localized, asynchronous events that occurred at variable spatial and temporal scales. Gap-scale disturbance events became frequent after about 40 years of forest development. The presence of soil charcoal indicated that fire had occurred in the Pogue Creek Natural Area in the past, but species composition and a lack of fire-scarred trees indicated that fire had not occurred during the development of the current stand. The majority of canopy gaps were caused by treefall (either windthrow or basal-shear) and half of all gaps were caused by the death of a Quercus individual. Gap ages ranged from 1 to 17 years with a mean of 7 years. Seasonality of death could be accurately determined for 17 gap makers and all but one of these trees died during the growing season. Strong wind associated with convective storms is the most probable disturbance agent in the forest. The fraction of land area in expanded gaps and true canopy gaps was 15% and 6%, respectively. The amount of land area in canopy gaps was highest for younger gaps and generally decreased with increased gap age. Most expanded and true canopy gaps had elliptical shapes and the majority of gaps were oriented perpendicular to slope contours. Significant positive relationships were documented between expanded gap size and the density of saplings, trees, and total stems. Only weak relationships existed between stem diversity and expanded gap size. Most of the canopy gaps documented were projected to close by lateral crown expansion rather than height growth of subcanopy individuals, but gaps still provided a means for understory trees to recruit to larger size classes. Over half of all trees located in true canopy gaps with intermediate crown classifications were Acer saccharum, A. rubrum, or Liriodendron tulipifera. Because the gaps documented were relatively small and close by lateral branch growth of perimeter trees, the most shade-tolerant A. saccharum has the greatest probability of becoming dominant in the canopy under the current disturbance regime. This study indicated that gap-scale disturbance processes have an influence on stand development and successional patterns of secondary hardwood forests in the absence of large-scale events