Disturbance history and stand dynamics along a topographic gradient in old-growth hemlock-northern hardwood forests of the Allegheny Plateau, USA.
RUFFNER, C.M.* 1 and M.D.ABRAMS 2
Southern Illinois University, Carbondale, IL 62901 USA 1
Pennsylvania State University, University Park, PA 16802 USA 2
Dendroecology and stand structure analysis were used to investigate how disturbance parameters vary across contrasting sites and how species respond to variable disturbance regimes in the Tionesta Research Natural Area on the Allegheny Plateau of northwestern Pennsylvania. Disturbance chronologies based on radial growth analysis were developed for upland, sideslope, and riparian sites to understand how these disturbances influence stand structure and long-term recruitment patterns. Exogenous disturbances such as tornades, glaze storms, and thunderstorm downbursts affect the region several times each century creating widespread pit-mound topography, large gaps, and uni-directional blowdowns. However, disturbance intensity was directly related to site elevation and exposure, decreasing from the upland to riparian sites. Upland sites (stand age approximately 300 years) experienced medium intensity disturbances (20% canopy damage) nearly every 30 years with four decades exhibiting heavy disturbances (40% canopy damage) resulting in higher importance of early succession taxa across these sites. Sideslope sites experienced medium intensity disturbances every decade with only one heavy intensity disturbance while the riparian site was impacted by medium intensity disturbances every 80 years with no heavy disturbances in the last 350 years. The reduced disturbance frequency of the lower elevation sites was largely responsible for the dominance of these sites by later successional hemlock and beech. Previous estimates based on witness tree line data suggesting disturbance rotations of 1000-2000 years for these forests appear too long. Based on tree-ring data, we suggest that medium intensity disturbances impact the Allegheny Plateau every 210-630 years. This discrepancy between historic data and tree-ring data suggest that disturbance intervals estimated from historical sources may be too long, inaccurately portraying the pre-European disturbance regime.
This abstract is being presented at: 12:00 PM in session:
Oral Session #42: Disturbance Ecology: Effects of Storms.