Disturbance history and stand dynamics along a topographic gradient in old-growth hemlock-northern hardwood forests of the Allegheny Plateau, USA

Dendroecology and stand structure analysis were used to investigate how disturbance parameters vary across contiguous and contrasting sites in the Tionesta Research Natural Area on the Allegheny Plateau of northwestern Pennsylvania, USA. Disturbance chronologies based on radial growth analysis were developed for upland, sideslope, and riparian sites. Exogenous disturbances such as tornados, glaze(ice) storms, and thunderstorm downbursts affect forests of the region several times each century creating widespread pit-mound topography, large gaps, and unidirectional blow-downs. Disturbance intensity was directly related to site elevation and exposure, decreasing from the upland to riparian sites. Upland sites (stand age approximately 350 y) experienced medium-intensity disturbances (>20% canopy damage) nearly every 30 y with four decades exhibiting heavy (severe) disturbances (>40% canopy damage), resulting in higher importance of early successional taxa on uplands. The sideslope site (stand age 350 y) experienced medium-intensity disturbances every decade with only one severe intensity disturbance. The riparian site (stand age 250 y) was impacted by medium-intensity disturbances every 80 y with no severe disturbances in the last 250 y. 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 data suggested disturbance rotations of 1000-2000 y for these forests. This study suggests that medium-intensity disturbances impact the Allegheny Plateau every 210-630 y. This discrepancy between historic data and tree-ring data suggests that disturbance intervals estimated from historical sources may be too long, inaccurately portraying the pre-European disturbance regime.