PARENT SESSION
Oral Session #7: Disturbance in Forests: Fire, wind, and other.
Presiding: G. Murray
Monday, August 5. 8:00 AM to 11:30 AM. Cochise Meeting Room, TCC.

Evaluating release criteria and climate impacts in old-growth hemlock.

Black, Bryan^*,1, Abrams, Marc¹, ¹ Penn State University, University Park, PA

ABSTRACT- New release criteria were developed to better quantify the disturbance history of an old growth hemlock (Tsuga canadensis) forest in central Pennsylvania. Percent changes in sequential 10-yr ring-width averages were used to mitigate the effects of tree age and short and long-term climate variations on radial growth. Influences of canopy position, growth history, and moderate-length climate events (i.e. prolonged drought) on percent growth change were tested and integrated into the release criteria. Tree age was not correlated with the magnitude of percent growth change peaks. In understory trees, percent growth change peaks were almost twice as large as those in overstory trees. However, the reason for this relationship is that response to a given disturbance is heavily influenced by the prior 10 years of raw growth. Percent growth change fell below a threshold of maximum potential release for a given value of prior growth. This boundary line formed a steep negative exponential trend with increasing prior growth. Thus, we propose criteria in which major, moderate, and minor releases are those percent growth change peaks falling within 60-100%, 40-60%, and 20-40% of the boundary line. Moderate-length droughts had the greatest effect on fast-growing trees, most of which were automatically excluded by our criteria. We believe that this method can extend beyond hemlock and have application in other species and forest types.

KEY WORDS: release criteria, hemlock, old growth, disturbance history