Patterns of coarse woody debris in old-growth hemlock-hardwood forests of northern Wisconsin and western Upper Michigan

Patterns of coarse woody debris (CWD) and associated features of tip-up mounds and canopy gaps were investigated in 25 old-growth hemlock-hardwood forests in northern Wisconsin and adjacent Michigan in relation to stand age, site characteristics, and landscape context. CWD was highly influenced by stand age. Stands with well-developed "old-growthness" have most of the following characteristics (1) maximum tree age ;I found little evidence for major influences of habitat type, slope, and boundary contrast on the patterns of CWD. A number of site and landscape factors are probably acting in combination with the patterns of local disturbance. Stands with richer soils show a weak trend toward greater amounts of CWD. Stands with moister soils have a greater density and area of recent mounds. There was no relation between slope and either area or density of mounds. Little of the total variation in CWD is explained by factors related to the exposure of the stand to wind disturbance. However, a greater proportion of wind-influenced uprooted or snapped gap-maker trees occurred as boundary contrast increased.;Based on a recensus of permanent plots in 15 hemlock stands, mortality rates for all species ranged from 0.2% to 1.6% of original basal area per yr, or from 2.0 to 8.4 trees per ha per yr. As documented by permanent plots, and a comparison of log origins versus gap-maker origins, time periods of at least 5 years increase the probability that stands experience a disturbance event. A disturbance event increases breakage and uprooting of live trees, elevating mortality above low, background levels, and increases snag framentation and collapse. Mortality type was not independent of tree species. Hemlock logs originated from tree uprooting more often and from snag collapse less often than expected. Evidence from stilt trees and from an exponential decay function based on wood age and density indicates that hemlock logs take approximately 200 yr for complete decay.