Patterns of coarse woody debris volume among 18 late-successional and mature forest stands in Pennsylvania1

Abstract

Coarse woody debris (CWD) provides vital habitat and resources for a broad variety of organisms in old-growth stands. Although many studies have quantified CWD, it remains unclear how variability in tree species composition relates to CWD and snag volume. If CWD and snags (standing dead trees) vary substantially with tree species abundance among different forest types, then the volume of CWD may change in response to shifts in tree species composition. Dominant canopy tree species vary significantly in their decay resistance, shade tolerance, and suppression of light transmission through the canopy; therefore, the rate of CWD input and output may vary with dominant tree species abundance. We hypothesize that CWD and snag volumes are positively related to the abundance of late-successional shade-tolerant canopy species and negatively related to the abundance of mid-successional oak species. To test this hypothesis, CWD was surveyed in 18 late-successional and mature stands across three physiographic provinces in Pennsylvania. The stands were classified by composition: ten as red oak-mixed hardwood and eight as hemlock-mixed hardwood. The hemlock-mixed hardwood stands (vs. the oak-mixed hardwood stands) had larger volumes of CWD and higher relative abundance of shade-tolerant species. CWD volume was positively correlated with the basal area of hemlock and all shade-tolerant species combined. In contrast, basal area of oak species was negatively correlated with the volume of snags and CWD. These results supported the hypothesis that CWD volume is greater in stands dominated by late-successional species, particularly hemlock, than in stands dominated by oak species. The volume of CWD did not change significantly with the abundance of any other tree species (e.g., red maple (Acer rubrum L.), beech (Fagus grandifolia Ehrh.)). Understanding the relationship between tree species composition and the volume of CWD and snags can provide insight into future CWD characteristics as pathogens dramatically change eastern forests (e.g., beech bark disease, hemlock woolly adelgid).