Coarse Woody Debris as a Function of Age, Stand Structure, and Disturbance in Boreal Newfoundland

We examined the structural attributes of coarse woody debris (CWD) in balsam fir and mixed balsam fir-black spruce forests in western and central Newfoundland. The purpose of the study was to document CWD abundance and structure within various stages of stand development, and to generate simple models to predict CWD availability as habitat for forest wildlife. Our objectives were threefold: (1) determine the temporal patterns of CWD following clear-cut disturbance, and compare these second-growth stands to old sites (>80 yr) of natural origin; (2) relate the temporal patterns of CWD to stand development; and (3) demonstrate how factors such as site conditions and disturbance may influence these temporal patterns. Our chronosequence included 19 second-growth stands with a mean tree age ranging from 33 to 80 yr, and 7 old-growth stands ranging from 87 to 110 yr. The volume of coarse woody debris (CWD), here defined as downed wood, was relatively low to intermediate early in the chronosequence (e.g., 32.3 m3/ha in a 36-yr-old stand), lowest in a 58-yr-old stand (15.2 m3/ha), and highest in an 80-yr-old stand (78.1 m3/ha). Results indicated that CWD volume followed the general “U-shaped” temporal trend observed in other forest systems. The presence of CWD early in the sequence was strongly influenced by residual hardwoods (i.e., birch) left by the original logging operation. The highest observed volume of CWD corresponded with stand senescence and appeared to be maintained within old-growth stands. Snag (standing dead wood) density was low within the youngest stands (<200 snags/ha), and peaked within mature second growth. Defoliation disturbance increased both the volume and the structural diversity of CWD within silviculturally mature second-growth stands. Results from our study indicate that CWD in stands >50–60 yr of age is not residual but generated primarily from regenerating tree structure. Factors affecting the rate of stem growth, e.g., site quality and initial stocking levels, should influence the accumulation and overall abundance of CWD within later stages of forest development. We therefore applied yield-density relationships as a method of examining stand-level dynamics of CWD.