Post-Clearcutting Chronosequence in the B.C. Coastal Western Hemlock Zone

The depth, mass, and N-capital of the forest floor were measured in three plots in a 400-year-old forest of Pacific silver fir (Abies amabilis [Dougl.] Forbes), western hemlock (Tsuga heterophyl-la [Raf.] Sarg.) and western red cedar (Thuja plicata Donn) on Southern Vancouver Island, as well as in three plots in each of four neighboring unslashburned clearcuts ranging in ages from 3 to 26 years old. Forest floor depth, mass and N content did not differ significantly during the first three years following clearcutting but had dropped by 35% to 40% of pre-clearcut values by the sixth post-clearcutting year. Although values of these ecosystem characteristics gradually increased thereafter, they were still about 20% below pre-clearcut values by year 26. Re-measurements of the 3-year-old, 6-year-old and 10-year-old sites after two years, as well as of 6 independent neighboring sites, confirmed that each of the five intensively studied sites were representative members of the same chronosequence. Average mass loss of cellulose disks placed in the three younger clearcuts was two-to-four times greater than mass loss in the 26-year-old clearcut and 400-year-old forest sites, suggesting that the decrease in forest floor mass was not only attributable to lower rates of litter input but also to changes in litter decomposition rates. The average seasonal moisture content was higher in the 4-year-old clearcut than in the other sites, and the average daily air and soil temperatures were higher in the 10-year-old clearcut than in the old forest stand. The change in micro-climate was believed to be responsible for increasing decomposition rates in the younger three clearcuts. We conclude that forest floor N is quite dynamic during stand establishment.