The density and distribution of Sitka spruce and western hemlock seedling banks in partially harvested stands in southeast Alaska

Abstract

This study’s objective was to document and describe the current seedling bank of Sitka spruce (Picea sitchensis (Bong.) Carr.) and western hemlock (Tsuga heterophylla (Raf.) Sarg.) stands in southeast Alaska that were partially cut between 1900 and 1984. We investigated the following: (1) What are seedling bank densities? (2) What are seedling size- and age-class distributions? (3) Do seedbed type, treatment, and understory vegetation affect seedling density and species composition? and (4) What are seedling growth rates? Density was high for both Sitka spruce (22,000 seedlings/ha) and western hemlock (223,000 seedlings/ha) and varied widely between sites. There were always fewer spruce than hemlock. Ninety-five percent of spruce and 94 percent of hemlock were less than 0.5 m tall. Spruce had a mean age of 8 years (range 1 to 41 years) and hemlock 19 years (range 1 to 110 years). Both species were four times as common on logs as on undisturbed forest floor. Under closed-canopy conditions at the 15 sites harvested between 1900 and 1958, the average annual height growth rate was 2.3 cm for hemlock and 1.7 cm for spruce. At the two open-canopy sites, harvested in 1983 and 1984, annual height growth rates increased to 8.2 cm for hemlock and 10.2 cm for spruce. Our results suggest that the seedling bank of both species is established and well stocked, thus providing advanced regeneration for the postharvest stand. Because forest managers have increasing concern about effects of clearcut harvesting in Pacific Northwest forest ecosystems and renewed interest in maintaining or restoring biodiversity, other silvicultural methods warrant examination.