Patterns of growth,mortality and biomass change in a coastal Picea sitchensis - Tsuga heterophylla forest

Abstract. Ten years (1979-1989) of growth and mortality were determined in a 130-yr old stand on the Oregon coast based on periodic remeasurements in 441000 m² plots. Western hemlock (Tsuga heterophylla) constituted 90 % of the individuals and 57 % of the biomass. Wind is a major form of disturbance in this area, creating both small discrete and large diffuse disturbance patches; wind therefore has a direct effect on the location and extent of regeneration. Rates of tree mortality were high for this coastal stand (2.8 %/yr), especially compared to similar-aged stands in the western and eastern Cascade Ranges. Though low in absolute density, Sitka spruce (Picea sitchensis) persisted in competition with the more tolerant western hemlock. Net production of bole biomass (4.9 Mg ha^-1 yr^-1) did not equal mortality (8.7 Mg ha^-1 yr^-1), and total biomass declined over the 10-yr measurement period from 499 to 460 Mg/ha; this trend may have begun as early as the mid-1950's at a peak biomass of about 600 Mg/ha. The decline may have been due to a positive feedback in which new gaps and enlarging gap perimeters exposed more and more trees to potential wind damage.