Forests of the Pacific coast are dominated by evergreen conifers that include the largest and longest-lived species of several genera (Abies, Picea, Pinus, Pseudotsuga, Sequoia, Sequoiadendron, Thuja, Tsuga). These forests develop structurally over centuries and can achieve higher biomass than any others. Douglas-fir (Pseudotsuga menziesii) forests are widespread in the Pacific Northwest, but relatively little is known about those occurring at low elevations in the western Olympic Mountains. We quantified aboveground structure, biomass, and leaf area of three one-hectare plots containing stands of 300 to 700-year-old trees. Our objectives were to examine structural development of these forests using a chronosequence approach and to quantify the spatial distribution of leaf area. Comprehensive aboveground sampling of trees, snags, logs, and understory vegetation was conducted in each plot, permitting us to compare this chronosequence with one located at higher elevations in Washington’s Cascade Mountains.
Results/Conclusions
Large Douglas-fir trees were the primary contributors to plot-level basal areas that ranged from 90 to 158 m2 ha-1 in the western Cascades. Total tree biomass was generally higher in the western Olympics ranging from 1399-1522 Mg ha -1 versus 1012-1476 Mg ha -1 in the Cascades. Olympic peninsula leaf area index values were moderately high between values of 8-10, with the Cascadian forests exhibiting consistently higher values (9-14). Old-growth habitat index values are similar among both regions and indicate a high degree of structural complexity within the Olympic plots. Aboveground structural attributes in old-growth stands of the western Olympic Mountains are similar to values calculated for old forests of the Cascade Mountains. However, key differences exist regarding the accumulation of coarse wood and number of shade tolerant trees suggesting variability in disturbance histories and developmental processes among the two regions. A better understanding of the variability in forest structure across western Washington enables increased ecological understanding as well as providing a basis for future management activities.