Spatial Relationship of Biomass and Species Distribution in an Old-Growth Pseudotsuga- Tsuga Forest

Old-growth forests are known for their complex and variable structure and function. In a 12-ha plot (300 m 400 m) of an old-growth Douglas-fir forest within the T. T. Munger Research Natural Area in southern Washington, we mapped and recorded live/dead condition, species, and diameter at breast height to address the following objectives: (1) to quantify the contribution of overstory species to various elements of aboveground biomass (AGB), density, and basal area, (2) to detect and delineate spatial patchiness of AGB using geostatisitcs, and (3) to explore spatial relationships between AGB patch patterns and forest structure and composition. Published biometric equations for the coniferous biome of the region were applied to compute AGB and its components of each individual stem. A program was developed to randomly locate 500 circular plots within the 12-ha plot that sampled the average biomass component of interest on a per hectare basis so that the discrete point patterns of trees were statistically transformed to continuous variables. The forest structure and composition of low, mediate, and high biomass patches were then analyzed. Biomass distribution of the six major species across the stand were clearly different and scale- dependent. The average patch size of the AGB based on semivariance analysis for Tsuga heterophylla, Abies amabilis, A. grandis, Pseudotsuga menziesii, Thuja plicata, and Taxus brevifolia were 57.3, 81.7, 37, 114.6, 38.7, and 51.8 m, respectively. High biomass patches were characterized by high proportions of T. heterophylla and T. plicata depending on spatial locations across the stand. Low AGB patches had high densities of A. amabilis and T. brevifolia. We presented several potential mechanisms for relating spatial distribution of species and biomass, including competition, invasion and extinction, distur- bance, and stand dynamics. Clearly, future studies should be developed to examine the details of how each process alters the spatial patterns of tree species with sound experimental designs and long-term monitoring processes at multiple scales. FOR .S CI. 50(3):364-375.