Development of canopy structure in Pseudotsuga menziesii forests in the southern Washington Cascades

We quantified stand-level structural diversity in eight stands of temperate coniferous forests that ranged in age from 50 to 950 years in the southern Washington Cascade Range. Stands were chosen based on the dominance, or former dominance in the case of the oldest stands, of Pseudotsuga menziesii. In addition, to avoid confusing patterns of structural development, only stands that had a single disturbance of origin were used. Within each stand, randomly located transects were laid out that were proportional in size to the dominant tree height. Tree heights and diameters ranged up to 90 m and 309 cm, respectively. A total of 5,737 trees among the 32 transects were mapped and measured for height, diameter, crown height, and four crown radii. The structure of the canopy in the younger stands is a simple, fairly even monolayer. This contrasts with the older stands, which have a very complex, highly diffuse canopy with extreme variability in both horizontal and vertical dimensions. The stands with the highest structural complexity were between 500 and 650 years old and retained from 17 to 76 Pseudotsuga stems/ha. Our oldest site had only one living Pseudotsuga/ha. Mean diameter of just the Pseudotsuga trees was less than 55 cm for stands less than 200 years and over 180 cm for the most complex old-growth stands. One measure of stand structural diversity, the standard deviation of dbh, ranged from less than 25 (stands less than 200 years) to values greater than 50 (most complex old-growth sites). A similar relationship exists when using the standard deviation of height. Large snags and logs important for wildlife were present only in the old-growth stands and were abundant in the most structurally diverse stands.