Structural Diversity and Epiphyte Distribution in Old-Growth

Douglas-Fir Tree Crowns

Joel P. Clement

Ecologists are gaining a better understanding of threatened old-growth Douglas-fir forests in the Pacific Northwest to more effectively manage and protect these unique ecosystems. Research has focused primarily on attributes of the forest at the landscape level rather than at the level of individual trees. Little data exists to quantify the attributes of old-growth tree crowns and their effects upon species composition and distribution. This study provides quantitative data describing both structural diversity and the distribution of epiphyte functional groups throughout old-growth Douglas-fir tree crowns. Structural dimensions and epiphyte biomass were sampled from throughout tree crowns to determine which structural attributes of the crowns played a functional role in influencing epiphyte distributions.

Large limbs accounted for 75% of the crown foliage volume and 50% of the limb surface area. Their distribution and length determined overall crown dimensions and structural diversity. I found the greatest concentration of epiphyte biomass in the foliage region of the outer crown and determined that height and limb size were the most significant attributes influencing epiphyte distribution. In particular, limb size had a strong effect upon the distribution of bryophytes regardless of height.

In addition to providing a "road map" for further ecological research into the specific relationships between structural components and the distribution of organisms, these results imply that the large living limbs play important structural and functional roles in these old-growth crowns. These roles should be considered by forest managers and conservation planners who determine the guidelines for structural retention and habitat restoration in disturbed forest landscapes.