Crown structure and the distribution of epiphyte functional group biomass in old-growth Pseudotsuga menziesii trees

Abstract

Epiphyte functional groups (alectorioid lichens, cyanolichens, other lichens, and bryophytes) were sampled in nine old-growth, canopy-emergent, Pseudotsuga menziesii trees along a riparian corridor in the Wind River Experimental Forest, Washington State, U.S.A., with the objective of determining epiphyte abundance and its relationship to crown structure. An additional objective was to develop a sampling design that reasonably captured the variation in epiphyte distribution so that total biomass could be estimated for an individual large tree, a design efficient enough to make description economically and logistically possible. Trees ranged in height from 51 to 66 meters and averaged 83 live and 79 dead limbs in a crown length of 40 meters. Diameter at breast height was a useful estimator of tree crown structural complexity. Epiphytes averaged 27.1 kg/tree, with alectorioid lichens (19.3 kg/tree) dominating the assemblages, followed by other lichens (3.3 kg/tree), bryophytes (2.6 kg/tree) and cyanolichens (1.9 kg/tree). The foliage region had the highest biomass of lichens (16.4 kg/tree), followed by the branches (8 kg/tree) and trunk (2.6 kg/tree). Alectorioid lichens predominated in the upper, middle and outer portions of the tree crown, whereas the lower and inner portion of the tree crowns had more equal distributions of all four functional groups. Relative height and limb size were the most significant structural attributes influencing epiphyte distribution. Limb size had a particularly strong effect on the distribution of bryophytes regardless of height. In old, canopy-emergent P. menziesii, the crown structural variables which determine epiphyte distribution and abundance are height, crown length, trunk surface area and exposure, distribution and abundance of small, medium and large branches, and distribution and exposure of foliated branches.