Tree Crown Structure and Vascular Epiphyte Distribution in Sequoia sempervirens Rain Forest Canopies

Abstract

Crown structure and vascular epiphytes were studied in eight large (82-97 m tall, 3.3-7.2 m dbh) redwood trees (Sequoia sempervirens) in old-growth temperate rain forests, using rope-based methods of access. The trees had complex individualized crowns consisting of multiple (12-62 per tree) reiterated trunks arising from other trunks and branches. Trunk-to-trunk and trunk-to-branch fusions were common, and the diameter of a trunk above a fusion was often greater than below the fusion. Thirteen species of vascular plants, including a spike-moss, three ferns, four shrubs, and five trees, grew as epiphytes. Many of the species were accidental epiphytes whose primary habitat was the forest floor. They grew in deep humus accumulations on large branches and in crotches formed by multiple trunks. Three species dominated epiphyte assemblages. The deciduous fern Polypodium glycyrrhiza was abundant on two of the trees, where it was always restricted to lower crowns. The evergreen fern Polypodium scouleri, the most abundant vascular epiphyte, occurred in the upper and lower crowns of all eight trees. The ericaceous shrub Vaccinium ovatum also occurred on all eight trees but was abundant on only four trees with large quantities of decaying wood in their crowns. Polypodium ferns were more frequent on living branches, while ericaceous shrubs were more frequent on trunks and dead branches. Complex crown structure clearly promoted humus accumulation and vascular epiphyte abundance, but much of the tree-to-tree variation in epiphyte distribution was attributed to differences in tree age, stand-level microclimate, tree health, and dispersal limitations.