Epiphyte communities on redwood (Sequoia sempervirens) in northwestern California

Abstract

We used rope techniques to access epiphyte communities on nine large and structurally complex redwoods (Sequoia sempervirens) occupying old-growth forest reserves of northwestern California. All species of epiphytic lichens, bryophytes and vascular plants were recorded, biomass of dominant vascular epiphytes (Polypodium scouleri and Vaccinium ovatum) was quantified, and tree crowns were mapped to estimate substrate surface areas. We employed a flexible, plot-based sampling regime defined by available microhabitats within height strata to search for epiphytes. All substrates were examined, including tree surfaces, canopy soils and perennially exposed surfaces of epiphytic vascular plants as well as forest floor vegetation, woody debris and terrestrial soils beneath the redwood crowns. Combined arboreal and terrestrial search efforts revealed 282 species, including 183 lichens, 50 bryophytes and 49 vascular plants. Beta diversities for plots aggregated by floristic group, stratum and substrate type were generally high, indicating a large proportion of infrequent species. Indirect ordination analysis suggested that an environmental gradient from exposed to sheltered habitats was the strongest factor controlling epiphyte community structure. Floristic groups, strata and substrates were highly segregated along the dominant compositional gradient. Chlorolichens, upper crown strata and redwood foliage occupied one end, while vascular plants, forest floor strata and terrestrial woody debris occupied the other end of the gradient. Indicator Species Analyses revealed that many species expressed affinities for particular substrates, including live vs. dead foliage, bark of small vs. large branches and limbs, bark of upper vs. lower surfaces of large limbs, bark of large trunks, bare wood, bryophyte mats, soils, non-redwood stems and terrestrial woody debris. Cluster Analysis identified seven groups of species with similar patterns of distribution across height strata and substrate types. Correlations between tree structure and species distribution suggested that structural complexity promoted epiphyte diversity within height strata. Surface areas of small live trunks, limbs and dead trunks were the best predictors of lichen species richness, Polypodium scouleri biomass and Vaccinium ovatum biomass, respectively. At least one new species (Calicium sp. nov.) was discovered, and two species (Buxbaumia piperi, Icmadophila ericetorum) normally restricted to terrestrial habitats were found as canopy epiphytes for the first time.