Structure and composition of corticolous epiphyte communities in a Sierra Nevada old-growth mixed-conifer forest

Abstract

Fifty white fir, red fir, incense cedar, Jeffrey pine and sugar pine were surveyed for corticolous bryophytes and macrolichens in the Teakettle Experimental Forest. Epiphyte abundances were estimated by percent cover in 5 m strata from ground-level to tree-tops. Gradients of bark pH within tree species and stand-level vapor pressure deficit (VPD) were also measured. Mosses had a strong positive association with white fir and proximity to perennial water. Clustering of trees in macrolichen space resulted in four significant groups: white fir, red fir, incense cedar, and Pinus. The lichens Ahtiana sphaerosporella and Letharia indicated the red fir group, Hypogymnia imshaugii and Melanelia elegantula the white fir group, and Nodobryoria oregana the Pinus group. Two nitrophilous lichens, Xanthoria polycarpa and Candelariella efflorescens were strongly positively associated with white fir, and absent from the Pinus species. Bark pH distinguished presence/absence of moss and composition of macrolichens among tree groups, while increasing VPD with height best explained within-group community structure. To foster epiphyte richness and diversity in this mixed-conifer forest, a heterogeneous mix of mature tree species should be retained when thinning, and mature trees in the narrowly confined riparian zone should not be harvested.