Patterns of woody plant epiphytism on tree ferns in New Zealand

Tree fern trunks provide establishment surfaces and habitat for a range of plant taxa including many understorey shrubs and canopy trees. The importance of these habitats for augmenting forest biodiversity and woody plant regeneration processes has been the subject of conjecture but has not been robustly assessed. We undertook a latitudinal study of the woody epiphytes and hemiepiphytes of two species of tree ferns (Cyathea smithii, Dicksonia squarrosa) at seven sites throughout New Zealand to determine (1) compositional variation with survey area, host identity, and tree fern size, and (2) the frequency of woody epiphyte and hemiepiphyte occurrence, in particular that of mature individuals. We recorded 3441 individuals of 61 species of woody epiphyte and hemiepiphyte on 700 tree ferns across the seven survey areas. All were facultative or accidental, with many species only ever recorded as seedlings. Epiphyte composition varied latitudinally in response to regional species pools; only two species occurred as woody epiphytes at every survey area: Coprosma grandifolia and Schefflera digitata. Five woody epiphyte species exhibited an apparent host preference to one of the two tree fern species surveyed, and trunk diameter and height were strong predictors of woody epiphyte and hemiepiphyte richness and diversity. Woody epiphytes and hemiepiphytes occurred on 59.7 ± 18.9% of tree ferns surveyed; yet mature epiphytes occurred on only 1.0 ± 0.6% of tree ferns. With the notable exception of some tree fern-Weinmannia communities, our data indicate that tree fern trunks are potentially not important as regeneration sites for most woody understorey and canopy species in New Zealand, instead acting more as sink habitats.