Community Ecology of Epiphytes and other Arboreal Plants in the New Zealand Eco-region

Epiphytes and other structurally-dependent plants have a spatial ecology and community structure intrinsically linked to that of the host trees in the forest, unlike fully terrestrial plants. Understanding of the ecological implications of this from a theoretical perspective is in its infancy. New Zealand’s south temperate rainforest, whilst not as species rich as tropical forests, hosts one of the richest temperate epiphyte floras. Our understanding of the ecological processes structuring the epiphyte communities of New Zealand forests is however lacking. Here, I present four key studies seeking to add to our knowledge of epiphyte community structure, host specificity and spatial ecology in the New Zealand eco-region.

First, I tested if seed size determined the likelihood of woody plant species occurring epiphytically on tree ferns (their arboreality) – Chapter 2. Arboreality was negatively related to seed size, with only smaller-seeded species commonly occurring on tree ferns. However, the effect of seed size reduced in later life history stages, as expected. These small-seeded species, most notably Weinmannia racemosa, appear to be utilising an alternative recruitment strategy by establishing epiphytically on the tree fern trunks.

Second, on Cyathea dealbata host tree ferns, I tested patterns of species accumulation, metacommunity network structure, and differences in vertical stratification (Chapter 3). Epiphytes and climbers followed a species accumulation model of succession between tree ferns of different sizes and between older and younger portions of the tree fern. The metacommunity network showed patterns of species co-occurrence and nestedness consistent with null expectations. Epiphytes of different habits and different dispersal syndromes show different vertical profiles of occurrence, with bird-dispersed species occurring more often near the top of the tree fern than other taxa.

To understand an unusual pattern in epiphyte between-host structuring, I quantified the relationship between epiphytic plant and sooty mould assemblages in New Zealand montane beech forest (Chapter 4). Due to the presence of host specific scale insects, the sooty mould was limited to two of three co-dominant canopy tree species. On these two host species, epiphyte richness was significantly reduced. The host size-richness relationship in these two species was also removed, with species composition significantly altered compared to the mould free host species. My results are consistent with the sooty mould amensally excluding the epiphytes and it can be considered as a part of a keystone species complex (with the host beeches and scale insects). This produces a strong pattern of parallel host specificity otherwise not seen in epiphyte assemblages.

Lastly, I compared the differences in spatial niche and host species diversity between three arboreal plants, with divergent ecophysiology, on Lord Howe Island (Chapter 5). These focal species were a dwarf mistletoe, an epiphytic orchid and an epiphytic fern. The mistletoe was restricted to thinner branches, and had a significantly different niche to both epiphyte taxa. The host diversity of the mistletoe and orchid both differed significantly from null model expectations. However, the epiphytic fern (Platycerium bifurcatum) had a host diversity consistent with null expectations.

Taken together, these studies increase our understanding of epiphyte community assembly in New Zealand and provide a platform to encourage further work in this field. They also provide results that expand understanding of spatial patterns between host and up vertical clines.