The distribution of epiphytes over environmental and habitat gradients in tropical and subtropical Australia

Epiphytes, plants which grow on other plants for support yet are not parasitic to their host, are a prominent feature in Australia’s rainforest. Despite this, very few epiphyte studies have been undertaken in Australia. This thesis examines the distribution of vascular and non-vascular epiphytes over two spatial scales, within the host tree and across elevation, and examines how gradients of light and moisture affect these distributions. This study focuses on the two epiphyte ‘hotspots’ in Australia, the tropical rainforests in the Wet Tropics Region (Far North Queensland) and the subtropical ‘Gondwana Rainforests’ (northern New South Wales). This thesis explores how the distribution patterns found in these two Australian ecosystems compares to those found for rainforest elsewhere in the world, with special reference to epiphyte distributions over continuous light and moisture gradients and broader zonation systems. Very little research examines the distributions of both moss and vascular epiphytes within the same study. In the subtropical site, vascular epiphytes and mosses were recorded from four height zones across five elevations between 300 and 1100 m above sea level (asl). Vascular epiphyte species richness was highest in the inner canopy (6.3 species), while mosses tended to have a uniform distribution over the height zones (3.8 - 5.0 species). Both moss and vascular epiphyte species richness peaked at mid-elevations (500 - 700 m), with moss richness peaking at a slightly higher elevation than the vascular epiphytes. Host tree characteristics (bark roughness, host size) explained very little of the species composition or richness of epiphytes. The strong patterns found in the species richness and composition of epiphytes over host tree and elevation gradients suggest that moisture, temperature and light may be one of the major influences on epiphyte distributions in this ecosystem. Moving beyond broad zonation systems, in the tropical rainforest site, the distribution of vascular epiphytes was examined over continuous gradients of light and humidity, using individual environmental measurements for each epiphyte surveyed. There was a strong partitioning of taxonomic groups over the light and vapour pressure deficit (VPD) gradient. Orchids had the highest average total transmitted light levels and VPD (27% and 0.43 KPa, respectively), followed by theferns (21% and 0.28 KPa) and then the other angiosperms (17% and 0.2 KPa). There was also strong partitioning of species within taxonomic groups, suggesting that microclimatic factors play an important role in the realized niche spaces of epiphytes within the tropical Australian rainforest. Epiphytes show a strong distribution of drought mitigating traits within the host tree, but few studies have examined distribution patterns of these traits over elevation gradients. We assessed whether epiphyte species that occupy comparable realised niche spaces within host tree and landscape scale gradients have similarities in taxonomy, morphology or physiology in the subtropical rainforest of Australia. Vascular epiphytes with Crassulacean Acid Metabolism and other drought-mitigating morphologies were common in the groups that occupied the most xeric situations. Vascular species with little to no drought-mitigating characteristics were common in groups that occupied moister situations. Moss morphologies were less congruent with environmental conditions than vascular plant morphologies. Broad zonation systems are often used in epiphyte research. The effectiveness of a widely used system, the Johansson zones, was tested. Vascular epiphytes were grouped by observed substrate and microclimatic attributes and assessed for correspondence to the zones. Twenty-four epiphyte species in the tropical rainforest site were agglomerated into four groups using Ward’s method. Group 4 was highly distinct and included shade loving species and nomadic vines from the lower zones of the host trees. Group 3 contained species from the most exposed habitats. Group 1 had higher light levels and lower substrate thickness than Group 2, yet both groups had close to identical distributions over the Johansson zones. This suggests that groups of epiphyte species may utilise different micro-sites within the same zone. While the Johansson zones are a useful tool in epiphyte studies, finer partitioning of habitat within the host tree may be missed. Overall, in both the tropical and subtropical rainforest sites, epiphytes exhibited predictable distributions, of both species and traits, over the host tree, elevation, light and VPD gradients. Thus, moisture and light have a major influence on epiphyte distributions in Australia. The patterns of vascular epiphyte distribution are similar to that reported in the international literature, however, moss epiphytes had distributions that were partly exceptional, perhaps due to mosses being able to inhabit small microhabitats within the host tree. Indeed, different communities of vascular epiphytes can coexist within the same zone of the tree in Australia, and perhaps more widely, due to fine scale patchiness of habitat. The conclusion that epiphyte distributions have a strong link with microclimate has important implications for their survival in the context of climate change. Much more study is needed on epiphytes in Australia, especially the bryophytes, in order to work out how to prevent biodiversity loss.