Vascular epiphyte assemblage structure and distribution patterns in the south-temperate zone
Vascular epiphytes, which are specialised to spend their entire life cycle within trees, are significant contributors to local ecosystem services. However, our current understanding of epiphyte distributions, co-occurrences, and general ecology lags far behind that of terrestrial plants. Furthermore, the majority of epiphyte research is undertaken in tropical forests, with comparatively few studies extending into temperate climates. As such, whether epiphytic plant assemblage structure varies geographically, or is influenced by area and isolation effects needs further scrutiny. In addition, how epiphytes are distributed in relation to host tree ontogeny and microclimates specific to south-temperate forests is poorly understood. Here, I attempt to bridge this gap by researching epiphyte distributions and assemblage structure in New Zealand, southern Chile, and Australia. In the first biogeographic study of epiphyte-host interactions, I determined if epiphyte-host network structure (i.e. nestedness, species co-occurrences, species specialisation) varied among New Zealand and Chilean temperate forests (Chapter 2). At the forest stand level, network structure was consistent with stochastic structuring, which suggests that dispersal and disturbances are important drivers of epiphyte distributions at a biogeographic scale. However, deterministic structure was observed in New Zealand networks with regards to nestedness (i.e. when specialists interact with generalists), which suggests that positive species interactions influence epiphyte distributions at a within-tree scale. Second, I determined whether the composition of plant communities residing in epiphytic birds’ nest ferns (Asplenium goudeyi) on Lord Howe Island, Australia, are influenced by fern size, isolation from a major propagule source and resident plant community richness (Chapter 3). Results suggest that plant communities are structured by dispersal. For one, there was a significant isolation effect on resident plant community richness. Additionally, wind-dispersed taxa were well represented in isolated ferns, while animal-dispersed taxa and taxa with no specific dispersal strategies were absent. This is the first study to test the combined effects of area, isolation and resident plant richness on epiphytic plant assemblage structure. Third, using Darwin’s geological theory of island ontogeny as a theoretical construct, I explored changes in epiphyte species richness throughout tree ontogeny (Chapter 4). Theoretical frameworks have helped bridge the gap between our understanding of vascular epiphytes and terrestrial plants, however, none have been implemented to guide investigations on epiphyte assemblage development. Based on the general features of island ontogeny, I found three stages of epiphyte assemblage development: (i) an initial stage where host trees are devoid of epiphytes, (ii) a second stage where trees acquire epiphytes into maturity, and (iii) a hypothetical stage where epiphyte assemblages follow a period of species decline following host tree mortality. In addition to these results, I found interspecific variation in the ontogenetic stage at which host trees become favourable for epiphyte establishment and the rate at which epiphyte assemblages develop. Lastly, I explored the systematic distribution of epiphytes and mistletoes in relation to microclimate gradients around the trunks of trees (Chapter 5). In addition, I tested the physiological responses of epiphytes and mistletoes to reductions in their most limiting resources to determine if the responses were consistent with their distribution patterns. The radial distributions of epiphytes and mistletoes were highly directional, and paralleled gradients of humidity, light and water. Additionally, the photochemical efficiency of epiphytes and CO₂ assimilation in mistletoe leaves decreased in plants growing in environments with lower water and light availability, respectively. However, mistletoe leaves still assimilated CO₂ in lower light conditions, which suggests a high plasticity of mistletoes to growing in a canopy environment. Despite over 120 years of recognising the importance of vertical microclimates on epiphyte distributions, this is the first systematic study of epiphytic plant distributions in relation to microclimate gradients around the trunks of trees. This thesis has increased our understanding of epiphytic plant assemblage structure, and how it is influenced by host tree species, isolation, area and resident plant species richness. In addition, this thesis has increased our understanding of the effect of host tree ontogeny and microclimate on epiphyte distribution patterns. Together, these studies may be built upon more broadly to further elucidate drivers of epiphyte assembly and distribution patterns.