Epiphyte ecology in urban forests: Community assembly, traits, ecophysiology and the role of tree ferns

Abstract

Vascular epiphytes are a conspicuous feature of New Zealand’s temperate rainforests. Epiphytes contribute to ecosystem functioning by increasing species diversity, enhancing habitat heterogeneity, and influencing ecosystem processes such as primary production and the cycling of water and nutrients. However, research on epiphytes in temperate regions remains limited compared to tropical regions, especially in urban environments. This thesis addresses key research gaps in our understanding of urban epiphyte communities by reporting results from four interconnected studies. Collectively, these advance our knowledge of the biology and ecology of indigenous epiphytes in two warm temperate cities on North Island, New Zealand.

In the first study, tree ferns were shown to augment native species richness in urban plant communities. Ground ferns and vascular epiphyte diversity were significantly higher in plots with tree ferns, illustrating strong facilitative effects. In contrast, light-demanding seedlings, saplings and trees were less frequent in plots with tree ferns, suggesting competitive interactions between these plant groups. Nonetheless, tree ferns can accelerate ecological restoration by increasing native plant diversity and providing an established canopy that helps suppress light-demanding invasive plant species. Community structure also differed significantly among site types in Hamilton: tree ferns increased total basal area, whereas restored sites supported more diverse tree and shrub communities. Associated shade-tolerant species, such as Schefflera digitata, suggest that enrichment planting of extant tree fern stands with such species may encourage a successional pathway towards conifer-broadleaved forest.

The second study investigated trait matching between epiphytes and host species. Root and rhizome characteristics among 10 epiphyte species showed varying associations with specific host preferences—either favouring tree ferns, woody trees, or exhibiting generalist tendencies. Species mainly associated with tree ferns had longer, more abundant rhizome hairs, while generalist epiphytes had wider rhizomes, longer roots, and longer, more abundant rhizome scales. Links revealed between epiphyte traits and host preferences suggest that biotic filters (e.g., host availability) may exert a greater effect on epiphyte assemblages than stochastic processes (e.g., dispersal limitation).

To better understand epiphyte tolerance to water and light availability, the third study involved an ex-situ greenhouse experiment. This revealed distinct physiological responses of four epiphytic ferns to water and light availability, which were associated with epiphyte microhabitat preferences. During a 12-week simulated drought, the species most closely associated with tree ferns (Asplenium polyodon) exhibited a rapid decline in photosynthetic capacity (Amax). The generalist Pyrrosia elaeagnifolia was drought-resistant, maintaining Amax and stomatal conductance (gs). Unexpectedly, the generalist Lecanopteris pustulata was not drought-resistant, exhibiting significant declines in Amax and gs. Asplenium flaccidum (associated with tree ferns in urban settings) was drought-avoidant, with low but constant Amax and gs. Light environments mimicking two distinct host microhabitats (tree ferns and woody trees) revealed greater shade tolerance in A. polyodon than P. elaeagnifolia, as indicated by predawn Fv/Fm. High light conditions exacerbated plant drought stress, as shown by reduced recovery compared with plants under low light.

Finally, epiphyte community data were revisited to explicitly test the influence of environmental predictors on obligate epiphyte diversity in constrained (i.e., urban forests) and unconstrained (i.e., non-urban forests) host communities. Obligate epiphyte communities in urban forests were more homogeneous than in non-urban forests, supporting fewer epiphytic shrubs, lycophytes and orchids. SEMs showed that urban epiphyte richness was indirectly influenced by forest age and tree fern basal area, and directly positively influenced by relative humidity stability and Dicksonia squarrosa basal area. In non-urban forests, epiphyte richness was positively associated with tree fern and woody tree basal areas. Epiphyte richness increased with host species richness and was influenced by host assemblage composition. Tree fern basal area positively influenced epiphyte richness in urban forests, although woody tree basal area had a negative relationship, which may reflect a lack of high-quality host species.

This research has expanded our understanding of epiphyte communities in temperate cities and how host assemblages, morphological and physiological traits, and environmental conditions influence them. It also provides a basis for implementing ecological restoration of diverse host assemblages that provide sheltered, damp habitat suitable for epiphytes. Future long-term studies on a wider range of species will be needed to build upon this research.