The influence of tree ferns on the composition and structure of New Zealand native forests

Abstract

Tree ferns are an abundant, ubiquitous and conspicuous feature of New Zealand’s indigenous forests, creating shady understories and substantially contributing to litterfall, amongst other effects. Previous research has hypothesized that tree ferns influence forests by suppressing the establishment of conifers in particular, and stalling successions. However, these hypotheses have not been examined, the importance of tree ferns to forest structure and dynamics in general is poorly understood, and variation in the ecological roles of different tree fern species has not been elucidated. The aim of this thesis was to determine the importance of tree ferns to New Zealand forests, and explore the mechanisms underpinning the different distributions and roles of the different species. First, the way in which tree ferns enter ecosystems was considered through a laboratory experiment on gametophytes of three New Zealand species. Second, using field data and manipulative experiments, I compared possible mechanisms by which tree ferns might influence community composition. Finally, the effects of different tree fern abundances on forest structure over 2,500 year periods were assessed using simulation modelling. Using an experimental approach I quantified gametophyte growth rates in varying orthophosphate concentrations and irradiance levels. These experiments suggested that gametophyte tree ferns of pioneering (Cyathea medullaris) and understorey (Cyathea dealbata and Cyathea smithii) species have different ecological strategies. Different gametophyte development rates suggest niche differentiation between understorey species. Though common in the landscape in northern New Zealand, I recognised and described a successional pathway based on C. medullaris as a pioneer canopy with abundant C. dealbata in the understorey. I present correlative associations, between tree fern basal area and dissimilarities across seedling communities, indicative of tree fern biotic influence on community assembly. This tree fern successional pathway leads to forests dominated by shade-tolerant broadleaf trees, and differs significantly from successional trajectories predicted under angiosperm pioneers such as Kunzea or Leptospermum. Observations on the effects of understorey tree ferns showed that they suppressed seedling densities by up to 50% suggesting long-term suppression of canopy species establishment. Also, conifers were present at significantly lower densities within the canopy drip-lines of tree ferns, consistent with previous observations. Experimental manipulations removing tree fern fronds led to a consistent increase in the establishment of shade-intolerant conifers. Using in-silico experiments, I showed that increasing tree fern densities was associated with increased height and age of angiosperms and conifers. Notably, increased tree fern density was associated with increased conifer persistence, contradicting assertions in the literature. Seedling suppression underneath tree ferns appeared to affect tree fern regeneration more than woody vascular plants, ultimately promoting canopy dominants over long time periods. Overall, I conclude that tree ferns are a fundamental component of the indigenous temperate rainforests of New Zealand playing a significant role in their structure and composition. Further research on the sporophyte niche, mortality and inter-specific gametophyte competition is recommended.