Abstract:
Multiple factors control species distributions, which are challenging to disentangle due to
their complex nature. Where the range limits of multiple species coincide biogeographic boundaries form, which is often coincident with rapid changes in environmental factors. The Kauri Line in New Zealand at 38°S is an example of a biogeographic boundary, which aligns with the southern limit of kauri (Agathis australis). The mechanisms behind the formation of
the Kauri Line however, have not been studied rigorously.
In this thesis, I aimed to analyse the existence and potential mechanisms explaining the Kauri Line. I first explored the strength of the biogeographic boundary in terms of average compositional changes in vegetation and rates in relation to latitude in New Zealand. The multivariate analysis employed identified a compositional split at the Kauri Line, and the coincidence of multiple range limits in a broad swathe centred on the Kauri Line, extending
~1.5° north and south. I found the boundary was weakened when high elevation sites (>370m asl) were excluded. Also, many species with southern limits coinciding with the Kauri Line were associated with A. australis, which creates distinctive habitats through soil modification. These results suggest an increase in mean elevation and the absence of A.
australis south of the Kauri Line may be contributing to its strength.
Second, I used species distribution modelling to quantify the climatic envelopes of plant species with different spatial distributions and to identify climatic variables that were strongly predicting the presence of species with southern limits near the Kauri Line. Species distribution modelling identified June solar radiation as the primary control on the presence of Kauri Line species. However, the predicted climatic ranges of the Kauri Line species in
terms of the six climatic variables tested differed, suggesting that multiple factors are setting
the southern limits of Kauri Line species and that the importance of each variable differs interspecifically. Finally, I evaluated the role of frost in setting the southern limits of Kauri Line species, by comparing frost-tolerance of seedlings of Kauri Line and more widespread
species through artificial freezing experiments on whole plants and electrolyte leakage tests on leaf samples. The southern limits of plant species and their frost tolerances were positively
associated, with Kauri Line species being intolerant of frosts. Overall, this thesis gives an
insight into the Kauri Line and the various factors that may lead to its formation, identifying
opportunities for further research on this boundary and others of its nature.