The role of New Zealand kauri (Agathis australis) in structuring forest community composition

Abstract

New Zealand kauri (Agathis australis) is known to exert a substantial influence on soil properties and nutrient cycling in its vicinity; modifying pH, available nutrient levels, and soil moisture regimes. There are also anecdotal accounts of distinctive plant associations that occur close to this species. This suggests a link between how kauri modifies the abiotic environment and the species composition of the forest it occurs in. However, little is known about the potential importance of kauri in shaping habitats, or the extent of its influence in shaping associated plant communities. This thesis quantifies the composition of the flora associated with mature kauri, identifying a significant effect by kauri on plant community composition. Plant species fell into three broad groups: species occurring predominantly within kauri stands, species most abundant where kauri is absent, and those with distributions unaffected by kauri. I then explored mechanisms that may enable kauri to affect the composition of its associated flora. A series of shadehouse and laboratory experiments examined the role of the chemical and physical environments of the soils formed beneath kauri in producing the vegetation patterns. I identified a negative effect of kauri soils on the growth of species not typically associated with kauri, and a neutral effect on common kauri-associated species. The low pH of the kauri organic horizon was found to have a considerable inhibitory effect on the germination and growth of species not typically found beneath kauri. In addition, the physical properties of this deep organic horizon result in comparatively low soil moisture levels during summer drought. Comparatively high drought resistance was found to be a characteristic common to the seedlings of species able to establish within kauri stands. Finally, species that occur on acidic soils typically have inherently low levels of nitrate reductase activity, however no evidence was found to suggest that kauri-associated species have lower abilities to metabolise nitrate than the understory species dominant in the surrounding forest. This study has identified kauri as a species of high ecological importance within its ecosystems; influencing plant community composition through its effects on microsite characteristics. The work contributes to global literature on the role of canopy species in habitat formation and vegetation dynamics.