In this thesis I investigated the mutualism between Kunzea ericoides (kanuka) and two groups of soil fungi, ectomycorrhizae (ECM) and arbuscular mycorrhizae (AMF). Mycorrhizal mutualisms, which are considered globally ubiquitous, are poorly understood over changing abiotic gradients. A field survey of K. ericoides assessed how the relationship with the soil fungi varieties altered over a hydrologic gradient. Arbuscular mycorrhizal colonisation was significantly improved by increasing rainfall levels and amount of surrounding kanuka, and negatively affected by increasing altitude. Ectomycorrhizal colonisation was not significantly affected by any measured variables and remained relatively constant across all circumstances, suggesting that it is the preferred fungal mutualist for this tree species. A glasshouse experiment was done to measure the effect of ECM inoculation on the growth and water usage of K. ericoides over varying moisture availability. The seedlings were planted in soil inoculated with both ECM and AMF (experimental) or AMF only (control) and then grown under varying levels of water stress. The experiment was replicated with two soil types, with soil from beneath adult manuka (Leptospermum scoparium) and soil from beneath adult kanuka (K. ericoides). ECM colonisation significantly increased as soil moisture decreased for both soil types. ECM inoculation also increased the root:shoot ratio, and drastically decreased water usage under drought conditions. There were some soil effects as the seedlings grown in manuka-soil achieving greater biomass than seedlings grown in kanuka-soil. This is possibly due to presence of pathogens or some type of legacy competition which the seedlings would experience growing near conspecifics. Overall, K. ericoides formed a dominant mutualism with ectomycorrhizae. These two both thrive in dry environmental conditions and have a suite of complementing abilities which possibly allow K. ericoides to expand it range into these dry habitat types. The increased benefit of these mutualisms at the hydrologic range limit of the species supports the importance of biotic interaction mediating environmental stress. Understanding the effects and response of mycorrhizal mutualisms are especially significant considering current climate change issues in New Zealand and worldwide.

Pollination and fruit set of four species of native New Zealand flowering plant species were examined through two field seasons. Bird exclusion, pollinator exclusion, natural and supplemental pollination treatments were initiated on individuals of Cordyline australis (Cabbage tree), Phormium tenax (Flax), Kunzea ericoides (Kanuka), and Pseudopanax arboreus (Five-finger). The species differed in the self-compatibility as well as in their floral syndrome. No species showed any evidence of pollen limitation, and two species. K. ericoides and P. arboreus set fruit from more than 70% of their flowers. The response of fruit set to treatment in C. australis varied from season to season, with birds appearing important to pollination in the first but not the second field season, while birds were important in pollination across both seasons for P. tenax. K. ericoides was resilient to treatment, setting high fruit set in every treatment, compared to P. arboreus which set high fruit set when pollinators had access, but low when all pollinators were excluded. No trends relating to fruit set or PLI and self-compatibility or floral syndrome were found. That there was no evidence of pollen limitation for any species, despite variation in fruit set from some treatments, indicates that these species are performing well and not at risk of decreased population size due to pollen limitation.

In New Zealand, native shrubs are considered an important potential carbon-sink in disturbed or abandoned land (e.g., pastoral land that is unsustainable for long-term pastoral agriculture). However, the impact of varying environmental drivers on carbon uptake from photosynthesis and carbon loss from respiration of a developing shrubland remains uncertain. In this study, the effects of both temperature and soil water content (θ) on photosynthesis and respiration were examined under controlled growth cabinet and field conditions in a pasture grass and the native shrub, kānuka (Kunzea ericoides var. ericoides). The purpose of the investigation was to assess the combined impacts of varying temperature and θ on canopy processes and to disentangle the effects of θ on photosynthesis and respiration for the two different plant types. A controlled growth cabinet study (Chapter 2) showed that θ had a greater effect on the short-term temperature response of photosynthesis than the temperature response of respiration. The optimum value of θ for net photosynthesis was around 30 % for both kānuka and the grass. Statistical analysis showed that the temperature sensitivity of photosynthetic parameters was similar for both plant types, but the sensitivity of respiratory parameters was different. Reduction in θ induced an inhibition of photosynthetic capacity in both plant types. The response of respiratory parameters to θ was not related to substrate limitations, however available evidence suggests that it is likely to be a species dependent plant mechanism in regulating the cost of maintenance due to reduced photosynthate assimilation and decreasing energy supply to support the activity of respiratory enzymes. Results obtained from a field study (Chapter 3) showed that photosynthesis and respiration in the grass and kānuka were sensitive to seasonal changes in temperature and θ. Photosynthetic parameters showed little acclimation following changes in seasonal growth conditions. In contrast, respiratory parameters tended to acclimate more strongly. Respiratory acclimation to multiple environmental conditions was characterised by changes in temperature sensitivity and a shift in the response of respiration to temperature, demonstrating the involvement of both ‘Type I’ and ‘Type II’ acclimation in both plant types. The results from controlled growth cabinet and field studies were used to drive a leaf level model that integrates the responses of photosynthesis and respiration to changes in temperature and θ and incorporates acclimation using variable photosynthetic and respiratory parameters (Chapter 4). This model was used to estimate the annual canopy carbon exchange of the grass and kānuka in response to seasonal changes and to predict changes in canopy carbon exchange under varying future climate change scenarios. The model highlighted the importance of considering seasonally-acclimated parameters in estimating canopy carbon exchange of both plant types to concurrent changes in multiple environmental variables. The overall results support the conclusion that understanding the combined effects of environmental variables on canopy processes is essential for predicting canopy net carbon exchange of a pasture-shrub system in a changing global environment. It has been shown here that the rate of increase in photosynthesis with increasing θ is greater than that of respiration which results in a progressively greater apparent carbon gain at moderate values of θ. Moreover, the impact of lower values of θ, which reduced the apparent sensitivity of respiration to temperature, may effectively decrease the rate of respiration during warmer summer months and enhance thermal acclimation via downregulation of respiration. Therefore, considering the influence of soil water conditions on the temperature sensitivity of photosynthetic and respiratory model parameters has important implications for precisely predicting the net carbon exchange of a pasture-shrub system.