Barriers to invasion: how do climate, microhabitat, and biotic interactions limit the establishment of Pinus radiata and Pinus radiata × Pinus attenuata in New Zealand high country? : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Pinus radiata (Monterey pine) is the most widely planted conifer in New Zealand with self-established populations found almost across the entire latitude of the country up to 1060 m in elevation. While invasion success is common in the lowlands and coastal areas, it is considered less common in the South Island high country due to its cold-intolerance. Cold-intolerance places elevation limits on P. radiata plantation establishment, and thus the species has recently been hybridised with a cold-tolerant congener, P. attenuata, for forestry purposes at higher elevations. As the establishment of both P. radiata and P. radiata × P. attenuata plantations is expected to increase with afforestation and carbon sequestration schemes, understanding the barrier(s) to invasion that limit their establishment in the high country becomes increasingly important. The aim of my thesis is to identify the climate and biotic barriers to invasion that may prevent naturalisation of P. radiata and P. radiata × P. attenuata in the Canterbury high country, and to assess the intensity of each barrier associated with early life stages (i.e. seed, emerged seedling, and established seedling). Biotic barriers were assessed through experiments along an elevation gradient of 600, 900, and 1100 m a.s.l., where elevation was a proxy of climate. The biotic factors assessed included post-dispersal seed predation, the influence of neighbouring plants on emergence and early establishment, and herbivory of established seedlings. Post-dispersal seed predation varied among elevations and microhabitats. There was no consistent preference between Pinus taxa or microhabitats among elevations. Microhabitat preferences in post-dispersal seed predation is likely a result of the foraging preference of introduced rodents and birds, and their interaction with the dominant vegetation. Temporal assessments found seed predation to be higher in spring compared to summer. Emergence of sown seeds was lowest in disturbed plots and highest within tussock grasses, while early establishment was highest in inter-tussock vegetation (both disturbed and undisturbed microhabitats) at 600 m and within shrubs at higher elevations. These results suggest neighbouring plants facilitate emergence across the elevation gradient assessed, but facilitate early establishment at higher elevations only. The survival of transplanted seedlings was over 90% for both taxa. Herbivory was most frequent and intense in the first year and at 600 m. Over the course of the study no level of herbivore incidence or intensity recorded induced mortality. I show that none of the potential biotic barriers assessed were sufficient to prevent naturalisation of P. radiata or P. radiata × P. attenuata up to 1100 m in the Canterbury high country, independently or collectively. My study did not identify an elevation limit to establishment for either taxon. Given both taxa are able to establish successfully, my study suggests naturalisation and invasion success are highly probable, with P. radiata × P. attenuata the higher risk taxon.