The vegetation pattern of Rangitoto

Abstract

The vegetation of the island of Rangitoto was examined in order to determine the current vegetation pattern and to identify the factors which have influenced the development of this pattern. Information about the order and dates of eruptive events was reviewed to gauge the length of time that the various surfaces of the island have been available as a substrate for the development of a vegetation covering. Available geological information, dating evidence, historical accounts, tree ages, and Maori evidence all point to a single period of eruptive activity, probably only several years in duration, around the mid- to late-1300's (A.D.). The order of eruption was probably production of the ash that covers neighbouring islands, followed by cinder cone building, followed by eruption of the lava flows. The lava flows were found to consist of clinkery aa flows, blocky aa flows, and a flow type referred to as Rangitoto slab flows. These flow types could be partially distinguished from one another, using discriminant analysis, on the basis of the length of the longest fragment on a 5x5m plot and the number of fragments on a lxlm subplot. A new transition sequence of flow types from pahoehoe to aa lavas is proposed for viscous lavas undergoing low rates of shear strain. The vegetation pattern of the lava fields was examine using TWINSPAN and CANOCO analysis of foliage cover percentage information gathered from 125 5x5m plots. It was found to consist of a successional sequence of vegetation arranged in a mosaic. The mosaic was found to relate strictly to the underlying lava flow surface. Large Metrosideros grow in crevices on slab flows and next to large boulders on aa flows. Smaller Metrosideros grow on small slabs on slab flows. Mixed species scrub is found growing on unbroken slab surfaces. The relative rate of colonisation of different types of flows under different climatic conditions was considered. The Metrosideros hybrid swarm was studied using morphometric information. The putative parent species of the swarm were found, using Principal Components Analysis, to be Metrosideros excelsa and Metrosideros robusta. The Rangitoto Metrosideros population was found, using Canonical Variates Analysis, to be the result of hybridisation, followed by introgression towards M. excelsa . The probable F1 hybrids grow in early successional stage vegetation. The major geographical trend is the tendency for backcrossed hybrids to grow on the eastern side of the island, suggesting eastern origins of M. robusta seeds. The impact of browsing animals on the vegetation was studied over five years in exclosures and corresponding control sites. Metrosideros foliage recovered slightly. Griselinia lucida and Cyathodes juniperina seedlings were recruited into the shrub layers in exclosures, but not in control sites. Astelia seedlings also benefitted from the absence of browsing pressure, as did Thelymitra longifolia. The distribution of browsing animal populations in relation to the vegetation pattern was studied using faecal pellet recruitment data gathered by the New Zealand Forest Service in 1984. Both wallabies (Petrogale penicillata penicillata) and possums (Trichosurus vulpecula) were found to be distributed principally according to the amount of palatable foliage available to each species in each vegetation type.