Montane outcrop vegetation of Banks Peninsula, South Island, New Zealand

Species composition patterns and vegetation–environment relationships were quantified for montane volcanic outcrops on Banks Peninsula. The flora of these habitat islands comprises 346 vascular plant species including 82 exotic species and 52 species that are nationally rare, regionally rare, or regional endemics. Both MDS ordination analysis and TWINSPAN results illustrated the high compositional and environmental heterogeneity across the outcrops. Multi-dimensional scaling revealed that primary environmental factors related to community composition comprise both regional-scale gradients of altitude and outcrop-scale gradients of slope steepness, soil pH, area available to plants, maximum vegetation height, and the percentage of the surrounding vegetation that is forest. Accordingly, TWINSPAN separated four outcrop communities that occur on steeper slopes, have less fertile soils and tend not to face north from three outcrop communities that have shallower slopes, more fertile soils and tend to face north. Types in the first group are more likely to be bordered by forest or taller shrublands, whereas those in the second group occur on outcrops primarily bordered by grasslands and support more exotic species. Within these broader groups, communities differ in their altitude and the size, soil depth and shading of the outcrops on which they occur. We describe the vegetation of the seven communities; this ranges from predominance of stunted trees and taller statured species such as Podocarpus hallii and Phormium cookianum to vegetation of shrubby species such as Heliohebe lavaudiana and Hebe strictissima, to short vegetation of native woodland and grassland species such as Polystichum vestitum and Rytidosperma corinum, to exotic pasture of clovers and exotic grasses. The percentage of species on an outcrop face that are exotic is well modelled by site factors, with exotics increasing as the surrounding matrix becomes more disturbed, slopes become more gentle, the percentage of shade on the outcrop decreases, and soil fertility increases. In contrast, nearby disturbance has little influence on the percentage or number of species that are rare on an outcrop face; rather rare species richness is more strongly related to outcrop area and lack of shade, echoing patterns observed for rare outcrop species elsewhere in the world. These results highlight the importance of considering the high compositional heterogeneity among outcrops and the influence of disturbance to surrounding ecosystems in guiding conservation planning.