Ranges of woody plant species and ferns on forested elevational gradients on Aotea-Great Barrier Island, New Zealand: the role of zones of permanent and temporary establishment

The arrangement of plant species along elevational gradients is prominent in the debate between individualistic versus community-unit concepts in plant ecology. We obtained elevational ranges (upper and lower elevation limits) for woody species and ferns on the three highest mountains on Aotea (Great Barrier Island), Aotearoa-New Zealand. These data allowed potential species composition to be obtained for any elevation and were analysed using regression, ordination, and classification. Both woody plants and ferns demonstrated linear declines in richness with increasing elevation. The three mountains were distinct in ordination space, and their differences increased with elevation and were most marked in the fern flora. Site classifications indicated a change in composition at c. 300 m for woody plants on Hirakimata. The elevational distribution patterns of individual species differed considerably between mountains. In particular, the higher-elevation flora of the highest peak (Hirakimata) was largely absent from the other two summits, despite these being within the elevational ranges of most of the missing species. This pattern supports Steenis’s (1972) hypothesis that species will be absent from, or ephemeral on, mountains not sufficiently high to intersect their zone of permanent establishment. A second observed pattern was that widespread forest species at lower elevations often did not extend to their potential upper limits on Hirakimata. These two patterns are clear for both woody plant species and ferns. Combining Steenis’s hypothesis with inferred redistribution of elevational vegetation during the Last Glacial and Holocene periods can explain these distributional patterns.