Effects of glacial climates on floristic distribution in New Zealand 1. A review of the evidence

Abstract

The hypothesis that the Otira Glaciation left a profound imprint on the present distribution of flora and vegetation is reviewed, in relation to an alternative hypothesis that patterns of endemism and disjunction stem from tectonic events extending back into the Tertiary. These hypotheses are not mutually exclusive, and both accept that there are “centres” of floristic richness in northern, central, and southern New Zealand that are separated by “gaps” showing relative poverty. Part 1 of the review concludes that endemic and disjunct taxa are neither especially associated with habitats that pre-date the Otira Glaciation, nor characterised by evolutionary antiquity. The Otira Glaciation is confirmed as a time of extreme environmental stress, which is likely to have caused more extinction in the “gaps”. The occurrence of cold-intolerant disjunct species in the southern and Nelson-Marlborough “centres” is seen not as evidence for glacial survival but as the result of post-glacial long-distance hybridisation with resident hardier species, followed by reconstitution of the less hardy species (see Part 2). The limited fossil evidence is consistent with post-glacial spread of lowland podocarp-broadleaved forest from the north of New Zealand, whereas the distributions of cold-tolerant beeches and some other species are consistent with spread from glacial refugia in several parts of New Zealand, including southern localities. The limits of the beech species against the southern “gap” are at right angles to the present environmental contours for geology, landforms, altitude, temperature, cloud cover, precipitation, and soils; and beeches are still spreading into this gap, from both north and south.