New Theory On History Of N.Z. Plant Species

Press, Volume CII, Issue 30108, 17 April 1963, Page 20

New Theory On History Of N.Z. Plant Species

A new theory to account for the distribution of New Zealand plant species is set out by Dr. P. Wardle, of the Botany Division, Department of Scientific and Industrial Research, Lincoln, in the first issue of the “New Zealand Journal of Botany.” Dr. Wardle believes that extreme climatic conditions during the Pleistocene iceage eliminated most species from all but a few refuges in the South Island and from practically the whole of the southern half of the North Island. Species on the outlying islands were not so severely affected. In the South Island, the chief refuges were in coastal Otago and Southland, at the eastern tip of Banks Peninsula, and in Nelson and Marlborough. The critical factor was low temperature at the height of the glacial phases. At these tames “there would have been dry grassland on the Canterbury foothills, but barren, unstable, braided riverbeds probably extended over the greater part of the plains.”

In the southern half of the North Island, < large numbers of species were eliminated because the mountains were not high enough to provide refuges for some of the alpine species during the warm interludes—sometimes warmer than now—which occurred during and since the ice-age, nor for some of the more tender species during the height of the glacial phases. Dr, Wardle, assuming the truth of theories that the New Zealand of the Miocene

pre-ice age period consisted of a low-lying archipelago extending far south of the present main islands, and that the climate in the latitude of the main islands was much warmer than now, suggests that the ancestors of the alpine and cool-tempera-ture species of the presentday mountains and uplands lived in the southern islands, and perhaps even in Antarctica. With mountainbuilding an dcooling climates, species from the south, perhaps aided by temporary land-bridges, migrated to New Zealand to occupy southern and mountain areas. In the extremities of the South Island, the range of habitats was sufficiently large for the species to maintain themselves during all the climatic fluctuations of the Pleistocene iceage. In putting forward his theory. Dr. Wardle runs counter to a theory put forward by Dr. L. Cockayne 30 years ago and generally held since. Dr. Cockayne thought a dry-climate flora evolved in the drier parts of the South Island during the iceage, citing as evidence the similarity of some New Zealand plant species, and of the youthful forms of other species, to plants from deserts. Dr. Wardle believes these “xeromorphic” traits of New Zealand plants are a response to conditions on the floor of New Zealand forests in dry or fairly dry situations.