Alpine Plants and Pleistocene Glaciation
The demonstration by Louis Agassiz and others that central Europe and North America had suffered under an Arctic climate at quite a recent geological date enabled Darwin to explain the discontinuous distribution of many alpine plants and
animals, on distant “mountain summits separated from each other by hundreds of miles of lowlands”. At the onset of a glacial period, he explained, temperate inhabitants retreated and were supplanted by Arctic ones, until a uniform Arctic fauna and flora covered the central parts of Europe. As the snow and ice retreated, Arctic forms followed the melting snow up the mountains “When the warmth had fully returned, the same Arctic species which had lately lived in a body together on the lowlands …… would be left isolated in distant mountain-summits (having been exterminated on all lesser heights) and in the Arctic regions.”74 Although the physical signs of glaciation in New Zealand have long suggested a similar history for our alpine plants it is only recently that concrete evidence of the former lowland distribution of mountain plants has been available. New Zealand paleobotanists, in the first instance Mr. D. R. McQueen, have, for example, found leaves and seeds of the alpine toatoa Phyllocladus alpinus and, apparently, a variety of Astelia linearis (now restricted to high altitudes) almost at sea level near Wellington, in deposits studied by J. W. Brodie75—deposits which Cotton and Te Punga had already diagnosed, on quite independent evidence, as the result of frozen-soil processes during a periglacial climate.76 Thanks to nuclear science, we now know that these alpine plants were living on the motorway near Porirua Harbour in a period of glacial advance about 20,800 years ago.
Darwin knew of the “direct evidence of former glacial action in New Zealand”, and believed that “the same plants found on widely separated mountains in this island, tell the same story” of former lowland distribution during the Glacial Period.77.
[Footnote] 74 Origin: 312.
[Footnote] 75 Brodie, J. W., 1957. N. Z. J. Sci, Tech., B38:· 623–43. Owing to Mr. McQueen's departure from New Zealand, the paleobotanical evidence has not yet been adequately documented. Miss R. Mason (Botany Division, D.S.I.R) has recently confirmed that seeds of Astelia linearis Hook. f. var. linearis are present in sample N 160/502, Tawa Flat (N.Z. Geol. Surv. B542).
[Footnote] 76 Cotton, C. A., and Te Punga, M. T., Trans. Roy Soc. N.Z., 82: 1001–31.
[Footnote] 77 Origin: 316
Fig. 6.—Lowland distribution of alpine plants during a glacial phase of the Late Pleistocene (about 20,000 years ago). Crosses show localities for fossil remains of two plants (Astelia aff linearis and Phyllocladus alpinus) of which the living representatives now live in the upper cold temperate and sub-polar belts (stippled and black) of the southern Tararua Range where P. alpinus is recorded only from Mt. Omega (O) and Mt. Renata (R).