Disjunction of Tree Species in Mountain Forests, Southern North Island, New Zealand: A Review of Paleobotanical Evidence

Abstract

Dominant trees of New Zealand forests, particularly Nothofagus, which has low seed mobility, show major distribution gaps, associated with past glacial and volcanic destruction of forest. In the southern North Island there are forest tree discontinuities distant from volcanic destruction and subject only to periglacial activity in the Pleistocene. Here there is absence of one taxon of Nothofagus and of some mountain coniferous trees, including podocarps, with bird-carried seeds, and Libocedrus, with winged seeds. The southern North Island, from 40° S to 41 ° 30' S, shows a progressive southward loss in montane and subalpine tree species. These species are common, often dominant, to the north of the area described, and in the northern South Island. In order of increasingly southern limits in the North Island they are Halocarpus bidwillii, Nothofagus solandri var. cliffortioides, Libocedrus bidwillii, Halocarpus biformis, and Phyllocladus alpinus. Their pollen and macrofossil history is reviewed. Libocedrus bidwillii was present in the southern North Island 80,000 yr ago, and Halocarpus and P. alpinus until ca. 10,000 yr ago. Various glacial episodes since 80,000 yr ago have severely limited their distribution. The postglacial reoccupation by forest of southern North Island sites was influenced by rapid climatic warming. This warming not only flooded the Cook Strait landbridge, cutting off tree migration from the south, but also allowed lowlands around the gorge bisecting the axial mountains to be occupied by temperate forest, effectively blocking access southward by L. bidwillii and N. solandri var. cliffortioides. The mountain podocarps, H. biformis and P. alpinus, now abundant just south of the gorge, are in a particularly cloudy climate, which lacks Nothofagus. Farther south, under less cloudy conditions, the rapid expansion of N. menziesii forest from lowland refugia could have excluded the two small podocarps, which were present in pollen records in the early postglacial.