Plant biogeography and the late Cenozoic history of New Zealand

Abstract

Patterns of regional endemism. vicariance. and disjunction in New Zealand higher plants are reviewed. These are discussed in relation to the post-Oligocene history of the geology. climate .. and vegetation. Previous explanations for su~h distrIbution patterns have centred on the disruptive effects of ice and severe climates during the Last Glaciation. and subsequent migration of plants from glacial refugia during the postglacial. It is conel uded that these explanations are largely Inadequate. It is suggested that many endemic. vicariant. and disjunct plant distributions are related to the large-scale modification of the New Zealand land mass which has occurred as a result of active tectonism since the Oligocene. The more stable regions of New Zealand (in particular Northland. northwest Nelson. and Otago) have retained di verse floras partly as a result of retention of older clements of the flora which more radically altered areas (southern North Island. central South Island) have tended to lose. The rapidly rising Southern Alps may have acted as a centre of speciation because of its provision of novel alpine and subalpine environments. Glaciations have aifec!ed distribution patterns mainly through permltt1l1g the wide spread of glacial environment specialists.

A late Otiran-Holocene paleoenvironment reconstruction based on cave excavations in northwest Nelson, New Zealand

“…Present evidence suggests that shrublands and grasslands were widespread during the Otiran on the West Coast of South Island, south of Westport (McGlone 1985). No information is available for northwest Nelson although Macphail (1983) has shown that the early Holocene vegetation was dominated by Nothofagus menziesii and Cyathea smithii later replaced by N.fusca group.…”

“…However, none of these studies was for western areas of northwest Nelson, and, because they were mostly concerned with glacial chronology, were done near former glaciers where grassland/ shrubland would have been much more likely to occur. As indicated by McGlone (1985), in these former studies,pollen samples were taken from sites of rapid sediment accumulation that were likely to have been sparsely vegetated.…”

“…It was hoped that potential changes m the faunal composition from the glacial to the Holocene could be described. Stratified deposits containing extensive faunas from this time, when the climate and vegetation cover were very different from today (McGlone 1985), were previously unknown in New Zealand, although several more limited faunal assemblages are known (see odd specimens dating from this period listed by Millener (1981) and Worthy (1987b)). …”

Environmental and floristic gradients in Westland, New Zealand, and the discontinuous distribution ofNothofagus

“…Most areas which lackNothofagusare characterised by maritime superhumid climates with comparatively small annu~temperature ranges and probably only rarely expenence drought. Even authors supporting the glacial refugia hypothesis realised the competitive advantages of podocarp-broadleaved forest species over Nothofagus under such climatic conditions (Wardle 1963;Burrows 1965), but assumed that this would only retard the process of re-immigration, McGlone (1985) proposed an alternative "tectonic" hypothesis which implies that the major floristic disjunction between the north-western and south-western South Island is the result of rapid large-scale environmental change since the Late Miocene. He assumes that the displacement of Nelson and Fiordland along the Alpine Fault in connection with the rapid rise of the Southern Alps created an extensive novel habitat in central Westland which is more suitable for the resident podocarp-broadleaved forest and related montane and subalpine vegetation types than to the otherwise widespread Nothofagus forests, which are excluded by biological competition.…”

“…Recently, several authors have expressed doubts about the concept ofLate Pleistocene extinction and still incomplete re-immigration of forest trees and suggest that the discontinuous distribution of Nothofagus is mainly the result of climate and competition (McGlone 1985;Sweetapple 1985;Wilson 1987). J.…”