Where and why have all the flowers gone? Depletion and turnover in the New Zealand Cenozoic angiosperm flora in relation to palaeogeography and climate

Abstract

The modern New Zealand angiosperm flora has many notable characteristics, such as a predominance of evergreen, perennial life forms, few nitrogen-fixing species, despecialised floral features and asymmetric genus—species relations. The origin of these features has been attributed to antiquity of the flora, isolation and/or environmental history. Using evidence from palynology and macrofossils, we investigate the characteristics of the mid–late Cenozoic angiosperm flora and the impact of environmental changes in land area and configuration, physiography and climate on the depletion and composition of the New Zealand flora. Climatic cooling, increasing isolation and tectonism have each acted as important environmental filters, contributing to regional extinctions and decreasing floral diversity, and inducing major turnover in the floristic composition of New Zealand. During the Miocene and Pliocene at least 15 families and a minimum of 36 genera were lost from the New Zealand flora. These included a range of life forms and physiognomically important taxa such as Acacia, Bombax, Casuarina, Eucalyptus, Ilex, many Proteaceae and several palms. The extinction and decline in richness of subtropical families was caused by the onset of cooling conditions in the Late Miocene—Pliocene, and exacerbated by the absence of significant land areas to act as refugia at lower latitudes. Many of these genera/families persist today on islands to the north (e.g. New Caledonia), reflecting mid-Cenozoic land conduits, and in Australia. The close floristic links with New Caledonia were probably maintained by intermittent island stepping-stones which facilitated interchange of subtropical taxa until the Late Miocene. The Pleistocene extinction of some genera, tolerant of warm-temperate environments (e.g. Acacia, Eucalyptus) may be a reflection of the fact that persistent mesic conditions favoured widespread dominance of dense rainforest during interglacials. The loss of these groups, containing diverse life forms and floral structures, suggests that many of the present characteristics of the New Zealand flora reflect strong selective pressures, mainly driven by climate change, in the Late Miocene, Pliocene and Pleistocene, rather than events of greater geological antiquity.