The formation of the oceanic temperate forests of New Zealand

Abstract

The pre-clearance New Zealand lowlands were largely in complex, liana-rich, multi-layered conifer–angiosperm forest. Here it is designated oceanic temperate forest (OTF), its climatic envelope is defined, global distribution is assessed and origin is reviewed. Often described as Gondwanan or relic, tree genera characteristic of the OTF are more likely to be shared with tropical regions to the north than with temperate fragments of Gondwana (southern Australia, southern South America). The OTF arose out of the warm temperate to subtropical forests of Zealandia that formed during the Palaeogene. Warm, fire-prone environments in the middle Miocene permitted an influx of Australian taxa. During the Pliocene/Pleistocene increasingly cool climates, less fire and loss of old, leached soils, reduced floristic diversity. Many low-nutrient specialists and arid-adapted Australian-origin taxa were lost, along with many now exclusively tropical genera. Reduction in ectomycorrhizal tall tree genera left only Nothofagaceae and myrtaceous Kunzea and thus OTF formations are largely arbuscular mycorrhizal. Lack of fast-growing, ectomycorrhizal cold-tolerant trees is a key reason for the marked physiognomic differences between northern temperate broadleaved forests and those of the New Zealand OTF. The pronounced oscillations in extent of dry, cool habitats and warm, moist habitats in this isolated archipelago during the Pleistocene had a profound effect on the composition and distribution of the OTF. We suggest two drivers have strongly modified both geographic and trait distribution in the flora. First, ‘glacial–interglacial asymmetry’—trees adapted to harsh glacial climates have more difficulty persisting in the face of strong biotic competition in warm, wet interglacial landscapes than trees adapted to interglacials do in persisting in climatically protected sites in cold, dry glacial landscapes. And second, the ‘Pleistocene ratchet’—the propensity of many forest species that lose range during a glacial (or interglacial) to fail to recover it entirely during the subsequent interglacial (glacial).