Freezing Resistance of Trees of the South Temperate Zone, Especially Subalpine Species of Australasia

Maximal resistance to winter freezing of trees of the South Temperature Zone, especially subalpine trees of Australasia, was assessed. Most of the tree species which grow in lower altitudes were marginally hardy to —10°. Subalpine and alpine shrubby species such as Podocarpus nivalis, P. lawrencei and Dacrydium bidwillii were the hardiest conifers in New Zealand and Australia, resisting freezing to —20° to —23°. This hardiness was comparable to that of conifers native to the warm temperate or temperate parts of Japan. In Nothofagus, the deciduous, subalpine N. antarctica of South America was the hardiest, resisting freezing at —22°. A New Zealand evergreen timberline species, N. solandri var. cliffortioides was marginally hardly to 15°. Of the Eucalyptus species, E. pauciflora which forms the alpine tree limit on the mainland of Australia was the hardiest, resisting freezing to —15° in the leaves. Other high—altitude angiosperm species tested mostly survived freezing to only —10° or —15°. Very hardy tree species that withstand freezing below —30° seem not to have evolved in the Southern Hemisphere, because the mild, oceanic winters did not provide the stimulus, and because hardy northern genera, with minor exceptions, have been unable to cross the barrier formed by the tropics.