Maximal resistance to winter freezing of trees of the South Temperate Zone, especially subalpine trees of Australasia, was assessed. Most of the tree species which grow in lower altitudes were marginally hardy to -I 0°. 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 hardy to -15°. Of the Eucalyptus species, E. paucijlora 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 tree genera, with minor exceptions, have been unable to cross the barrier formed by the tropics.

The available physiological evidence suggests that ontogenetic ageing of E. Grandis seedlings involves a direct and quantitative association between decreased rooting ability of stem cuttings and increased levels of a rooting inhibitor in the tissue forming the base of the cutting. As detected by bioassay, this inhibitor is present only in adult tissue, which very rarely forms roots from stem cuttings. It is absent in easily rooted seedling stems of all Eucalyptus species tested, but it is also absent in the easily rooted adult tissue of the exceptional species E. deglupta. The ability of seedling cuttings of E. deglupta to root very easily in water provides an appropriate bioassay for monitoring the presence of inhibitor in other Eucalyptus species.

No abstract

The events related to the leaf requirement (induction) in the late pea cultivar Greenfeast are separate from events (evocation) between induction and initiation of the first flower. Photoperiod influences induction, whilst vernalization influences evocation. Vernalization represses synthesis of the graft-transmissible floral inhibitor present in unvernalized plants where it delays rapid evocation. Nothing is known of the biochemistry of the floral inhibitor although there is some preliminary histo-chemical evidence which suggests an interesting relationship between presence of inhibitor and RNA content in the apex.

A catastrophic frost in the Bonang valley, Victoria (alt. 660 m), during Aug. 1982 caused severe damage to naturally occurring trees growing within a contour-related inversion layer. All eucalypts and acacias on the valley floor and up to 6 m above it on the lowest slopes were either killed outright or suffered complete crown loss. Between 6 and 25 m above the valley floor, tall Eucalyptus viminalis. trees escaped damage if their crowns were positioned well above the inversion layer. Neighbouring trees of the slightly shorter original codominant E. radiata were so severely damaged that they were either killed or survived crown destruction only by production of trunk epicormics. Despite being wholly positioned within the inversion layer, the shorter trees of E. pauciflora generally survived without much damage, although leaf injury continued to develop for several years in some trees. Results of the final assessment 5 yr after the frost indicate that the lowest surviving E. viminalis trees, together with severely damaged E. radiata. trees, now constitute an inverted treeline separating forest above from dead trees and grassland below. This treeline is related to the 666 m contour, 6 m above the valley floor, and forms a very abrupt boundary because of the predominance of large undamaged trees of E. viminalis. The consequences of this rare catastrophic frost have implications for subalpine ecology in south-eastern Australia, especially the genesis and stability of inverted treelines.