Seasonal cycle of tolerance to low temperatures in three native woody plants, in relation to their ecology and post-glacial history.

The post-glacial pollen sequence for inland regions of the eastern South Island is from grass and Coprosma _ Dacrydium (probably D. bidwillii) _ Phyllocladus (probably P. alpinus) _ Podocarpus and Dacrycarpus _ Nothofagus (solandri and/or fusca). If this is interpreted simply as a response to climatic warming, the position of Dacrydium is anomalous, as at present it does not often reach altitudes as high as Phyllocladus and Nothofagus solandri var. cliffortioides. Nor can its position be explained in terms of pedogenesis, for it is characteristic of overmature soils that are leached and often boggy.
Dacrydium bidwillii stands are mostly situated on valley terraces subject to severe frost. This suggests (1) that this species is more tolerant of winter cold than either Phyllocladus alpinus or Nothofagus s. cliffortioides even though the latter reach the alpine tree limit, and (2) that early post-glacial times, as well as being generally cooler than now, may also have been characterised by severe temperature inversions. To test these hypotheses, twigs of the three species were collected from mature plants throughout one year, and subjected to artificial freezing. Temperatures were dropped gradually, and samples removed at intervals of 1-4¡C. Species were consistently in the following order of increasing resistance to freezing damage: Nothofagus collected at 800 m, Nothofagus at 1200 m, Phyllocladus at 1200 m, Dacrydium at 700 m, although the hardiness of each species varies according to season.
These results are in good agreement with patterns of damage in the field although, in Nothofagus at least, natural damage during winter is manifested as "frost desiccation". Experimental results are also consistent with present ecology and relative abundance in the pollen record.