Frost as a Factor Influencing the Growth and Distribution of Subalpine Eucalypts

Abstract

An examination was made of the effect of natural frosts on pole stands of eucalypts growing within and surrounding a shallow depression at Snug Plains (alt. approx. 600 m) in south-eastern Tasmania. Marked differences in microclimate occurred between the slopes surrounding the depression and the base of the depression. The most severe frosts were experienced by the site at the base of the depression, and during a cold spell in June 1983 a record minimum temperature of - 22°C was recorded just above the radiating surface at this site. Pronounced vertical stratification of the air occurred (up to 9°C per m) and a difference in minimum temperature of 7.3°C was recorded over a distance of 200 m between a ridge-top site and the site at the base of the depression. Cooling rates of up to 6.5°C per h were recorded during these severe frosts.

The effect of frost in determining the distribution of six local species of Eucalyptus was examined by comparing the damage caused to mixed pole stands of five of the six species by the cold spell in June 1983. The order of frost sensitivity for fully hardened pole stands from the most resistant to the most susceptible was E. gunnii > E. coccifera > E. johnstonii > E. delegatensis > E. pulchella. The natural distribution of these species was closely related to the minimum temperature recorded at the various sites. Intense frosts during June 1983 caused marked changes in the dominance of mixed stands even though relatively few individuals were completely killed. These results suggest that rare, exceptionally severe frosts may play an important role in determining the distribution of subalpine eucalypts.