Frost Resistance of Subalpine Eucalyptus Species. I. Experiments Using a Radiation Frost Room

Abstract

Frost resistance of seedlings of four subalpine Eucalyptus species was studied using a frost room that simulated natural radiation frosts. Seedlings from populations at and above valley-bottom treelines were examined. With cooling rates similar to those of natural frosts, initial freezing was detected within the temperature range – 2.1 to – 6.9 °C, with a mean initial freezing temperature of -4.5°C. Seedlings in an unhardened state, produced by a temperature regime of 23°C day / l8°C night, had very little freezing resistance, with complete leaf death being produced by exposure to - 6° for 4 h. A hardening regime of 16/4°C, with 12-h-photo- and thermoperiod, enabled E. pauciflora seedlings to harden in 3-4 weeks to the extent that the mean leaf damage produced by a frost of - 8.5 °C was about 50%. Hardening for longer periods under this regime enabled seedlings to withstand frosts below - 10°C with very little damage. A 9/4°C regime was much more effective than a 919 °C regime in promoting hardening. E. pauciflora leaves sprayed with water before a frost were slightly more damaged than dry leaves. The greater damage appeared to be the result of the slightly lower temperatures of the wetted leaves during the frost. Two damaging frosts, occurring within 2-3 days, produced slightly more damage than that which would be predicted by adding the damage that would have been produced by each frost acting separately. E. stellulata and the 'pendula' form of E.pauciflora, the populations extending furthest down gradients of increasing frost severity to the valley bottoms, had the greatest seedling frost resistance. Treeline populations of E. rubida and the 'forest' form of E. pauciflora were intermediate in resistance, and E. dalryrnpleana, a species that did not extend to the treeline, was least resistant. Examinations of variation in frost resistance of E. pauciflora seedlings from parent trees sampled along two environmental gradients produced two different results. Frost resistance increased markedly down a transect from forest form of E. pauclflora on a hillside to pendula form of E. pauciflora near a valley bottom, but a corresponding increase did not occur down another transect where the pendula form was absent.