Sakai and Wardle: Freezing Resistance of New Zealand Trees and Shrubs

Forty-two native woody species were selected to represent the range of temperature-related ecological gradients in New Zealand. Twigs collected in July (mid-winter) were sent by air to Sapporo, Japan, where they were artificially hardened and then tested for freezing resistance. Although only one plant of each species was sampled from each site in anyone year, results were consistent from year to year. Freezing resistance of the plants correlates well with their natural distribution; for leaves this ranges from -4 C or warmer for broad-leaved species not extending south of 39S, to -18C to -25 C for the hardiest high-altitude shrubby conifers. Ecotypic differences are also apparent. Air temperatures only occasionally reach or exceed the freezing resistance of local native species, but ground frosts are more likely to affect seedlings in frosty depressions. Other temperate, southern hemisphere, evergreen trees grown at Christchurch, New Zealand, show similar freezing resistances to native species, but two deciduous species of Nothofagus are hardier than evergreen Nothofagus. Even the hardiest New Zealand trees and shrubs compare only with trees growing in warm temperate, lowland districts of Japan.