Abstract
For approximately a fortnight at the beginning of July 1996, following heavy snowfalls to sea-level, the agricultural districts of Southland, West and South Otago were subjected to a prolonged period of intense, record, low air temperatures created by a sequence of anticyclone weather systems. The environmental conditions created by this extreme, short-term, climatic event, referred to in this area as the Big Freeze, are considered responsible for widespread, but varied, mortality amongst shelterbelt trees in subsequent years.
Shelterbelts are essential components of the agricultural landscape, yet there has been little research into their vulnerability to climatic perturbations. The vast majority of research concerning tree mortality is conducted upon trees within forest settings, with only a limited number of examples considering mortality in other settings, including amenity plantings. Thus, the mortality that occurred amongst shelterbelts following the Big Freeze provides a unique opportunity to investigate extensive, pre-senescence tree death in a managed environment. By the use of a postal survey and field investigation of 11 case study shelterbelts, this study examines the pattern of mortality and the relative contribution that species, habitat, landscape features, and anthropogenic factors made to that pattern.
The results indicate that the genetically-determined cold tolerance of a species was largely responsible for the interspecific response of shelterbelt trees to the Big Freeze. Accordingly, the broad pattern of mortality was related to variations in the intensity of temperatures across the study area. Farms in close proximity to the coast and the Southern Lakes experienced lower levels of shelterbelt mortality than farms further inland because of the positive thermal influence that substantial water bodies had on temperatures during the Big Freeze. Eucalyptus and native shelter species, which had the least cold tolerance of all species investigated, were significantly more susceptible to mortality than were the hardier exotic conifers and deciduous species. The intraspecific pattern of mortality amongst eucalypt shelterbelt species was primarily controlled by the distribution of low air temperatures that was in tum determined by the shape of topography and its influence on the meteorological phenomena of in situ cooling, katabatic air flow, and cold air pooling. The study also showed that shelterbelts trimmed in late summer or autumn before the Big Freeze were more likely to experience mortality.
Although the occurrence of another climatic event similar to the Big Freeze is likely to be rare, it is concluded that to avoid tree mortality, species should be planted in locations where conditions suit their natural tolerance characteristics.