摘要：New Zealand treeline species have low frost tolerance compared to their northern hemisphere counterparts, and appear susceptible to out-of-season frosts. However, foliage from high altitude trees is rarely directly measured. This study compares seasonal frost tolerance of mature treeline trees with local temperatures to assess whether frost affects their performance. Photosystem efficiency and seasonal frost tolerance (temperatures causing 10% and 50% foliage mortality, LT 10 and LT 50 , respectively) were measured on foliage from four native and one exotic species across the treeline ecotone. For all species, photosystem efficiency and frost tolerance were lower in spring and summer than in autumn. Frost tolerance changed with altitude only for exotic Pinus contorta in spring. Spring frosts regularly exceeded LT 10 for all species. In all seasons over the last 20 years, the minimum temperature experienced was at least 4 °C warmer than the LT 50 ; however, east of the Main Divide, a 1-in-40 year extreme minimum temperature in summer reached LT 50 levels. This study suggests frosts may cause some foliar damage, especially in spring, but the effects of frosts on mature trees are unlikely to control the position of the New Zealand treeline.
