Seasonal frost hardiness inLeptospermum scopariumseedlings from diverse sites throughout New Zealand

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Abstract
Seasonal variation in frost hardiness was evaluated in a number of populations of the indigenous shrub Leptospermum scoparium (manuka). The seed was collected from wild populations at sites ranging from North Auckland to mid Canterbury but germinated and grown outdoors in a common environment in Palmerston North. For comparison, an Australian species, L. rupestre, was included. One-year-old seedlings were exposed to controlled frosts on four occasions throughout a year. Visual damage was measured four weeks after the frosts as the percentage of foliage damage, and frost hardiness was determined as that temperature causing an average of 30% leaf damage. The seasonal minimum frost hardiness in midsummer was similar for each population at about 4°C,whereas the maximum winter hardiness varied from about -5°C in a low altitude (200 m), northern population to -8°C in a high altitude (1000 m), southern population. Leptospermum rupestre had a comparable maximum frost hardiness to the most hardy L. scoparium populations. However, there was little seasonal change in frost hardiness in L. rupestre since the summer minimum was about -7°C. Frost hardiness in L. scoparium was positively correlated with latitude and altitude of origin of each population in both autumn and midwinter, indicating that ecotypic variation in capacity for hardening has arisen across the wide distributional range of the species. However, maximum expression of frost hardiness, particularly in the high altitude, southern populations, may not have been fully realised in this study.