Frost tolerance of six seed orchards of Acacia mearnsii (black wattle) and the effect of developmental stage and tree size on frost hardiness

Abstract

ABSTRACTFrost damage is the most important abiotic risk in Acacia mearnsii (black wattle) plantations in South Africa. Current A. mearnsii planting stock cannot tolerate temperatures below −4°C, and therefore there is an urgent need to develop frost-tolerant seed. This study investigated the frost hardiness of seedlots from six South African A. mearnsii seed orchards located at altitudes of 1120–1458 m asl and latitudes of 26.7–29.8°S that were established with cold-hardy Australian provenances and/or local landraces from cold sites. Frost damage was assessed in four field trials established on two different dates in 2015. Three sites experienced extensive frost events, with frequency and intensity varying among sites. Minimum temperatures ranged from −6.4°C to −7.7°C across sites. The seedlot from the coldest, highest altitude orchard showed superior frost hardiness to other seedlots from lower altitude and warmer orchards of similar genetic origin. Thus, site climatic characteristics are important for the development of cold hardiness in A. mearnsii seed orchards. Frost hardiness in young A. mearnsii trees was significantly related to age and tree size. At four months of age, trees were highly susceptible to frost damage irrespective of tree size and orchard origin (60–71% frosted trees). Frost hardiness increased with age and was very high at c. nine months across orchards (1–8% frosted trees at one site and 0–2% at another site). At nine months, although the extent of frost damage was low, this was inversely related to tree size. The percentage of trees that recovered from frost damage also increased with age of frost damage, this being 21% at the site frosted at four months of age and 65–67% at the sites frosted at nine months of age. Finally, there were no differences in tree growth at 14 months suggesting that, unlike cold hardiness, the differences in environment between the orchards did not affect the growth performance of the offspring.