South Africa's climate is characterised by the occurrence of regular droughts. An assessment of drought mortality in KwaZulu-Natal during 1999 could not provide accurate information regarding the effect of species or planting density on mortality because of confounding species-site allocations. Thus, following the 1999 drought event a trial was established in a drought-prone area, planted to a number of genotypes that were deemed to be more tolerant to water stress than Eucalyptus grandis. Each species was planted at stand densities of 816, 1 600 and 2 066 stems ha-1, where 1 600 stems ha-1 represented the current operational prescription. The aim of the trial was, firstly, to find a genotype that would yield more utilisable timber and, secondly, to determine the effect of planting density on tree mortality and growth. It was hypothesised that planting at a lower density will reduce stand-level growth slightly, but that it will reduce tree mortality under drought conditions. Serious drought symptoms were observed in this trial during October 2010. After the drought event that occurred towards the end of the rotation, the survival of the E. grandis x E. camaldulensis hybrid clone (98.7%) and E. dunnii (72%) was superior to that of E. grandis (52%). Across all genotypes the survival percentage on the densely planted plots (2 066 stems ha-1) was significantly lower than on the other two planting densities (1 600 stems ha-1 and 816 stems ha¡V1). Both E. dunnii and the E. grandis x E. camaldulensis hybrid clone had better or similar growth, and showed fewer symptoms of water stress than E. grandis. This clearly illustrated the importance of selecting genotypes that can tolerate water stress for establishment on drought-prone sites. The risk of drought mortality can be further reduced by planting trees at a lower density.
Keywords: defoliation, drought mortality, fast-growing eucalypt plantation, water stress
Southern Forests 2013, 75(4): 189–198