Silvicultural intervention during re-establishment to reduce mortality in eucalypt plantations, South Africa: early results

Abstract

High mortality negatively affects rotation-end yield in pulpwood eucalypt stands. Most forest companies in South Africa aim to achieve +90% survival. To ensure optimal survival, companies implement preventative measures such as the use of high-quality plants and optimum re-establishment practices, with blanking the only curative/post-planting practice used. However, blanking is expensive and may result in variable stand growth. To test the viability of alternative mitigation measures, a trial was planted in 2019 to Eucalyptus grandis × Eucalyptus nitens in the KwaZulu-Natal Midlands. It consisted of a 2 × 2 × 2 Factorial with 2 additional controls, replicated 4 times and arranged in a randomised complete block design. Two levels of Mortality (Moderate; High) were combined with Blanking (Yes; No) and Coppice + Interplanting at 6 months (Yes; No). These six treatments were compared to a Low mortality control (< 10% = optimum), or where the trees were replanted at 6 months to simulate crop-failure with subsequent Low mortality (6 months) (< 10%). Mortality, tree performance (HT, DBH, BA) and treatment variability were measured up to 24 months. The two control treatments recorded 84% survival, with the mean of all the other treatments combined 48%. Blanking (54%), or Re-planting at six months (85%) resulted in higher survival than Coppicing + Interplanting (35%). Winter frost and/or lack of rain is suspected to have caused higher mortality of young coppice shoots. Tree growth was negatively impacted in those treatments where trees were Re-planted at 6 months, Blanked, or Coppiced + Interplanted. Overall, the best treatment in terms of BA was the Control (low mortality from planting) indicating the importance of using high-quality plants and re-establishment practices. BA was positively improved in the Blanking treatments. Blanking remains a viable alternative where mortality exceeds the desired threshold, albeit with treatments that have more variable tree growth.