The influence of tree improvement and cultural practices on the productivity of <i>Eucalyptus</i> plantations in temperate South Africa

Abstract

The South African forestry industry is dependent on a resource of fast-growing, low-cost wood to remain globally competitive. Land availability and the legislative requirements for afforestation in South Africa are limiting factors for future growth and therefore increasing the productivity per unit area of existing land offers the largest potential to  improving forest productivity and reducing the unit cost of wood production. Operational gain integrates all the components of the deployment process that have an influence on final stand yield, and these include tree breeding efficiency, propagation efficiency, plant use efficiency, matching species to sites, stand density and early  growth through cultural practices. The aim of this study was to gain increased understanding of the interactions between intensive levels of silvicultural management and the deployment of improved genotypes of a number of commercialEucalyptus species, across a range of sites, with varying levels of productivity. To demonstrate the influence of factors associated with productivity gains, a series of five trials were established, on five different sites, within the warm and cool temperate regions of KwaZulu-Natal. At each site and for each factor, a treatment that would exhibit superior productivity was deliberately chosen over a second treatment representing inferior but not uncommon operational practice. The four main factors tested at each site were species (recommended relative to an alternative choice), genetic level (unimproved relative to some level of improvement), planting density (recommended 1 667 stems ha^–1 relative to a suboptimal 1 111 stems ha^–1), and silvicultural intensity (combination of fertiliser, water and optimum weeding termed ‘high’, relative to a suboptimal level ’low’). Significant productivity improvements at approximately five years (mid-rotation) were demonstrated for the main factors across site productivities and species. Silvicultural treatments contribute slightly more to productivity improvements than tree improvement and species choice. In this trial series, achieving high stocking and fertilising with adequate weed control influenced Eucalyptus productivity to a greater degree than deploying improved genetic material and matching the correct species to the site. The influence of genetics and species has increased over the past three years. In general, interactions between the main factors remain absent, highlighting the additive nature of productivity  improvements for each of the main factors.

Southern Forests 2009, 71(2): 85–93