The relative contributions of tree improvement and cultural practice toward productivity gains in Eucalyptus pulpwood stands

Increasing yield per hectare and associated reduction in the unit cost of wood is an important component of sustained global competitiveness for forest products exported from South Africa. The genetic improvement of planting stock and maximising growth through cultural practice are two strategies for achieving higher yield. The aim of this study is to increase understanding of the interactions between intensive levels of silvicultural management and the deployment of improved genotypes of a number of commercial eucalypt species across a range of sites with different productivity classifications. To demonstrate the relative contributions of different factors associated with productivity gains, five trials of standard design (2×2×2×2 factorial) were planted on five different sites. For each trial 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 factors tested at each site were species (first choice for the site relative to a sub-optimum second choice), genetic level (unimproved or land-race material relative to first- or second-generation material), planting density (1111 or 1667 stems per ha) and silvicultural intensity ("high" or "low"). An ANOVA for volume per hectare at age two and a half years confirmed significant productivity improvements for the main factors across site productivities and species. Silvicultural treatments were the largest contributors to productivity differences. When measured at this age, achieving high stocking, fertilising and/or maintaining adequate weed control influenced stand productivity to a greater extent than deploying improved material and matching the correct species to the site. On all sites a lack of interaction between factors predominated. This means that the productivity gains associated with each factor are additive.