The above-ground biomass composition and nutrient content of commercial Eucalypts and their use in managing sustainable intensive forestry in South Africa.
A study was undertaken of the biomass composition of ten eucalypt species growing on five sites in the timber growing regions of southeastern Mpumalanga. The species comprised E. deanei, E. dunnii, E. elata, E. fastigata. E. grandis, E. macarthurii, E. nitens, E. saligna, E. smithii and E. viminalis. The average tree height was 19.3 m and dbh 15.9, with stocking at 1494 stems/ha and merchantable yield at 153 t/ha. The sites had been established under commercial conditions seven years earlier as a series of identical site-species trials, and differed widely with respect to air temperature and effective precipitation. The soils were derived from granite, and either shallow and rocky (Glenrosa form) or deep and friable (Inanda forms). Sample trees were felled at the height of summer, separated into the biomass components of leaves, branches, on-tree dead matter, bole bark and bole wood, and weighed for fresh mass. Sub-samples were taken of each component for laboratory determinations of moisture and nutrient content, as well as bole wood density. Statistical analysis of the results show significant differences between species and sites as well as site species interactions for the tree mass of biomass components, and nutrient composition and water content. While overall E. smithii and E. dunnii produces the largest biomass, there are strong site interactions due to the varying degrees of site specificity required by different species. The proportions of the different biomass components differ between species and across sites, with the bole mass fraction increasing as growth rate improves. Biomass moisture is highest in bark and leaves, especially for the Viminales species group of the genus Symphyomyrtus. This group also has the highest wood density, while density within the stem is remarkably uniform for most species. Density generally increases with tree vigour. Nutrient concentrations are greatest in leaf and bark and least in bole wood, and are strongly influenced by soil nutrient status. There are considerable differences between species, with the subgenus Monocalyptus showing generally the lowest values. Models are developed to estimate biomass components from tree mensurational parameters and their nutrient content from soil variables. Nutrient budgets are developed on an equivalent tree mass basis to estimate the quantity of nutrients required to grow biomass. Nutrient quotients of the biomass produced per unit of nutrient are calculated to compare the efficacy with which biomass is produced per species, and comparisons made for average growing conditions across the study region. While the most economical subgenus is Monocalyptus, the most efficient of current commercial species is E. smithii, followed by E. nitens, E. macarthurii, E. grandis and E. dunnii. Nutrient export during harvesting exceeds natural inputs, is exacerbated by 60% on average when retaining bark on logs, and more than doubled if slash is burnt. Further research is required into estimating soil reserves and extending the study to other site types.