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Pulp yield is the mass percentage of cell wall material remaining after pulping. Pulp is made up of wood fibres that have been separated from each other by chemically dissolving lignin or by mechanically tearing the fibres apart. For chemical pulping numerous benefits are associated with processing species that yield greater percentages of pulp for a given amount of timber:
A reduction in wood requirements
A reduction in chemical use
Improved digester efficiency in chemical pulping
Increased production capacity.
The significance of density and pulp yield to a pulp mill can be illustrated by comparing E. grandis to E. smithii. Samples collected from a species trial in the KZN Midlands were processed and E. smithii yielded an additional 14,7 kg in fibre per tree when compared to E, grandis.
SITE SPECIES MATCHING
Timber growing regions in South Africa cover a wide range of growing conditions with regards to latitude, altitude climate, geology, soils, topography and biotic factors. To cover this site diversity, it is necessary to recommend a variety of species that can grow well under prevailing conditions.
Wood quality can be strongly influenced by genotype-site interactions. This is an important factor as we become more orientated towards producing quality rather than simply quantity. It is very important to match species to site conditions that suit that particular species. For example, E, grandis grown under optimal conditions (high rainfall, deep soils, optimum temperature) will give higher pulp yields than trees grown on marginal sites.
Table 2 summarises the species with acceptable pulping characteristics that are suitable for growing on the range of sites found in the summer rainfall region of South Africa. Members should limit their species choice to those listed in this table. This will enable NCT, in the future, to market species pure batches to customers.
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