|dc.description.abstract||Eucalyptus grandis was introduced into South Africa in the 19th century, and has since become the most important of the hardwood plantation tree species grown for pulp. Until the late 1980's E. grandis was virtually the only eucalypt species grown. In order to meet the increasing demand for pulpwood in South Africa, forestry companies need to increase their timber output from an existing land base (Brown and Hillis, 1984; Kimmins, 1994 and Little and Gardner, 2003), or alternatively extend the planting of favourable alternative tree species into areas previously considered unsuitable for forestry due to unfavourable climatic conditions. From 1984 the major timber companies expanded their plantation forestry into the colder, frost-prone highland areas of western KwaZulu-Natal, the northeastern Cape and southeastern Mpumalanga Highveld. As E. grandis was not tolerant to severe frost, E. macarthurii amd E. nitens were planted in these areas as alternatives (Schonau and Gardner, 1991). As much of the earlier research had been centered around the development of silvicultural standards for E. grandis, it became necessary to test these for the different eucalypt species. Two trials were therefore established to E. marcarthurii and E. nitens with the following objectives: - to extend current recommendations to include different species, - to determine the degree of interaction between different silvicultural standards (genetic improvement, fertilisation and weed control), - to determine the effects of weeds, fertilisation, genotype and regeneration method (seedling vs coppicing) on the initial and long term growth, uniformity, tree straightness and survival of cold tolerant eucalypts, - cost effectiveness of various methods for re-establishing E. nitens and E. macarthurii. Genetic improvement played an important role in the establishment and initial
growth of E. macarthurii and E. nitens. The improved treatments outperformed
the unimproved treatments in terms of tree growth until canopy closure. At the last measured date when the trees were six years of age, the E. nitens improved
seedlings were still significantly better in terms of basal area when compared to
unimproved seedlings. The initial positive effect of genetic improvement of E.
macarthurii seedling however, was not sustained. Genetic improvement of E.
macarthurii and E. nitens also had a positive effect on tree straightness and
survival when the trees were assessed at five years of age. The genetic
improvement of both species also showed to be a viable option to produce an
optimum timber output at a lower cost when regeneration is carried out by means
of replanting with seedlings.
Fertilisation also showed positive effects in terms of the establishment and initial
growth of E. macarthurii and E. nitens. At six years after planting, the basal area
of E. macarthurii seedlings without fertiliser was still significantly lower than any
one of the other treatments. However, the initial positive effect fertiliser had on
the growth of E. nitens seedlings decreased to a non-significant level at six years
after planting. Fertilisation of E. macarthurii and E. nitens had a positive effect on
tree straightness and survival when the trees were assessed at five years of age.
The fertilisation of E. macarthurii seedlings also produced an adequate amount of
timber at a relatively low cost.
The controlling of weeds did not have an impact on tree performance initially or
after canopy closure for either E. macarthurii or E. nitens. This is due to the lack
of weed growth at these high altitudes at which the sites were planted. Little and
Schumann (1996) found that eucalypts could tolerate an aboveground weed
biomass of up to 2000 kg ha (-1) before there were any severe losses in growth due
to competition. At both these trials, the weed load did not reach these levels in
order to compete with the trees.
No significant interactions between any of the treatments were detected at both
these sites at any stage.
At the last measured date, there were no significant differences in terms of tree
growth between the coppice and seedling treatments for either E. macarthurii or
E. nitens. Regeneration by means of E. macarthurii and E. nitens coppice had a
positive effect on tree straightness and survival when the trees were assessed at
five years of age. Re-establishment by means of coppice for both E. macarthurii
and E. nitens was also shown to be by far the most cost-effective way at present
to produce an adequate amount of timber. Coppicing was shown to be the least
costly way to produce a m2 ha(-1) of timber provided the right species are coppiced,
and optimum density levels are obtained.||en