G. van Wyk

Classification and dynamics of a southern African subtropical coastal lowland forest

The composition and dynamics of the Dukuduku coastal lowland forest were investigated by means of ordination techniques. Size-ciass distributions on data from 200 plots were analysed and we also interpreted aerial photographs. An initial classification suggested that there were six widespread communities. Ordination of data of potential canopy species, found in the canopy and in the sub-canopy, was done separately tor five of the six communities. A successional trajectory became apparent from this analysis. Understorey composition in a particular community was often more similar to the canopy composition in the next community in the hypothesized time sequence, than to the composition of its own canopy. Analysis of size-class distributions of canopy species for each community supported the concept of a successional gradient. Common canopy species found in early successional communities tended to have negative exponential size-class distributions, whereas these same species were less frequent and had flatter size-class distributions in the later successional stages. Based on the results of the ordinations and size-class analysis, a conceptual model of forest dynamics is presented. It is hypothesized that large-scale disturbances, such as fire, are the dominant forces driving the dynamics of this coastal lowland forest.

Genetic Improvement of Timber Yield and Wood Quality in Eucalyptus Grandis (Hill) Maiden

SYNOPSIS Results of a partial diallel progeny test of Eucalyptus grandis assessed at ages 5, 36, 60 and 105 months are presented. Large genetic variations among full-sib families were found for all traits studied, e.g. the mean tree volume per family at 105 months varied from 0,226 m3 to 1,325 m3. Estimates of additive genetic variance (σ2 a) for volume production was slightly less than dominance variance (σ2 d) but for stem and crown form, and for taper σ2 a clearly exceeded σ2 d. Estimates of individual tree heritabilities were moderately high, varying from 0,28 to 0,54. Reciprocal effects were not significant. Genetic values, breeding values and specific combining ability values indicate that the relatively high level of additive genetic variation can be utilised for predicting which parent will produce the best offspring. Such offspring could be particularly useful for vegetative propagation. Genetic correlations are favourable among most growth traits, indicating that simultaneous improvements are po...

Successional pathways in disturbed coastal dune forest on the coastal dunes in north-east KwaZulu-Natal, South Africa

The floristic composition and structure of secondary dune forest, established on old lands and in grassland, was investigated on the coastal dunes of the north-eastern coast of KwaZulu-Natal. Two distinctly different pathways from disturbance to forest were identified. One is typified by an even-aged stand of Acacia karroo, the other by bush clumps in a grassland matrix. The bush clumps are initiated around a single tree, normally Syzygium cordatum, which then acts as a nurse for the establishment of other woody species. The species composition and structure of the bush clumps were the same, regardless of whether the area where it established was identified as an old fieid or grassland. The bush clumps had a substantially higher density of both woody plants and woody species than did the A. karroo stands.

Inbreeding Effects in Eucalyptus grandis in Relation to Degree of Relatedness

SYNOPSIS Introductory remarks refer to the detrimental effects of selfing in E. grandis. How do other types of inbreeding such as the mating of half-sibs or full-sibs compare with out-crossed material? Results from two trials are presented comparing growth performance of selfed, half-sib, full-sib, polymix, open-pollinated and control families with each other. Even though the trees are still young and sample sizes may be too small, it is evident that selfing and full-sib mating are not desirable. The difference between half-sib mating and supposedly out-crossed families is less evident, suggesting that more in-depth study may be required for these comparisons.

Reaction Wood in Pinus taeda—A Preliminary Report

SYNOPSIS Excessive compression wood has been reported from most young Pinus taeda stands in the summer rainfall region. Huge financial losses are predicted and the Forestry Branch of the Department of Environment Affairs has stopped planting the species until the probability of incidence has been markedly reduced. Climate, site and silvicultural practices do not appear to be causative. An inherited susceptibility in certain seed orchard families has been found which usually coincided with susceptibility to black pine aphid (Cinara cronartii) infestation. The most susceptible families have been removed from the seed orchards and attempts are being made to get the black pine aphid under biological control.

The Economic Benefits of Forest Tree Breeding

SYNOPSIS The application of research results will increase volume production of forestry plantations and improve the quality of the raw material considerably. This paper illustrates that only one aspect of forestry research, namely tree breeding, will result in volume improvement of 5 to 30%. What does this mean to the industry in terms of money? It is shown that the discounted value of tree improvement potentially available to the industry, is more than R30 million per annum for the six major species grown in South Africa. By contrast, the cost of this research to the forest industry is very low. The present total research budget of less than 1,0% of the Industrys total annual turnover could be raised many fold and still leave the industry with vast “research profits”. If we are serious about self-sufficiency in timber production as stated in our national forest policy, then tree breeding must be our starting point because it will provide increased volume and more versatile and adaptable plantation trees.

The status of tree breeding in South Africa

Summary The status of tree breeding in the Republic of South Africa is presented with the emphasis on statistics rather than description of trials and techniques. A complete list of species and provenance trials is presented indicating the search for new breeding material. Figures of phenotypic selections indicate the large number of species actively being worked on. Seed orchards have been established and good seed production is obtained for some of the major commercial species. Finally, a table summarises the number of progeny trials in the R.S.A. to indicate the establishment of populations for ongoing selection and breeding.

The Identity of Trees Currently Known as Eucalyptus grandis in the Republic of South Africa Based on Isozyme Frequencies

SYNOPSIS Pattern analyses of isozyme frequency data from Australian populations of E. grandis Hill ex Maiden and E. saligna Sm. populations in the Republic of South Africa known as E. grandis, (including a derived population from Zambia) show that the latter are mostly within the range of Australian E.grandis. There is evidence that the source of the South African populations was centered in New South Wales or southern Queensland.

Does Tree Improvement Influence the Harvesting and Transport of Trees

SYNOPSIS Tree to tree variation in well adapted stands is used to identify trees with high volume production, with straight trunks, and with acceptable crown features. The inclusion of timber properties as another selection criterion ensures that trees are grown which are above average in both volume production and wood quality. An increase in straightness and volume production results in bigger logs so that yields and productivity are increased and handling costs reduced. Desirable branch features limit the amount of debris in the plantation and ensure a better quality product to the sawmiller. To the pulpmiller, increased wood density results in more cellulose per unit volume.

Analysis of Stable Isotopes of Xylem Water from Plantation Trees in E. Transvaal and Zululand Indicate they Utilised Ground water During Drought of 1992

SYNOPSIS We investigated the stable isotopic signature of xylem sap of mature plantation trees and the associated ground water at sites in Zululand and E. Transvaal. The survey took place mainly during the drought of 1992. Results indicated a range of plant isotopic signatures. In many instances a great similarity existed between tree water and ground water in terms of isotopic content. This suggests that plantation trees utilised ground water during drought.