Josua H. Louw

Forest structure in selected South African forests: edaphoclimatic environment, phase and disturbance

Analysing data from 903 permanent sample plots situated in medium-moist and moist forests in the southern Cape, South Africa, we explored factors controlling forest structure. Pronounced subcanopy stem density persistence (well-stocked subcanopy forest matrix) and stem density packing (comparatively high stem densities of relatively large-sized trees) were found in the moist, less seasonal (quasi-tropical) Tsitsikamma forests. These attributes of structure were linked to the prevailing dystrophic, less seasonal conditions and the associated metabolic vertical growth orientation. The cool, moist and seasonal (quasi-temperate) Knysna forests had lower densities of relatively large-sized trees at the canopy level (stem density intolerance). This was attributed to the lateral growth mode and extended persistence of the trees involved. The warm, seasonal (quasi-subtropical) Outeniqua forests, on relatively nutrient-rich soils, had high stem densities at the canopy level relative to the subcanopy stratum; due to a combination of low subcanopy tree persistence, fast ingrowth of trees into the canopy stratum, which were then lost to mortality before they reached large sizes (high canopy tree turnover). Persistence of the multi-species subcanopy forest matrix supported asynchronous establishment and death of individual trees. Typical for tropical-type forests, the development of trees towards maturity (phase) was associated with a spatially fine-grained disturbance regime. A metabolic performance trade-off model was developed and provided an ecophysiological framework for the interpretation of forest structure and its underlying dynamics. This explanatory model indicated causal links between intraspecific metabolic tactics of trees in response to their edaphoclimatic environment and associated attributes of forest structure. Some implications of the findings for tropical forest management are discussed.

A stratification of the South African forestry landscape based on climatic parameters

The benefits of high-resolution forest site classification systems have been widely acknowledged in South Africa as a prerequisite for decision-making in forestry management. Although substantial research and development work have been done on categorising and mapping the soil and geological environment, surprisingly little has been done to improve our understanding of the role of climate in plantation ecology and to incorporate climatic variables into spatial databases. A stratification of the forest landscape of South Africa is proposed, using growth days and growth temperature as primary variables. The appropriateness of these variables is discussed in terms of its relation to plant physiology and landscape ecology. The provinces of KwaZulu-Natal and Mpumalanga are used as a case study for demonstration purposes. A total of nine growth day classes and 17 growth temperature classes were identified. An analysis of variance illustrated a significant difference between all classes in terms of related factors expressing moisture and temperature regimes, indicating the relevance of growth days and growth temperature for identifying areas of homogenous climatic conditions. The covariation between growth days and growth temperature and factors expressing moisture and temperature, respectively, also indicates the usefulness of these variables for landscape stratification in complex biophysical environments such as South Africa. The application potential of this stratification system is discussed in terms of its value as a resource inventory, as well as for species choice, site quality prediction, growth modelling, site ecology analysis and various aspects of risk management in plantations. It is proposed that this system be regarded as a first step towards a more holistic multifactor national forest site classification system.

The use of near-infrared scanning for the prediction of pulp yield and chemical properties of Pinus patula in the Mpumalanga escarpment area of South Africa.

Near-infrared (NIR) scanning technology is regarded as a potential tool for rapid determination of wood properties, which can substitute time-consuming and costly traditional methods. Pinus patula is the most important softwood species in South Africa, and this study is aimed at developing NIR calibration models for quick prediction of its pulp yield and chemical composition. A total of 85 trees from 17 plots, covering the range of site conditions in the Mpumalanga escarpment area, were sampled. Two samples were taken from each tree: a 1 m billet above breast height and a 20 mm disc at breast height. The billet was pulped using the kraft pulping process to determine pulp yield. The disc was ground into sawdust and the chemical composition was determined using conventional wet chemistry. Sawdust was scanned on a NIR spectrophotometer to produce NIR spectra. Calibration models to predict pulp yield, cellulose and lignin content were developed by applying chemometrics and partial least squares regression. Validation and determination of prediction accuracy of the models were performed using independent data. The prediction of cellulose and lignin were acceptable with correlations of determinations (r 2) of 0.71 and 0.70 respectively. Standard errors of prediction were generally low (less that 0.86) for all the models. The prediction r 2 for both total and screened pulp yield were only 0.62. Although the cellulose and lignin models can be used with confidence, the expansion of the sample size for follow-up research must be considered in order to increase the variability of tested wood properties and improve the prediction strength of the models. The NIR calibration provided in this study can contribute to the efficient examination of forest site-to-wood quality relationships that would enhance precision forest management and wood processing efficiency.

Whole-tree sap flow responses to soil water and weather variables for Pinus radiata and three indigenous species in a southern afrotemperate forest region

In a water-scarce country such as South Africa, timber and fibre production often stands in conflict with catchment water yields. The optimal provision of both is sought. Forest hydrological experiments improve our understanding of the soil–plant–atmosphere continuum and facilitate the development of rapid estimation techniques through extrapolation. To quantify and characterise the influence of local environmental conditions on single-tree water use, whole-tree daily sap flow for three indigenous afrotemperate tree species (Podocarpus latifolius, Ilex mitis and Ocotea bullata) and one introduced and intensively managed tree species (Pinus radiata) was measured. Hourly sap flux, soil water content and weather data were collected for a year in an indigenous forest and a plantation stand in the southern Cape afrotemperate region. Correlation analyses for whole-tree daily sap flow and environmental variables related to water and energy availability were done. Cross-validation was then employed to test combinations of non-covarying independent parameters for the prediction of whole-tree sap flow responses to environmental variables. Generalised linear models were developed for each species. For the Pinus radiata plantation species, separate models were developed for tree specimens in the subdominant, average and dominant diameter classes. The dominant diameter class model had the best R2 value of 0.80 (p < 0.00). For the indigenous tree specimens, the Ilex mitis model had the highest R 2 (0.65, p < 0.00). Validation of the models with independent data indicated positive and significant Pearson correlation coefficients for the observed versus predicted daily sap flow values, with the dominant diameter Pinus radiata specimens showing the highest correlation value (r = 0.91, p < 0.00), whereas those for the indigenous species were r = 0.63, p < 0.00; r = 0.78, p < 0.00; and r = 0.66, p < 0.00 for Podocarpus latifolius, Ilex mitis and Ocotea bullata, respectively. The results indicate that variable-specific linear models can be employed to estimate whole-tree sap flow and water use as a function of environmental factors with an acceptable degree of accuracy for both introduced and indigenous tree species.

Variation in selected solid wood properties of young Pinus patula from diverse sites in the Mpumalanga escarpment area in South Africa

Regression analyses identified ‘Growth Days’ (an index expressing site moisture availability) as the only site variable contributing significantly to the prediction to wood density (R2 = 0.57), whereas the model predicting grain angle included only ‘Altitude’ (R2 = 0.60). These results surfaced during an investigative study to quantify various sources of variation in wood properties and to quantify the effect of a number of site factors on wood properties of Pinus patula grown in the Mpumalanga escarpment area of South Africa. For this purpose, 10 trees were sampled from each of 17 diverse sites for wood property analyses. The effects of site, distance from the pith and differences between trees within site on wood density, transverse shrinkage, grain angle and dynamic modulus of elasticity were investigated. The site factors considered included a wide variety of soil and climatic factors. The effect of radial distance from the pith and differences between individual trees within sites were highly significant, accounting for most of the variation in wood properties. Although the effects of a number of site factors were statistically significant, they generally explained relatively small but important variation in wood properties among sites. The study not only quantified the effects of important sources of variation on a few key wood properties, but it also revealed that the extent of differences between sites can be explained in terms of some specific site factors. It is envisaged that the results will contribute significantly towards the refinement of current forest site classification systems for improved decision-making with respect to wood quality in intensively managed plantation systems.

Single-tree water use and water-use efficiencies of selected indigenous and introduced species in the Southern Cape region of South Africa

In South Africa, the development of a plantation tree industry using fast-growing introduced species was accelerated by the limited extent of indigenous forests. However, concerns about the impacts of plantations on the country’s limited water resources has initiated forest hydrology research and subsequent regulation of the industry since 1972. The forestry industry’s continued efforts to sustainably meet fibre and timber demands for the country’s growing economy have prompted questions whether indigenous tree species can provide an additional low water-use form of forestry. Single-tree water use and water-use efficiencies of three indigenous species (Ilex mitis, Ocotea bullata and Podocarpus latifolius) and one introduced species (Pinus radiata) in the Southern Cape region of South Africa were quantified. The heat-pulse velocity method was used to collect hourly sap flow data over a 12-month period. Hourly weather and soil-water data were concurrently recorded and tree growth rates were determined for the year. Biophysical water-use efficiency was calculated as the ratio of utilisable biomass gained per volume of water transpired. Patterns of water use through the year were different for the different species. Pinus radiata had higher transpiration volumes and water-use efficiency levels than the indigenous species. The most transpiring Pinus radiata tree had a transpiration volume that was 4.7 times that of the most transpiring indigenous tree. Indigenous species’ relatively lower water-use efficiencies were more a consequence of slow growth rates and not high water-use rates, which could be attributed to competition for resources in the dense indigenous forest. Potential implications for further hydrological research on the development of water-use-efficient tree production systems are discussed.

The value of six key soil variables for incorporation into a South African forest site classification system

The intensive nature of management practices in the exotic monoculture plantations of South Africa requires reliable decision support systems. Recent socio-economic developments, the need for optimal forest productivity, as well as increasing awareness of broader ecosystem values and environmental risks, highlight the importance of a unified approach to forest site classification and evaluation. This paper highlights the value of specific soil characteristics and its application value for a range of silviculture and management aspects related to site-specific forestry. A limited set of six soil variables are proposed for ecological mapping of forest landscapes at high-resolution operational-level scales. The variables proposed are parent material, soil classification, effective soil depth, depth limiting material, topsoil organic matter and topsoil texture. Each variable is discussed in terms of its significance as well as relevance to plantation forestry in South Africa. Shortcomings in our knowledge base and research requirements are highlighted, and the format of incorporation into a national forest site classification system is proposed. This paper will contribute to unity of purpose and understanding of forest site classification and evaluation in South Africa, and will promote management frameworks and decision support systems with desired environmental, economic and social benefits.