The Namibian savannah ecosystem has experienced considerable anthropogenic pressures, which have resulted in the disruption of key ecological processes, and consequently, the proliferation of a dense vegetation structure, commonly known as bush encroachment. Approximately 45 million hectares of land have been affected, which has caused a significant decline in the grazing capacity, poor economic returns for the farmers, and loss of suitable wildlife habitat. Restoration thinning, a method that involves the selective removal of excess trees/shrubs, has been applied to counteract the negative effects associated with this phenomenon.
This thesis aimed to assess the effect of thinning on (a) the activity patterns of local ungulates and predators, (b) the encroaching woody vegetation in terms of (i) regeneration, and (ii) structure, abundance, and habitat sighting lines, in a savannah habitat in the north-central region. Generalised linear mixed-effects models (GLMM) and linear mixed-effects models (LME) were used for statistical analysis. Results showed that thinned areas had overall greater wildlife activity. Thinned areas also had reduced tree/shrub abundance, which was significant for the mature height classes. Natural regeneration was rapid in thinned areas, where the abundance of young cohorts was 34% greater than non-thinned areas. In the thinned areas, red umbrella thorn (Vachellia reficiens) was significantly reduced but umbrella thorn (Vachellia tortilis) numbers increased. Tree/shrub densities in the thinned areas fell within the commonly accepted range for a 400 mm rainfall area (600–750 tree equivalents (TE) ha–1). Thinning significantly modified the dense thornbush to an open vegetation structure with a low woody canopy cover that favours grass growth and provides greater sighting lines for open savannah wildlife. This thesis demonstrated that thinning was effective in controlling bush encroachment and could be used as a method to restore other affected areas. However, periodic post-thinning management is recommended to control the established samplings.
Fast-growing, high-yield tree plantations are an increasingly significant source of wood in the tropics. In these areas, improved wood productivity is an important economic goal. In Costa Rica and other countries in Central America, most of the tree plantations, especially those of advanced age, have not had the productivity expected of them. In the general context of a closer relationship between management practices and quality wood production, the general objective of this research was to develop a set of intensive management scenarios that could lead to alternative timber production practices with attainable and promising economic returns.
Data for the different studies were collected in different regions of Costa Rica, covering almost all climatic conditions where T. grandis plantations have been established and where the species has adapted well. The study consisted fundamentally of the measurement of growth and yield parameters at the stand level and of the analysis of wood properties at the individual tree level and the interrelationship between silvicultural management and site conditions.
High intensity, timely thinnings yielded both individual tree and stand volume, i.e. the objective of high individual tree growth was met without a severe reduction in stand yield. Pruning up to a reasonable height and on time caused no reduction in tree growth and stand yield. Moreover, pruning may improve stem form (higher form factor, lower stem taper) and heartwood content.
Important characteristics, such as heartwood content and wood density, were found to be related more to tree age than to silvicultural management practices, especially at early plantation stages.
Growth scenarios for 20 and 30 year rotations with the objectives of high individual tree growth and high stand growth were developed for high, medium, and low quality sites. Different site classes, production objectives, rotation periods, and discount rates resulted in marked differences in the financial profitability projections of the developed scenarios.