Timber yield regulation in the indigenous forests of the Garden Route
The indigenous forests of the Garden Route form the largest forest complex in South Africa. Approximately 60 500 ha of forest lie scattered in patches of varying sizes separated by towns, farmlands, exotic plantations and natural fynbos vegetation on the narrow coastal strip south of the Outeniqua and Tsitsikamma mountain ranges, from Mossel Bay in the west to the Tsitsikamma River in the east, a distance of about 225 km. Two national forest types occur in the Garden Route. Most of the forests are Southern Cape Afrotemperate Forests. They display great variation in forest structure, species composition and species density due to the rugged topography and variations in soils, microclimate, disturbance history and other factors. Various sub-types have been identified. Western Cape milkwood forests are generally species-poor and occur as small patches at low altitudes along the southern coast (Mucina & Rutherford, 2006).
by Graham Durrheim
|Harvesting an indigenous hardwood in a Southern Cape forest.|
South African National Parks (SANParks) manages approximately 40 000 ha of indigenous forest as part of the greater Garden Route National Park and associated areas. Extractive resource use from terrestrial ecosystems is an important component of the management of the Garden Route National Park, both historically and to comply with new policy directives to accommodate the need for access to resources, to optimise socio-economic benefits to neighbouring communities, and to generate income for the organisation. The resource use programme makes provision for the harvesting of timber and non-timber forest products, as well as the harvesting of fynbos products (SANParks, 2010).
Timber has been harvested from the indigenous forests of the Garden Route since at least the 1770s. The government attempted to control the harvesting early on, and policies and practices were developed and implemented over decades to curb over-exploitation, culminating in a conservation-orientated, multiple use management approach and a sustainable timber harvest control system. Currently, the timber of mature trees of several canopy species is harvested mainly for the manufacture of high quality furniture and ornaments.
Timber yield regulation
Yield regulation is the science of determining inter alia what species and products you may harvest, where, when, and how much. Nature conservation remains the primary aim of management in the Garden Route forest areas, so a timber yield regulation system was developed that has minimum ecological impacts and mimics the natural turnover and disturbance mechanisms at work in the forests as closely as possible so as to ensure an optimal sustainable yield. The result was a single tree selection system based on natural mortality patterns.
Timber harvesting may only take place in areas where it is compatible with other long-term management objectives. It is only carried out in accessible medium-moist and moist high forest types where harvesting is feasible. The other forest types are either ecologically too sensitive or do not have sufficient trees of marketable size. Approximately 9 200 ha of forest is zoned for sustainable timber utilisation. However, timber is not harvested in the ecologically sensitive parts of these areas, including areas that are too steep or too wet, so the area actually harvested is less than this, approximately 6 500 ha.
The indigenous forests, with their mixture of trees of various sizes, ages and species, are managed for timber production on a different system to exotic plantations. There is no clearfelling, but rather a selected removal of trees on a ten-year cutting cycle, ie. a number of trees are harvested from a particular area only once every 10 years. Each timber production compartment is scheduled for harvesting in a particular year so that approximately one tenth of the total effective area is harvested annually (600-650 ha/a). These timber production compartments are scattered throughout the area.
A crucial component of any timber yield regulation system is the method used to select trees for harvest. The system used in the Garden Route forests is a single tree selection system that was developed locally, known as the Senility Criteria Harvesting (SCH) yield regulation system. Harvest tree selection criteria have been compiled for each of the main canopy species (see Table 1). Visible signs of senility and decline, such as crown dieback, structural damage and stem decay, are used to identify trees for harvesting that are senile and have a short remaining life expectancy, ie. trees that are busy dying. A tree complying with one of the criteria for that species is identified as a candidate for harvesting. Severity of crown dieback is generally the best indicator of senility, but other parameters such as structural damage, degree of stem decay and incidence of epicormic shoots also tend to be related to senility, depending on the species. Trees displaying advanced signs of senility often have low vigour (low growth rate).
The harvest tree selection criteria were compiled after an intensive survey in a long-term research area at Diepwalle Forest, for which reliable increment, ingrowth and mortality data exists. Over 2 400 canopy trees of the main canopy species were evaluated for possible visible signs of senility, low vigour or reduced life expectancy. Species-specific harvest selection criteria were then compiled that provided harvests equivalent to the natural rate of mortality. The application of these criteria on a 10-year cycle should thus produce a harvest that is equivalent to the natural mortality over a period of 10 years for each species in that area. All of the monitored trees have been re-evaluated three times over a period of 15 years and the criteria refined to predict mortality more accurately.
Harvest tree selection
The application of the SCH system in the field, namely the selection of trees for harvesting, is known as marking. Before the marking process starts, the compartment to be harvested is divided into strips by making narrow parallel cutlines, 15-20 m apart, through the undergrowth of the compartment. The marking team walks up and down the strips, systematically covering the whole effective area of the compartment. All trees with a diameter at breast height (dbh) of 30 cm or more are assessed according to the selection criteria. Trees complying with the criteria are candidates for harvesting if they contain utilisable timber.
All candidates of the more popular 'full use' species, namely stinkwood (Ocotea bullata), common yellowwood (Podocarpus latifolius), Outeniqua yellowwood (P. falcatus) and hard pear (Olinia ventosa) are marked for harvesting. Due to the limited market for the other 'lesser' species, including ironwood (Olea capensis subsp. macrocarpa), cherrywood (Pterocelastrus tricuspidatus), white pear (Apodytes dimidiata), assegai (Curtisia dentata) and quar (Psydrax obovata), only the most accessible candidates with the best timber are marked for harvesting, subject to volume quotas and marking standards established for each species. Non-utilisable trees that comply with the criteria but have no market value are left to die standing without any additional treatment.
Trees selected for harvesting are marked with a strip of coloured biodegradable tape (blue – large utilisable trees with big crowns that will unduly damage the forest when felled should be topped before felling; yellow – utilisable trees with small crowns that will not cause unacceptable levels of disturbance can be felled without prior topping). The tree is also blazed with a hatchet, in case the tape is removed by baboons. A strip of tape is also hung in the cutline opposite the tree to make it easier for the harvesting team to find the tree again. The species, diameter and harvesting instruction is recorded for each marked tree and the position plotted with a GPS.
Once the marking operation has been completed, a report containing a list of trees marked on each cutline, together with a summary of the expected utilisable volumes for each species and a map of the tree positions, is sent to the forest estate manager and harvesting may commence.
The forest is very sensitive to disturbance, in particular, soil compaction and drainage disturbance. Reduced-impact logging techniques were introduced into the southern Cape forests in the 1960s and have been applied consistently since the early 1980s, with a continuous development and improvement in techniques and equipment. Large crowns are removed before trees are felled, a procedure known as topping. Trees are felled carefully to reduce damage to surrounding trees. Logs are not cut to maximum length, but rather to facilitate handling and reduce damage within the forest.
The extraction of the logs from the forest is only carried out when the soils are sufficiently dry, as wet soil is much more prone to disturbance and compaction. Heavy machines are only allowed to operate on permanent slipping paths, on which there is little further destruction to the organic soil layer possible. Inside the forest, slipping is done with heavy draught horses or by cable and winch from a machine on the slipping path.
The harvested timber is taken to depots where it is sorted into lots, graded according to quality and usually sold on public auction, sometimes on tender. Approximately 1 100–1 600 m³ of indigenous timber, as well as approximately 1 000 m³ of the exotic Australian blackwood (Acacia melanoxylon), are harvested annually. The quantity and quality per species can vary from year to year. The main buyers are furniture manufacturers and timber merchants.
As an integral part of the yield regulation system, the effects of timber harvesting are monitored in timber utilisation areas by means of permanent sample plots, full count monitoring and post-harvest audits. Long-term monitoring of yields, growth rates and regeneration in both harvested and unharvested forest stands is carried out to ensure that harvest levels and mixes of products are sustainable.
The management of these forest areas received international recognition with the awarding of Forestry Stewardship Council (FSC) certification in December 2002. SANParks is currently exploring options to outsource some of the timber harvesting operations so as to ensure the economic sustainability of timber harvesting operations without compromising forest conservation objectives.
Mucina, L. & Rutherford, M.C. (Eds.) 2006. The vegetation of South Africa, Lesotho and Swaziland. Strelitzia 19. South African National Biodiversity Institute, Pretoria.
SANParks, 2010. Draft management plan for the Garden Route National Park, http://www.sanparks.org/parks/garden_route/about/docs/grnp_mgmnt_plan.pdf (Accessed 25 August 2010).
Seydack, A.H.W., Vermeulen, W.J., Heyns, H.E., Durrheim, G.P., Vermeulen, C., Willems, D., Ferguson, A., Huisamen, J. and Roth, J. 1995. An unconventional approach to timber yield regulation for multi-aged, multispecies forests. II. Application to a South African forest. Forest Ecology and Management 77: 155-168.
Published in October 2010