Armin H.W. Seydack

An unconventional approach to timber yield regulation for multi-aged, multispecies forests. I. Fundamental considerations

Abstract Sustainable harvesting here implies that similar amounts and types of products (dimensions, quality, species) continue to be harvestable at periodic intervals in perpetuity. Harvesting must therefore be organised in such a way that it remains within the renewability capacity of the forest system both in respect of growth as well as the success and type of regeneration. Coupled with the demands for environmental services and maintenance of biodiversity, this can only be achieved if harvesting remains spatiotemporally in accord with the natural disturbance regime. A yield regulation system must define how many and which trees are to be harvested. This determines the spatiotemporal impact of harvesting. Regarding the controversy between mono- and polycyclic approaches it is concluded that either of the two classes of systems may be appropriate or sustainable provided the associated intervention does not deviate substantially from the natural disturbance pattern (spatial component) and the parameters of renewability are not exceeded (harvesting intensity and frequency). Sustainability, as defined, is often jeopardised by harvesting being too concentrated in space or time in an attempt to promote logging economics, induce regeneration or enhance productivity. The long-term net growth of a primary forest is zero. Stagnant or overmature trees are often taken to be visually unidentifiable. On this basis a harvestable yield can only be achieved through the transformation of the forest into net growth phases through clearfelling (monocyclic system) or productivity enhancement measures and harvest tree selection according to minimum harvestable diameters (polycyclic systems). In contrast, the yield regulation approach advocated here involves periodic mortality preemption through the selective harvesting of overmature or senile trees proportional to species-specific turnover.

An unconventional approach to timber yield regulation for multi-aged, multispecies forests. II: Application to a South African forest

Abstract The development and implementation of the senility criteria yield regulation system to the multi-aged, multispecies forests of the southern Cape, South Africa, is described. The described yield regulation system aims to maximise the sustainable yield with minimised artificial impact on the ecology and minimised managerial input. The felling cycle is 10 years, which is still considered compatible with the prevailing natural disturbance regime. Harvestable trees are not defined by minimum harvestable diameters, but according to signs of senility or reduced remaining life expectancy (e.g. percentage crown dieback or base rot). These criteria are determined for canopy species in accordance with species-proportional turnover rates. The gross increment of the stand is thus realised as harvest through periodic mortality preemption by harvesting those trees which have reached a defined level of senility or reduced life expectancy. Advantageous characteristics of the senility criteria yield regulation system relate to guaranteed sustainability, compatibility with nature conservation aims, optimal productivity and managerial simplicity and flexibility. The potential of the system for wider application is discussed.

Competition effects in an afrotemperate forest

BackgroundInformation about competition responses is mainly available for monospecific stands or mixed stands with a small number of species. Studies on complex multi-species and highly structured forest ecosystems are scarce. Accordingly, the objective of this study was to quantify competition effects and analyse competition responses in a species-diverse afrotemperate forest in South Africa, based on an observational study with mapped tree positions and long-term diameter increment records.MethodsThe sensitivity to competition was analysed for individual species and involved the calculation of the slope of the linear relation between the value of a competition index (CI) and diameter growth as a measure of sensitivity. In a next step different competition indices were combined and tree diameters were grouped in three classes as surrogates for canopy status and ontogenetic stage.ResultsFive competition indices were found to be effective in showing sensitivity to competition for a number of canopy and sub-canopy species. Significant linear regressions were fitted for 18 of a total of 25 species. Species reactions varied significantly in their sensitivity to the different CIs. The indices were classified as belonging to two groups, those that responded more to local crowding and those that are more sensitive to overtopping, which revealed species-specific sensitivities to both factors. The analysis based on diameter classes revealed that species clearly changed their sensitivity to crowding or overtopping depending on diameter. Canopy and sub-canopy species showed distinct differences in their reactions.ConclusionsThe application of multiple CIs brought novel insights relating to the dynamics of afrotemperate forests. The response patterns to different competition indices that focus on crowding and overtopping are varied and tree diameter dependent, indicating that oversimplified assumptions are not warranted in the interpretation of CI- growth relations.

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.

Long-Term Antelope Population Monitoring in Southern Cape Forests

SYNOPSIS The results of population monitoring of two forest antelope species in the Knysna forests (South Africa), the bushbuck (Tragelaphus scriptus) and blue duiker (Philantomba monticola) are presented. Most monitoring was done by faecal pellet group counting, but some track counts, strip transect counts and direct observation recording were also carried out sporadically. Both faecal pellet group counting and strip transect count methods were found suitable for population monitoring of the two forest antelope species. The results reveal meaningful patterns of habitat preferences and population changes over time (1970–1997). Blue duiker populations declined sharply between 1970–1980 and have remained low since. Bushbuck densities increased from 1970–1990, but were low again in 1997. The current state of the monitoring programme is outlined and recommendations are made for its improvement.

Regulation of Timber Yield Sustainability for Tropical and Subtropical Moist Forests: Ecosilvicultural Paradigms and Economic Constraints

Components of yield regulation systems are discussed which are required to ensure successful stand regeneration and sustainable timber yield optimization in tropical and subtropical moist forests. Existing yield regulation/silvicultural systems were classified into four groups: uniform systems, tropical shelterwood systems, manipulistic selection cutting systems and naturalistic selection cutting systems. The performance of these systems, in particular of the relatively high impact systems generally practiced in the tropics, was assessed and problems relating to inadequate regeneration, unsustainable timber harvest levels, inadequacies of the residual growing stock and neglect of silvicultural interventions are identified. Most systems, as practiced, are potentially subject to sustainability risks due to forest matrix destruction effects, disruption of functional interspecies dynamics and disruption of regeneration processes. A forest matrix invasion model, which is considered to explain tropical forest dynamics ecologically more closely than the successional disturbance model generally invoked for temperate forests, is outlined. Based on this model, timber yield regulation characteristics for continuous cover primary forest management were defined. They include large-tree productivity realization, relative high harvest maturity thresholds and residual growing stock levels, and protection of the forest matrix through reduced impact logging.

Trade-off relationships between tree growth and defense: a comparison of Ocotea bullata and Curtisia dentata following bark harvesting in an evergreen moist South African Forest

Key messageTwo artificially debarked Afrotemperate tree species showed different trade-offs between wound closure and compartmentalisation of decay in the stem. One species had a relaxed trade-off but inefficient defence, and the other showed more efficient defence but a substantial trade-off.AbstractBark stripping for medicinal use is a common cause of damage in several indigenous tree species in natural forests of South Africa. Ocotea bullata and Curtisia dentata are in high demand for their bark for medicinal purposes. The study aimed at revealing intra-specific and inter-specific differences of tree growth rates wound closure and decay containment responses ten years after experimental bark harvesting. The results obtained on computer tomography scans of twenty trees showed that C. dentata had significantly higher decay percentages compared to O. bullata, indicating that O. bullata was able to more efficiently contain decay than C. dentata. While decay in O. bullata was confined at the wounded tissues, decay in the C. dentata extended below and above the wounded area. Intra-specifically, O. bullata showed strong positive correlations between tree growth rates and wound closure rates, and wound closure rates with relative volume of decayed wood, indicating that individuals with higher rates of growth and wound closure suffered higher percentages of decay. Results confirmed an intra-specific trade-off between growth rate and defense investment, with fast growing trees showing high percentage decay (poor compartmentalization of decay). Inter-specifically, however, growth versus defense trade-off did not present itself. For C. dentata, no correlations were found between rates of wound closure and percentage decay. The findings and conclusions derived from this study reveal complex, species-specific responses to damage. This study highlights the need to gain an in-depth understanding of underlying morphological, phylogenic, physiological characteristics of species to further explain the observed species differences.