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Population structure
The size-class distribution of the Elaeodendron transvaalense population at Tshirolwe is illustrated in Figure 5.3. The population status resembles the typical reverse Jshaped curve. Three ideal types of size-class distribution can be recognized for tree populations (Peters 1996, Cunningham 2001). The typical reverse J-shaped curve or negative exponential curve indicates continuous recruitment of young stems, the bellshaped curve indicates a lack of seedlings and young plants and the straight horizontal line indicates relatively low numbers of seedlings and young plants. In a closedcanopy environment the reverse J-shaped curve as displayed in Figure 5.3 is considered to indicate species which are tolerant to shade or competition while the bell-shaped curve or straight line curve will indicate shade-intolerant or competitionintolerant species. The fact that most of the adult individuals are harvested leaves the population in danger of not producing seeds due to poor health. In their study on Pterocarpus angolensis Desmet et al. (1996) found that the most important requirement for the survival of these populations was the continued presence of mature, reproductive individuals. It is also important for seedling size-class to recruit into adult size-classes without being harvested. On a plant community level it has been established that the majority of species increasingly resides in the smallest size-class (Niklas et al. 2003a, Guedje et al. 2007) and that in fact species richness is a size-class dependent phenomenon. Large sizeclass individuals in rare species are found in small numbers thereby attributing to the rareness of the species. The fact that the E. transvaalense population sampled has few individuals in the large classes shows that it is not abundant and that it may become increasingly rare in the near future. A high abundance of individuals in smaller size classes, which lead to an inverse Jshaped size class distribution, is generally regarded as an indicator of adequate regeneration and population maintenance (Peters 1996, Condit et al.1998, Lykke 1998, Niklas et al. 2003a, Ganesan and Siddappa 2004). The abundance of seedlings is therefore a manifestation of successful seed germination and establishment in the E. transvaalense population. The position of the centroid found to be 49.12 cm, which was left-skewed in relation to the midpoint of the circumference distribution of 130 cm stem circumference, confirmed the healthy status of the population. It was clear that except for the smallest size class (0 – 20 cm), all the size classes had a high proportion of individuals harvested (Figure 5.3). In many size classes all individuals showed signs of harvesting.
The linear regression on the natural logarithm of the density in the size classes against the size class midpoint (Figure 5.4) produced a significant linear regression (r² = 0.678; y = -0.014x + 4.279; p= 5.38×10-4). The slope and Y-axis intercept of this equation can in future be used to compare other populations of E. transvaalense under different harvesting regimes. It can also be used to monitor and compare the same Tshirolwe population over time to detect changes in population structure (Gaugris and Van Rooyen 2011). Although long-term population monitoring data would be optimal to detect trends in population structure, Kohira and Ninomiya (2003) have indicated that there is merit in using the size-class distribution with single-year data. Furthermore, a range of techniques has been devised to obtain as much information as possible from single surveys. The assessment of population structure with single-year data gives an essential head start for conservation efforts with a small amount of resources.
Harvesting
Forty eight percent of the Elaeodendron transvaalense individuals sampled were not harvested (Table 5.1; Figure 5.3). Most of the unharvested individuals were seedlings. The large number of unharvested seedlings indicates that the population should potentially be able to recover if harvesting intensity is reduced, although it still needs monitoring. In contrast, most of the larger size classes showed that 100% of the individuals had signs of harvesting. Some individuals showed severe bark removal with some of the individuals ending up dead due to harvesting pressure. Harvesting area increased with an increase in stem circumference (Table 5.1, Figure 5.6, r2 = 0.6219 and y = 0.1437x – 0.1662). This is understandable because large trees have more available bark to harvest. Overharvesting could be the reason for the absence of any individuals either harvested or unharvested in the larger than 140 to 160 cm circumference size class in the studied Elaeodendron transvaalense population. The three size classes most affected by the bark removal practices were the >180-200, >200-220, and >220-240 cm circumference classes (Figure 5.6). These three size class categories also constituted 30% of the individuals that showed 100% crown mortality. When the ratio of harvested area : stem circumference was plotted against the different size classes it was clear that some of the smaller size classes were experiencing the same high harvesting pressure as the larger ones. It is clear that harvesting of medicinal materials is also done on young individuals.
Chapter 1 INTRODUCTION
1.1 Thematic background
1.2 Problem statement and rationale for the study
1.3 Study aim and objectives
1.4 Structure of the dissertation
References
Chapter 2 LITERATURE REVIEW
2.1 Historical development and current state of medicinal plant use
2.2 The concept of sustainable use
2.3 Size-class distribution
2.4 Matrix modeling
2.5 Plant conservation target areas
References
Chapter 3 STUDY AREA, MATERIAL AND METHODS
3.1 Study area
3.2 Description of the species investigated
3.3 Methods
References
Chapter 4 AN EVALUATION OF THE EXTENT AND THREAT OF BARK HARVESTING IN THE VENDA REGION, LIMPOPO PROVINCE, SOUTH AFRICA
Abstract
4.1 Introduction
4.2 Study area
4.3 Materials and methods
4.4 Results and discussion
4.5 Conclusions
4.6 Acknowledgements
References
CHAPTER 5 POPULATION BIOLOGY OF ELAEODENDRON TRANSVAALENSE JACQ. IN THE PRESENCE OF HARVESTING
Abstract
5.1 Introduction
5.2 Study area
5.3 Materials and methods
5.4 Results and discussion
5.5 Conclusions
5.6 Acknowledgements
References
Chapter 6 POPULATION BIOLOGY OF BRACKENRIDGEA ZANGUEBARICA OLIV. IN THE PRESENCE OF HARVESTING
Abstract
6.1 Introduction
6.2. Species and study area
6.3 Materials and methods
6.4 Results and discussion
6.5 Conclusions
6.6 Acknowledgements
References
Chapter 7 IS THE PRESENT BRACKENRIDGEA NATURE RESERVE LARGE ENOUGH TO ENSURE THE SURVIVAL OF BRACKENRIDGEA ZANGUEBARICA Oliv.?
Abstract
7.1 Introduction
7.2 Study area
7.3 Materials and Methods
7.4 Results and discussion
7.5 Conclusions
7.6 Acknowledgements
References
Chapter 8 SYNTHESIS AND MANAGEMENT RECOMMENDATIONS
Abstract
8.1 Introduction
8.2 Discussion
8.3 Conclusions and recommendations
References
Chapter 9 REFERENCES
