The population dynamics of Acacia nitlotica subsp. kraussiana in Northern KwaZulu Natal
Taylor, Janet Mary
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Globally the thickening of woody plants is on the increase, creating many issues within these areas of encroachment. Within KwaZulu-Natal, one such species, Acacia nilotica, is part of this phenomenon. A. nilotica is currently creating an issue over a large portion of the province. An increase in tree density lowers the amount of available grazing which reduces the stocking numbers of grazing herbivores. A better understanding of the drivers, phenology and growth of this plant will assist in providing information to control the establishment and increase of tree density of this plant species. Focussing on a smaller area in KwaZulu-Natal, uMkhuze Game Reserve, in the northern region of the province, was chosen as the study area. This reserve was chosen as the study area as it is currently experiencing an increase in A. nilotica density as well as having good historical management records and imagery. The Maxent species distribution model was used to determine the extent and potential distribution of A. nilotica within KwaZulu-Natal. For this study A. nilotica presence data, collected from surveys done by the KZN Department of Agriculture and Environmental Affairs, along with environmental variable data (rainfall, altitude, geology and mean minimum temperature) were used to model the potential distribution of the plant. Results indicate that a large portion of the province has the potential for A. nilotica occurrence. This distribution map has created an awareness of this plant for land users within the province. It also illustrates that a possible increase in temperature and rainfall from global change may provide more areas within KwaZulu-Natal with the potential to inhabit A. nilotica. Within uMkhuze Game Reserve, the plant communities were surveyed, described, classified and mapped. From this the approximate areas and tree density of A. nilotica were estimated. Seventy sites were surveyed with the 5 measurements for the BECVOL procedure. Site data were run through the BECVOL 3-model and grouped into one of the 12 identified vegetation communities. Of these 12 communities, A. nilotica were found to be in 8 of them. This is a possible 73% of the park that has the potential to contain A. nilotica. Tree density for these vegetation classes ranges from an estimated 950 to almost 7 000 plants/ha, while tree volume ranged from 5 200 to approximately 15 900 ETTE/ha. These results provide good baseline data for the vegetation of the reserve and illustrate the variety that an area can have. Different vegetation communities require different management practices and therefore this vegetation map illustrates these rough boundaries. An assessment of the monthly plant phenology was undertaken to gain insight into the inter-seasonal changes of A. nilotica. Browsing herbivores rely on the available browse of deciduous plants like A. nilotica, where forage becomes limited during the winter season. This understanding also provides insight into the biology of the plant for easier control. Thirty-two trees were marked and monthly leaf, flower and seedpod phenology was recorded on a qualitative scoring basis. Four branches within each of these trees were marked and repeat monthly growth measurements were taken. The leaf phenology had a seasonal pattern that seemed to be linked largely to temperature and photoperiod. New leaves budded in late September to October and leaves started to senesce by June. A. nilotica produces summer (October to February) flowers and seeds followed thereafter (March to June). The major plant growth was also found to be in the warm, high rainfall season. A distinct link was noted between tree stem circumference (age) and tree branch growth. These phenological seasons are useful in guiding management decisions for both the stocking of game in the reserve and for the control of A. nilotica. Finally, all the knowledge gained from this study and other literature sources were pulled together to develop a conceptual understanding on how best to aid in the control of this plant species. This conceptual understanding provides detailed information into the different life stages of the plant and a ‘tool kit’ of suggestions into the management and control of this plant. Through this research, an integrated management plan using a combination of holistic management measures is regarded as the recommended form of control for this plant species.