Impacts of Acacia melanoxylon R.Br. (blackwood) on the environmental conditions and phytosociology in forest communities in Harkerville forest, South Africa
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There have been very few studies on the impacts of Acacia melanoxylon on the environmental conditions in forest communities in South Africa. The aim of this study was to determine the effects of this exotic species on the environmental conditions within forest communities in Harkerville forest, located between Knysna and Plettenberg Bay in South Africa. The effects on the nutrient, moisture and organic content of the soil were studied, as well as the effects on canopy closure and the light penetration to the forest floor. In addition, potential sources of invasion were also identified in the area. It was determined that blackwood did not influence the moisture and organic matter content of the soil, it did however affect the phosphorus and ammonia content. Phosphorus was found to be significantly lower in the vicinity of the blackwood trees. The soil nitrate concentration was significantly higher. Canopy closure was significantly reduced in the invaded areas, which had resulted in a significant increase in light penetration to the forest floor. This was attributed to blackwood's preference for open areas. Major sources of blackwood seed identified in the area, were a very dense stand of blackwood on private land and blackwood plantations in the area. The phytosociology was determined not to be different between the invaded and non-invaded. The study determined that the blackwood altered the tree layer by reducing the number of tree species that comprise the layer. It was concluded that blackwood had minimal impact of the on the environmental conditions measured. Even though the differences in the environmental conditions measured were significant, they were slight. The major issue identified was the dispersal of seeds from nearby sources . Fortunately the seeds do not germinate readily under the closed canopies in the forest. Management and monitoring of blackwood trees is prominent in the area, the adult blackwood trees are ring-barked and the seedlings removed within the vicinity of water courses.