Vachellia sieberiana var. woodii a high-altitude encroacher: the effect of fire, frost, simulated grazing and altitude.
Russel, Jennifer Mary.
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ABSTRACT There is increasing evidence that savannas and grasslands throughout the world are experiencing bush encroachment. The replacement of grassy biomes with woody biomes has serious consequences for net primary productivity. The grasslands of South Africa are not exempt from this phenomenon. Despite this, the drivers of the tree:grass dynamics are still robustly debated. In mesic and moist savannas and grasslands, the tree:grass balance appears to be maintained mainly through disturbance such as fire, frost and herbivory or a combination of disturbances. Other factors such as competition for resources may play a modifying role. High altitude grasslands are frequently within a climatic zone that would support trees, yet trees are absent. The answer as to what mechanism excludes trees from these grassy biomes continues to elude researchers. Very often low temperature is cited as a possible mechanism. Vachellia sieberiana var. woodii is a typical savanna tree which is absent from high altitudes. However, it has been encroaching into the grasslands along the escarpment of the Drakensberg, KwaZuluNatal, South Africa, over several decades, although is still excluded from the top of the escarpment. I acquired aerial photographs and satellite images covering the Van Reenen’s Pass area, north-western KwaZulu-Natal, dating from 1955 to 2015. These images confirmed that V. sieberiana was increasing in density along the escarpment, but that no V. sieberiana was present on top of the escarpment, despite the successful establishment of other tree species. The photographs and images also suggested that V. sieberiana was extending its range into higher altitudes. Because fire, frost and herbivory are generally thought to be the determinants of the structure of grasslands and savannas, I conducted field trials along the altitudinal gradient on Van Reenen’s Pass, investigating the effect of these determinants on the establishment of transplanted V. sieberiana saplings at three different altitudes. Competition for resources were briefly taken into consideration, although they were not the main thrust of the project: soil nutrients and root gaps. Soil moisture was not a concern as the area is what is defined as mesic. The transplanted saplings were smallest at the high-altitude site and largest at the low-altitude site after two growing seasons in the field. The response of the saplings to the various treatments was not consistent at the three sites. There was no response to the treatments at the highaltitude site; a significant response to fire, frost and simulated grazing at the mid-altitude sites; and a significant response to frost and simulated grazing at the low-altitude site.