Another look at the savannah conundrum : evidence of niche segregation and competition avoidance with grasses in establishing trees
Bachelor Thesis
2014
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University of Cape Town
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Abstract
The savannah vegetation, although iconic, is a curiosity due to the coexistence of the two dominant, but remarkably different life-forms: trees and grasses. However, despite an apparent competitive advantage grasses do not dominate over trees in these systems and this presents a conundrum: how are savannah trees are able to establish in the face of such strong competition from grasses? As this is when trees are most susceptible to competition from grasses. This makes the knowledge of establishment phase critical for understanding the tree-grass co-existence. Despite this, there is a paucity of knowledge of this phase and the long-term aspects of the competitive interactions between trees and grasses, where the focus should be on seedling establishment and survival rather than biomass accumulation This study investigated the natural levels of tree establishment amongst differing amounts of grass as we sought to identify the limitation, both aboveground for light and belowground for resources, imposed by grasses on tree establishment. To uncover this limitation we searched for naturally established Acacia nigrescens (the dominant savannah tree species in our research area) seedlings at 12 sites in the Limpopo Province, South Africa. We also collected data on seedling health to determine the effect of proximate grass canopy on their performance. Finally, we compared the root lengths of established seedlings and co-occurring grasses to determine if seedlings were rooted in the same soil horizons as grasses. We hypothesised that tree establishment would be facilitated by low levels of grass biomass, as the lower the grass biomass, the more gaps would be present in the grass layer for seedlings to establish. We found that establishing seedlings were more resilient to grass competition than had previously been appreciated, as we found established seedlings (at a density of 9 seedlings per 100m2) with a grass canopy cover of 65%. We also found no correlation between our measure of grass competition and seedling performance. Finally, we found that even very young tree seedlings consistently root deeper than co-occurring grasses, despite establishing on different geologies, soils and across a rainfall gradient (p < 0.0001) - demonstrating that this resilience to grass competition is due to a specialised rooting strategy present in seedlings that allows them avoid belowground competition with grasses. In light of this, we propose a dynamic two-niche hypothesis of savannah tree-grass co-existence.
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Reference:
Lewis, J. 2014. Another look at the savannah conundrum : evidence of niche segregation and competition avoidance with grasses in establishing trees. University of Cape Town.