Above- and belowground competition in Savanna systems.
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Date
2008
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Abstract
The structure and composition of savanna vegetation is influenced by resource availability
and disturbance. Grasses, a major component of savannas, influence this resource
availability by competing directly with trees for light, water and soil nutrient resources.
The direct causes of bush encroachment are not always apparent, but are commonly
ascribed to overgrazing and consequent decreased grass competition. The interaction, both
above and belowground, between tree and grass seedlings and the surrounding grass sward
is dependant on many factors, such as soil depth, seedling species and sward composition.
These factors, as well as the presence or absence of defoliation, in the form of grazing or
fire dictate whether the system will remain in a transition state as savanna or move towards
a stable woodland state. The major competitive effects experienced by the tree seedlings
were dependant on grass species and nutrient level. A. nilotica was affected by
aboveground competition while A. karroo was affected by belowground competition. E.
capensis caused the greatest decrease in A. karroo plant biomass. Both E. capensis and H.
hirta had large competitive effects on the aboveground biomass of A. nilotica, while S.
africanus had the greatest effect on belowground biomass. Increasing nutrient availability
resulted in an increase in the competitive effect exerted on A. karroo, while little to no
change was seen in the competitive effect exerted on A. nilotica. Soil depth constrained
plant size in both tree species. The intensity of belowground interactions on tree biomass
was unaffected by soil depth, while aboveground competition had a significant effect on
shallow soils. Belowground competition was also of greater importance than aboveground
competition in dictating tree seedling height. Grass seedlings growing on all three soil
depths differed in mean mass, with E. racemosa having the least mass and T. triandra
having the greatest. Simulated grazing by cutting the surrounding sward resulted in
biomass increases in all three grass species. Changes in savanna composition and structure
are thus likely to be influenced by initial species composition and soil depth and soil
nutrient composition. While grazing creates niches for grass seedling establishment, heavy
grazing has been observed to increase grass seedling mortality. Encroachment is thus more
likely to occur on intensively grazed shallow and deep soils than on medium depth soils.
This highlights the importance of ensuring the grass sward remains vigorous by resting and
monitoring stocking rates to ensure veld is not over-utilized. It is then possible to maintain
some form of tree-grass coexistence at a level where available grazing is not compromised.
Description
Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2008.
Keywords
Competition (Biology)--KwaZulu-Natal., Grasses--KwaZulu-Natal., Acacia--Seedlings., Soil depth--KwaZulu-Natal., Savanna ecology--KwaZulu-Natal., Range management--KwaZulu-Natal., Theses--Grassland science.