Leaf nutrient draw back as a strategy for tree grass co-existence in the savanna biome

Bachelor Thesis


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University of Cape Town

As there is still a major debate on how trees and grasses manage to coexist in the savanna biome, the niche separation by phenology hypothesis (Scholes and Archer 1997) was tested by observing the seasonal variation in carbon and nitrogen concentrations in leaves and young twigs of two deciduous species, Combretum apiculatum and Terminalia sericea. The study was carried out in the sub-tropical savanna biome in southern Kruger National Park, South Africa. Photosynthetic readings show a decrease in leaf activity towards the end of the growing season. There was no significant change in the carbon concentrations over the months for both species sampled but there was a significant decrease in leaf nitrogen for C. apiculatum. T. sericea experienced relatively no nitrogen draw back until a sudden drop at the end of June. Translocated nitrogen in C. apiculatum accumulated in large amounts in the peripheral twigs (branching order one) compared to T. sericea, where nitrogen only increased at the end of June in the same location. There was also a corresponding significant change in the carbon:nitrogen ratio in C. apiculatum, especially in the leaves, showing a decrease in palatability as nitrogen was drawn out. The results observed did not become more pronounced in site 3, which had the highest soil moisture content. Soil moisture availability therefore does not appear to influence the degree of nitrogen reabsorption from the leaves. The hypothesis proposed is that deciduous trees use stored nitrogen from their senescing leaves to leaf out earlier gaining a competitive advantage over grasses, which are waiting for nitrogen to be mineralized in the soil with the onset of the summer rain. This study therefore supports the niche separation by phenology hypothesis to explain how these two growth forms coexist in the savanna biome.