Relationship between Tree Density and Grass Dry Matter Yield in a Southern African Savanna

The investigation was carried out on a densely wooded area in the Mopani savanna of South Africa. Seven plots were subjected to different intensities of tree thinning, ranging from a totally cleared plot (0 %) to plots thinned to the equivalent of 10 %, 20 %, 35 %, 50 % and 75 % of the leaf biomass of a control plot (100 %). The grasses responded positively to the tree thinning in terms of total dry matter yield. The yields between tree canopies were generally of the same order or higher than under tree canopies, with the yields where trees have been removed initially the highest. At high tree densities, yields differed little between seasons of varying rainfall. No quantitative advantages of C. mopane trees on the grass layer could be established, and the highest grass DM yields were recorded in plots where all trees were removed. The relationship between grass DM yield and tree density (expressed as Evapotranspiration Tree Equivalents ha-1) was curvilinear, best described by the exponential regression equation

The Influence of Tree Thinning and Tree Species on the Dry Matter Yield of Grasses of a Bush Encroached Semi-Arid Savanna in South Africa

The Acacia erubescens-Combetum apiculatum dominated savanna of South Africa is water-limited and an increase in woody plant abundance suppresses the grasses. This is the major reason why thinning or total clearing of all woody plants is often considered by landowners. The objectives of this study were to investigate the influence of intensity of tree thinning and tree species on grass yields in a semi-arid South African savanna

Grazing Impacts on Rangeland Condition in Semi-Arid South-Western Africa

The savannah biome, consisting of a dense herbaceous layer and a relatively open woody layer in competitive balance, constitutes 64% of the land surface of Namibia, an arid country in south-western Africa, and is used mainly for extensive cattle and sheep ranching. About half of the savannah area is affected by dense to moderately dense bush-thickening, resulting in a ten-fold decrease in the rangeland’s grass-based carrying capacity and a concomitant loss in meat production of about US$115 million per year (De Klerk, 2004). Bushencroached areas typically have densities \u3e 2 000 bushes/ha with \u3e 90% belonging to a single species. High grazing pressure by specialist grazers, such as domestic cattle, is often blamed for rangeland degradation. There is an urgent need to understand the dynamics of bush encroachment and devise grazing strategies to contain it

Preheating in Derivatively-Coupled Inflation Models

We study preheating in theories where the inflaton couples derivatively to scalar and gauge fields. Such couplings may dominate in natural models of inflation, in which the flatness of the inflaton potential is related to an approximate shift symmetry of the inflaton. We compare our results with previously studied models with non-derivative couplings. For sufficiently heavy scalar matter, parametric resonance is ineffective in reheating the universe, because the couplings of the inflaton to matter are very weak. If scalar matter fields are light, derivative couplings lead to a mild long-wavelength instability that drives matter fields to non-zero expectation values. In this case however, long-wavelength fluctuations of the light scalar are produced during inflation, leading to a host of cosmological problems. In contrast, axion-like couplings of the inflaton to a gauge field do not lead to production of long-wavelength fluctuations during inflation. However, again because of the weakness of the couplings to the inflaton, parametric resonance is not effective in producing gauge field quanta.Comment: 10 pages, 9 figure