Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Woody biomass production ecology in Kalahari communal areas of Botswana

Auteur : Sekhwela, M. B. M. 

Université de soutenance : University of Queensland

Grade : Doctor of Philosophy (PhD) 2000

Résumé partiel
The savannas in Southern Africa have remained comer stones of subsistent livelihood in rural areas of countries in this region for millennia. Despite the observed depletion of preferred tree species for fuel and other wood needs around settlements, ecological information on wood resources in savannas under intensive communal land use remains scanty. The current study has investigated vegetation structure, production ecology and environmental factors influencing it in two areas with different rainfall regimes but comparable Kalahari sandy soils, in the savannas of Botswana. Study sites were located near Tsabong (25°56.65’S 22°27.72’E) with a rainfall of 300 mm yr-1 and near Maun (20°09.60’S 23°46.63’E) with a rainfall of 500 mm yr-1. The main objectives were to determine the production rates of economically important woody species and the major abiotic determinants of production with a view to contributing to sustainable management of the ecosystems studied. Ecological, ecophysiological and micrometeorological methods were used to describe and understand the interactions between plant processes and influential climatic and other abiotic factors, resulting in the measured plant growth.

The baseline information showed significant differences between vegetation in the low and high rainfall areas, with 5 and 24 species of woody plant found, respectively, in the two areas. The woody vegetation cover and densities also differed accordingly. The three economically important species studied (.Acacia luederitzii, Acacia erioloba and Acacia mellifera) contributed 72.9 and 20.6 % of tree densities at the Tsabong and Maun sites, respectively. At Maun, the bush encroachment species Dichrostachys cinerea and Rhus tenunervis contributed the highest proportions of 35.3 and 14.7 % to woody species density, respectively, as a result of overgrazing in the past. However, the important three species contributed the bulk of the standing woody biomass (96.6 and 85.6 %) with total standing fresh biomass of 14 and 23 t ha-1 at Tsabong and Maun, respectively. Comparably low levels of monitored major nutrients were found at both sites with 1507 and 1341 μg/100g nitrogen, 71 and 105 mg/100g phosphorus measured at Maun and Tsabong, respectively. The availability of these nutrients was highest during the rain season (November - March) and was related to soil moisture in the topsoil layers, consistent with general observations on intermittent nutrient release in semi-arid and arid environments. Other studies observed the influence of livestock on nutrient dynamics of grass covered areas and areas below tree canopies, and some observations in this study are comparable with those reported studies.

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