ANALYSIS OF SOIL ORGANIC-CARBON AND VEGETATION COVER TRENDS ALONG THEBOTSWANA KALAHARI TRANSECT

Determination of trends in soil organic carbon (SOC) and vegetation co ver along savanna ecosystem moisture gradients is critical to the unde rstanding of ecosystem functioning and global change. Field results fr om 57 sites along the Botswana Kalahari Transect (BKT) showed general increases in both SOC and vegetation cover components along the temper ature/moisture gradient. However, details in both SOC and woody cover trends revealed an area of relatively declining values in the central Kalahari. Image classification using TM single band data as input valu es confirmed the existence of an anomalously low woody cover area in t he north-central Kalahari, within an area mapped as northern Kalahari tree and shrub savanna. It is postulated that the occurrence of such a zone of low shrubs in a gradient of otherwise increasing tree height and structural complexity may result mainly from edaphic factors, beca use of the co-incidence of near surface calcrete. However, the effects of fire and heavy grazing also influence the extent of low above-grou nd biomass, at least locally. Attempts were made to examine the extent to which soil organic carbon and vegetation components can be predict ed using Thematic Mapper (TM) imagery, to increase potentially the num ber of sites in the BKT database. As a result of correlation and regre ssion analyses, it was determined that the imagery-based SOC indexes o f the literature were not useful in the Kalahari environment because o f the extremely low SOC values encountered. Results of multiple regres sion analysis confirmed that woody vegetation cover could be predicted mainly through the use of TM3 single band radiance values throughout the BKT even during a non-drought period. Woody vegetation cover could be predicted to a lesser extent by the Transformed Normalised Vegetat ion Index. As high correlation coefficients were found between woody v egetation cover and SOC, the density of woody cover as derived from TM 3 data may be taken as a surrogate for relative SOC abundance, Because this method is indirect, field checking will be required to verify re sults. (C) 1998 Academic Press Limited.