The role of landscape and parent material on regolith under timber plantations at Highflats, KwaZulu-Natal, South Africa

Abstract

Regolith is an important component of the Earth's critical zone. Little investigative work has been done in southern Africa on regolith characteristics as a component of the critical zone, and in particular the importance to forestry plantation productivity in the eastern hinterland of South Africa. Regolith cores were extracted from five positions across two slopes underlain by sandstone and tillite to characterize these regolith profiles and ascertain the effect of slope position and parent material on regolith development. Regolith profiles were characterized and analysed on hill crest and lower midslope positions at both sites. The profile descriptions noted the degree of weathering, rooting activity, and mineral composition. The deepest in situ weathering of regolith was found on the hill crest positions; sandstone crest (24.0 m) and tillite crest (8.2 m) respectively. The accumulation of colluviated sediment on lower slopes also resulted in a thicker regolith at the tillite site, despite the less intensely weathered bedrock. These findings were consistent with earlier geophysical and pedohydrological studies of the area and the regolith profiles are consistent with those described for a range of other sedimentary lithologies in similar regions in sub-tropical and tropical landscapes The results show that tree roots utilize the saprolite and saprock below soil horizons and the nature of these layers indicate the full regolith profile should be considered in the water and nutrient supply available to forest plantations. Current concepts of ‘effective rooting depth’, as widely used by South African forestry, are inadequate for understanding ecosystem processes in forest plantations.