Influence of land use on phytomass accumulation in Highland Sourveld grassland in the southern Drakensberg, South Africa

Abstract

Transformation of indigenous vegetation for production purposes impacts negatively on biodiversity but does this necessarily equate with a decrease in phytomass accumulation, which may influence carbon sequestration? Phytomass accumulation was studied for common land covers of beef ranching or dairy farming under private tenure, plantation forestry, crops and rangelands of communal areas, and conservation areas in the southern Drakensberg. Annual turnover in above-ground phytomass (AGP) was estimated by harvesting at seasonal extremes, or at bimonthly intervals for grasslands and pastures. Peak root biomass was estimated by coring. Tree biomass (44792 g m−2) of pine plantations unsurprisingly dwarfed total AGP (litter plus standing) of all other cover types, but total AGP of plantations even excluding trees was highest because of litter accumulation. Total AGP ranged from 82–1423 g m−2 in mid-summer and 215–1008 g m−2 in mid-winter. Total AGP was otherwise highest for commercial maize, followed by kikuyu pasture, conservation grassland, Eragrostis pasture, commercial rangeland, rangeland and maize under communal tenure, and least for annual ryegrass pasture. Intensively used pastures were therefore superior, equivalent, or inferior to indigenous grasslands in terms of phytomass accumulation, depending on the pasture and grassland compared. Excluding plantations, total AGP consisted mainly of standing AGP. Graminoids contributed >83% of standing AGP for all cover types except communal maize (54%); necromass contributed 33–47% for grasslands or pastures in mid-summer, and 86–93% for all in midwinter except ryegrass pasture (49%). Root biomass varied sixfold among cover types (648–4128 g m−2) and contributed greater than 75% to total phytomass (root, litter, standing AGP) for all cover types except maize (46%). Although soil stocks of carbon are usually far greater than vegetation stocks in grassland, significant increases in vegetation stocks can be achieved through choice of land use and management.