Satellite‐derived vegetation indices were used to identify a geochemical mound of higher active greenness in the Karoo National Park, Beaufort West, South Africa. We determined whether this mound was occupied by plants with higher CO2assimilation rate. Plant cover on and off the mound was determined. Three woody species with high cover were selected for further investigation. Two deep‐rooted species,Rhigozum obovatumBurch. andEriocephalus ericoides(L.f.) Druce, showed greater net CO2assimilation rates on the mound. Net CO2assimilation rates for the third species,Pentzia incana(Thunb.) Kuntze were similar both on and off the mound. In an attempt to find a mechanistic basis for the elevated CO2assimilation rates, the relationships between soil factors, foliar nutrients and CO2assimilation capacity were also examined. Our results suggest that the elevated net CO2assimilation was not mediated via improved soil or plant water relations on the actively greening mound, nor through a difference in nitrogen levels in the soil or plant material, but possibly by way of the higher sub‐soil phosphorus levels measured from the geochemical mound. Genotype and cover cannot alone be used for rangeland condition assessment since localized elevated soil nutrient status (patchiness) contributes to greater photosynthetic carbon gain which may confer superior browsing responses to plants occurring on these mounds.