Carbon sequestration on the subtropical dunes of South Africa: a comparison between native regenerating ecosystems and exotic plantations

Abstract

Rehabilitation and revegetation of mined coastal sand dunes on the east coast of South Africa makes sense. It recovers ecosystem services such as carbon sequestration. The outcome of rehabilitation, which covers a third of the mined area, is a secondary coastal dune forest similar to the forests in the region. The aim of revegetation, on the other hand, is to establish Casuarina equisetifolia J.R.Forst.&G.Forst. plantations on the remaining two thirds of the mined area, for the small-scale production of charcoal. The ratio of these two post-mining land use options has consequences for the carbon sequestration potential of the mined area. As growth rate could be a reflection of carbon sequestration rate, this study compared the growth rate of Acacia kosiensis P.P.Swartz, the species that dominates rehabilitated stands, to that of C. equisetifolia in response to rainfall. The carbon sequestration potential of the two post-mining land use options was subsequently evaluated by measuring carbon storage in wood, the herb layer, litter and soil in different-aged stands. I also compared the financial potential of the two land use options. Tree ring analysis could not be applied to the two species. No correlation between growth and rainfall could be found either. On average, A. kosiensis grew twice as fast as C. equisetifolia. Carbon storage in the wood, herb layer, litter and soil in rehabilitated stands of known age (7, 11, 17 and 21 years old) differed from the revegetated stands (8, 12, 16 and 19 years old). More carbon was stored in the revegetated stands than the rehabilitated stands. I attribute this primarily to the relatively larger wood and litter components of the former. C. equisetifolia, however, is harvested for charcoal production after about sixteen years of age, thus releasing most of the carbon stored in wood. The present ratio of rehabilitation to revegetation (1:2) is not optimal for long-term carbon sequestration. Rehabilitation costs more, but the income potential thereof, as determined in this study, is less than that of revegetation. This, however, does not reflect the true financial potential of the two land use options. The financial analysis performed in this study only considered income from the sale of timber, charcoal and carbon credits. It excluded other potential benefits of the rehabilitation of coastal dune forests. These may include the contribution to biological conservation and ecological services such as dune stabilisation and water purification. Coastal dune forests also provide habitat for a variety of organisms adapted to live in them. All these have a value. Their inclusion in a detailed cost-benefit analysis could render rehabilitation as the more financially efficient option.