Carbon sequestration and trading potential in semi-arid South Africa : a Karoo case study

[摘要] ENGLISH ABSTRACT: The succulent thicket plant community of South Africa, particularly Portulacaria afra (in this study referred to as spekboom), occurring in the Thicket Biome, sequestrates an exceptional amount of carbon dioxide (due to its photosynthetic properties), particularly for a warm, semi-arid region and in this capacity is more akin to forest ecosystems. Spekboom has additional favourable characteristics over forested systems regarding carbon trading (CT), namely: economic water use; potential for combating desertification and poverty in arid environments; and ability to withstand stand-replacing fire (spekboom doesn't burn) which improves its attraction as a commodity in CT and the payments for ecosystem services (PES) industry.Landowners interested in using the capacity of their land for carbon sequestration (CS) are challenged to calculate their plant communities' sequestration ability and biodiversity potential and therefore to quantify the carbon credits to be sold. The aim of the study was to quantify the CS ability of a selected property unit by vegetation area and to establish the carbon credits value that can be sold in the carbon market. It commenced by highlighting the problem of global warming and its effect on climate change in South Africa. CS is described as a process and a solution to decrease high and rising atmospheric CO2 levels, and its use in the carbon market to attain a monetary value to promote the growing of vegetation or to protect and conserve biodiversity that will enhance carbon stocks.The study site is Bosch Luys Kloof Private Nature Reserve (BLK PNR) in the semi-arid Karoo. Because CS potential is known to follow a rainfall gradient, one expects the semi-arid Karoo to have a low CS potential. However, this area contains spekboom, an extraordinary thicket type that sequestrates carbon at similar rates to forest ecosystems and should therefore reap the benefits of CT. Yet, accurate mapping of this vegetation is critical for accurate carbon stocks assessment. The CS potential of all vegetation communities in the study area had to be established to estimate the carbon stocks in the whole property unit.ArcGIS was used to map the vegetation communities (sub biomes) and eCognition to refine mapping of the Gamka Thicket (containing spekboom) through an object-orientated approach to automated vegetation mapping. For accuracy assessment a heads-up digitized map was created for comparison. The true surface area was calculated for the vegetation classification to ensure accurate area accounting on the mountainous terrain and this calculated area of the mapped vegetation was used to convert area to carbon sequestration potential. By examining the different markets and trading mechanisms for trading in the carbon market, using CS, a marketing strategy for the land units was advised.Results show that due to BLK PNR's history of overstocking, spekboom remains degraded on the study site. This creates an opportunity to restore the vegetation with funding through CT. The sequestration potential of spekboom on BLK PNR was determined through a regional differentiation comparison. The regional differentiation comparison identified that rainfall amount and carbon accumulation are inversely proportional, therefore more arid conditions (<200mm a year, or a dry season longer than seven and a half months) cause spekboom to switch to Crassulacean acid metabolism (CAM) due to water stress, so increasing the rate of carbon accumulation. In wetter environments, where spekboom is not under water stress, it continues in carbon fixation of 3-phosphoglycerate (C3) having a lower carbon accumulation. This is a remarkable finding as CS is known to follow a rainfall gradient: in this instance spekboom is an exception to the norm.Mapped results showed that eCognition classified spekboom poorly, yielding between 64% and 69% correspondence to the accurate manually classified map. As expected three-dimensional area comparative results show that the true surface area on complex terrains was 10% higher than the original (and surveyed) land area of the estate.Results indicate that there are substantial carbon stocks for CT on BLK PNR. This study recommended the most practised form of CT (restoration) for BLK PNR. Over a period of 30 years of restoration on BLK PNR about 46 000 tons of carbon could be sequestrated. At the current price of carbon credits this could accumulate over R3.7 million over the 30-year restoration period. Combining restoration with conservation strategies into one project type has additional favourable characteristics as it takes into account the CS potential of all vegetation communities on the land unit and the ecosystem services it promotes. Conservation of BLK PNR would yield 758 000 tons of carbon. Calculated through emission abandonment, this could be valued at R8-10 million. The economic importance of the vegetation community's biodiversity on BLK PNR and significant differences between biomes were recorded and implies a total intrinsic value in excess of R830 000 per annum.To date, pilot projects elsewhere have successfully acquired carbon credits for avoided deforestation through the climate, community and biodiversity alliance (CCBA). However, owing to the pilot stage status there is little literature that substantiates the calculation of the monetary value of conservation. This is a fertile area for further research.

[发布机构] Stellenbosch University