Carbon sequestration and environmental effects of afforestation with Pinus radiata D. Don in the Western Cape, South Africa

A three-step methodology to assess the carbon sequestration and the environmental impact of afforestation projects in the framework of the Flexible Mechanisms of the Kyoto Protocol (Joint Implementation and Clean Development Mechanism) was developed and tested using a dataset collected from the Jonkershoek forest plantation, Western Cape, South Africa, which was established with Pinus radiata in former native fynbos vegetation and indigenous forest. The impact of a change in land use was evaluated for a multifunctional, a production and a non-conversion scenario. First, the carbon balance was modelled with GORCAM and was expressed as (1) C sequestration in tC ha−1 year−1 in soil, litter, and living biomass according to the rules of the first commitment period of the Kyoto Protocol, and (2) CO2 emission reductions in tC ha−1 year−1, which includes carbon sequestered in the above-mentioned pools and additionally in wood products, as well as emission reductions due to fossil fuel substitution. To estimate forest growth, three data sources were used: (1) inventory data, (2) growth simulation with a process-based model, and (3) yield tables. Second, the effects of land use change were assessed for different project scenarios using a method related to Life Cycle Assessment (LCA). The method uses 17 quantitative indicators to describe the impact of project activities on water, soil, vegetation cover and biodiversity. Indicator scores were calculated by comparing indicator values with reference values, estimated for the climax vegetation. The climax vegetation is the site-specific ecosystem phase with the highest exergy content and the highest exergy flow dissipation capacity. Third, the land use impact per functional unit of 1 tC sequestered was calculated by combining the results of step 1 and step 2. The average baselines to obtain carbon additionality are 476 tC ha−1 for indigenous forest and 32 tC ha−1 for fynbos. Results show that the influence of the growth assessment method on the magnitude of C sequestration and hence on the environmental impact per functional unit is large. When growth rate is assessed with the mechanistic model and with the yield table, it is overestimated in the early years and underestimated in the long term. The main conclusion of the scenario analysis is that the production forest scenario causes higher impacts per functional unit than the multifunctional scenario, but with the latter being less efficient in avoiding CO2 emissions. The proposed method to assess impacts on diverse components of the ecosystem is able to estimate the general tendency of the adverse and positive effects of each scenario. However, some indicators, more specifically about biodiversity and water balance, could be improved or reinterpreted in light of specific local data about threat to biodiversity and water status.