Land-use change impact on time-averaged carbon balances: rubber expansion and reforestation in a biosphere reserve, South-West China

The expansion of rubber plantations (Hevea brasiliensis Müll. Arg) is a major driving force for deforestation and forest degradation in mountainous South-East Asia. in China, the unclear distinction between natural forest and commercial plantations has permitted the expansion of rubber into protected forest areas of nature reserves. However, the impact of such land-use changes on landscape carbon balances within nature reserves has rarely been investigated. We selected the Naban River Watershed National Nature Reserve (NRWNNR) in Xishuangbanna, China as a case study area and evaluated carbon stocks using the Rapid Carbon Stock Appraisal (RaCSA) method based on tree, plot, land use and landscape assessments, integrating field sampling with remote sensing and GIS technology. Six dominant land use systems were surveyed: lowland forest (<800 m above sea level), highland forest (>800 m a.s.l.), lowland rubber, highland rubber, bush and grassland and agricultural crops. Land use type-specific time-averaged carbon stocks (Cta) were calculated based on total above ground biomass (AGB). Soil organic carbon (surface 30 cm) and below ground biomass were measured, but not included in Cta calculations due to large uncertainties unrelated to land use change. The Cta of lowland and highland rubber plantations were 58 Mg C ha-1 and 28 Mg C ha-1 respectively, showing larger carbon sequestration potential than nonforest land use types (agricultural crops, bush and grassland) but much lower than Cta of natural forest (156–185 Mg C ha-1). Sensitivity analysis of Cta variability showed that forest C stocks have the largest influence on landscape carbon balance. Time series analysis of land-use and land-cover maps (1989, 2007, 2012) demonstrated that during 23 years, the whole landscape of the nature reserve (26,574 ha) gained 0.644 Tg C. Despite rubber expansion, biosphere zoning strategy (i.e. experimental, buffer, core zones), and reforestation activities conducted in NRWNNR were able to enhance the carbon stocks.