Carbon stock assessment for a forest-to-coffee conversion landscape in Kalikonto watershed (East Java, Indonesia): Scaling up from plot to landscape level

The impacts on net sequestration of CO2 or net release to the atmosphere of CO2, of a change in land use from natural forest to tree-based agricultural system can be rapidly estimated by measuring the carbon (C) stocks of both land use systems in a Rapid Carbon Stock Appraisal (RACSA). Aim of this study was to assess the aboveground C stocks at landscape level for the Kalikonto sub-watershed (Malang regency, East Java), using land use maps of 1990 and 2005. During that period remaining natural forest was reduced by 2.6% per year and by 33% on aggregate, while the total area of annual crop and of settlements increased by 2.5%/year (aggregate 45%) and 1.1%/year (aggregate 18%), respectively. The area of tree plantations and agroforestry were reduced about 0.6%/year (aggregate 10%). Plot-level measurements of aboveground C stock in June –December 2008 covered the eight land use systems (LUS) most commonly found in the study area: 1) remnant natural forest, 2) bamboo forest, 3-5) 3 types of plantation i.e. pine (Pinus merkusii), mahogany (Swietenia mahogany) and dammar (Agathis dammara); 6) multistrata shaded coffee with fruit and timber trees, as well as nitrogen-fixing shade trees; 7) simple shade coffee (using Gliricidia sepium as shade tree); and, 8) annual cropping systems (vegetable and food crops). The natural forest in Kalikonto area has been severely disturbed as shown by a low aboveground C stock of about 136 Mg ha-1. Aboveground C stock in coffee-based agroforestry systems was lower, at about 44 Mg C ha-1. The time averaged C stock of tree plantations (pinus, mahogany, and damar mostly aged 25-40 years) was estimated to be 85 Mg C ha-1. The estimated time averaged-C stock (above-ground) in annual crops was only 2 Mg ha-1. Extrapolation of C stock at plot level to watershed level were done by multiplying the area of each land cover with its time-averaged aboveground C stock. Within 15 years, C lost for the whole watershed (23810 ha) was estimated to be 27750 Mg yr-1 or equivalent to a yearly C loss of 1.17 Mg ha-1. Carbon lost from natural forest was about 0.92 Mg ha-1 yr-1, tree plantations lost 0.15 Mg ha-1 yr-1. Carbon lost from coffee-based agroforestry systems was relatively small, about 0.03 Mg ha-1 yr-1. Planting more diverse shade trees in coffee-based agroforestry system may increase the role of coffee gardens in net C sequestration at the landscape scale.