INSIGHTS Estimating the Aboveground Biomass of Bornean Forest Sugeng Budiharta 1,2,8 , Ferry Slik 3 , Niels Raes 4 , Erik Meijaard 1,5,6 , Peter D. Erskine 7 , and Kerrie A. Wilson 1 1 Australian Research Council Centre of Excellence for Environmental Decisions, School of Biological Sciences, The University of Queensland, Brisbane, Queensland 4072, Australia 2 Purwodadi Botanic Garden, Indonesian Institute of Sciences, Jalan Surabaya-Malang Km. 65, Pasuruan, East Java 67163, Indonesia 3 Faculty of Science, University Brunei Darussalam, Jalan Tungku, BE1410, Brunei Darussalam 4 Naturalis Biodiversity Center, Section Botany, Leiden 2333 CR, The Netherlands 5 Borneo Futures Project, People and Nature Consulting International, Country Woods House 306, Jalan WR Supratman, Ciputat, Banten, Indonesia 6 Center for International Forestry Research, P.O. Box 0113 BOCBD, Bogor 16000, Indonesia 7 Centre for Mined Land Rehabilitation, The University of Queensland, Brisbane, Queensland 4072, Australia ABSTRACT zWe apply a process-based model, called 3-PG (physiological principles for predicting growth), to estimate aboveground biomass for the primary forests of Borneo. Using publicly available soil and climate data, and parameterized with physiological traits of Bornean forest, the modeled aboveground biomass and basal area showed statistically significant relationships with field-measured data from 85 sites across four major forest types. Our results highlight the possibility to expand the application of 3-PG to forests of varying condition, which would facilitate inclusion of modeled forest biomass data for developing a Tier 3 carbon inventory system for Borneo. Abstract in Indonesian is available in the online version of this article. Key words : Borneo; carbon sequestration; carbon-accounting system; forest growth; forest type; process-based model. THE KYOTO PROTOCOL UNDER THE UNITED NATIONS FRAMEWORK CONVENTION ON CLIMATE CHANGE (UNFCCC) requires every sig- natory nation to standardize their carbon-accounting systems (United Nations 1998). All 44 Annex 1 countries have submitted a National Inventory Report that contains a toolbox of method- ologies for calculating sources and sinks of carbon (UNFCCC 2013). Non-Annex 1 countries are requested to submit a volun- tary National Communication biennially, which outlines progress with the development of a carbon inventory system. Since 1999, only four non-Annex 1 countries (of 154 countries) have updated the third National Communication and only Mexico has completed the fourth and fifth reports (UNFCCC 2013). Previous attempts to measure carbon dynamics associated with the tropical forests of Southeast Asia have mostly focused on regional quantification of stocks and losses generalized across forest types (Gibbs et al. 2007, Saatchi et al. 2011, Harris et al. 2012, Carlson et al. 2013, but see Berry et al. 2010). This could potentially fail to capture carbon gains at a local scale, leading to overestimation of net carbon emissions. These approaches would be classified as Tier 1 and Tier 2 due to the lower resolution of information associated with the stock change methods employed (IPCC 2006). Tier 3 approaches require detailed forest inventory data across a variety of forest types, complemented with region- specific process-based model(s), which have been field validated (IPCC 2006). To our knowledge, there have been no Tier 3 approaches trialed in Southeast Asia. This study is the first application of a process-based model, 3-PG (physiological principles for predicting growth), to predict aboveground biomass (AGB) dynamics of the primary forests of Borneo. The 3-PG model estimates stand development based on physiological processes that are simplifications of plant-environment interactions (Landsberg & Waring 1997). This model has been used for single-species plantations in temperate and subtropical regions (Landsberg & Sands 2011) and has resulted in accurate estimates of the growth of highly diverse forests in the Australian wet tropics and Amazon (White et al. 2006, Nightingale et al. 2008). We aim to apply the 3-PG model to obtain baseline esti- mates of the upper limits of AGB accumulation for the tropical forests of Borneo. We evaluate the performance of the model, and appraise the utility of the approach for developing a Tier 3 carbon-accounting system for Borneo. MODEL DESCRIPTION.—The 3-PG is a process-based model, which calculates forest productivity from absorbed photosynthetically active radiation and canopy quantum efficiency, constrained by atmospheric vapor pressure deficit, soil characteristics, and tem- perature (Landsberg & Waring 1997). The model consists of five Received 14 November 2013; revision accepted 19 May 2014. 8 Corresponding author; e-mail: s.budiharta@uq.edu.au ª 2014 The Association for Tropical Biology and Conservation 1 BIOTROPICA 0(0): 1–5 2014 10.1111/btp.12132