Prediction of decomposition of litter under eucalypts and pines using the FullCAM model

Abstract

The FullCAM model was designed, by the Australian Greenhouse Office, for full carbon accounting in forests at project and national scales, and links the empirical carbon tracking model CAMFor to models of tree growth (3PG), litter decomposition (GENDEC), and soil carbon turnover (RothC). We collated data from 385 studies of decomposition of foliage, bark and wood litter in eucalypt and pine forests for calibration and validation of two alternative sub-models available within FullCAM for decomposition: empirical algorithms in CAMFor, and the process-based GENDEC (requiring additional data on the chemical properties of litter and nitrogen availability). Decomposition rates were derived using short-term litterbag studies to determine mass loss after 1 year, or using long-term rates of litter accumulation from known amounts of litterfall inputs. Half of the data from the litterbag studies were used for calibration of CAMFor, while GENDEC was calibrated using litterbag studies where the chemical properties of litter components had been measured. Remaining data for litterbag studies not used for calibration, and litter accumulation studies, were used for model validation. CAMFor accounted for 60–67% of the variation in decomposition of foliage, bark and dead wood of eucalypts and pines, while GENDEC accounted for only 53–56% of this variation. Therefore, despite its simpler structure, decomposition was more accurately predicted using CAMFor than GENDEC. Results suggested that it was important to account for the effect of temperature and rainfall on decomposition, but utilising data on the chemical composition of litter was less useful. Performance of the two sub-models was comparable when decomposition rates were calibrated together with the fraction of carbon in the fast, medium and slow pools of litter simulated by GENDEC, rather than this partitioning of carbon between pools being restrained by actual values of lignin, cellulose and soluble content of each component of litter.