Assessing the carbon sequestration potential of managed forests: a case study from temperate Australia.
The concept of assessing forests for carbon sequestration is well established. Operationally, estimating a forests' potential to sequester carbon requires comparing its current carbon state with a prediction of its carbon carrying capacity (CCC). Assessment of CCC is, however, problematic. Mathematical models can be used, although traditional modelling techniques, where parameters are estimated from empirical measurements, are usually limited by a lack of field data. For example, estimates of carbon residency times in vegetation and soil are not generally available, nor are they easily measured. Alternative methods are required. Current carbon stocks in 17 previously logged field sites were measured by field survey. CCC for those sites was then predicted using a terrestrial carbon model, calibrated with measurements from mature, unlogged vegetation of a comparable forest type. Model parameters were estimated using 'model-data fusion' methods, where the model is inverted and field measurements of the carbon stocks (the model outputs) are used to calibrate the model parameters. Spatial variation was included through functions defining landscape-scale effects on plant growth relating to topographic influences on light and soil water availability. Current above-ground carbon stocks (living plus litter) varied with management history, averaging 273±30 tC ha-1 (mean±SE). Model-predicted CCC was 445±13 tC ha-1, yielding a carbon sequestration potential of 172±31 tC ha-1. Model simulations predicted the recovery of an average site to take 53 years to reach 75% carrying capacity, and 152 years to reach 90% carrying capacity. Extrapolation of these results to 7 Mha of comparable managed forests in the same region suggested a potential carbon sink of 680-895 Mt C. Synthesis and applications. In this study we have demonstrated that forests recovering from prior logging have the potential to store significant amounts of carbon, with current biomass stocks estimated to be approximately 60% of their predicted carrying capacity, a value similar to those reported for northern temperate forests. Although sequestration activities often focus on the aforestation and reforestation of previously cleared land, our results suggest that, where appropriate, native forest management should also be considered when developing terrestrial carbon management options, and for terrestrial carbon accounting more generally.