Estimating temporal changes in carbon sequestration in plantings of mallee eucalypts: Modelling improvements

Establishment of mallee eucalypt plantings on cleared agricultural land is currently the predominant method of reforestation for carbon sequestration in Australia. Investment in establishing and maintaining such plantings relies on having a cost-effective approach for providing un-biased estimates of yield in biomass and carbon sequestration. The Australian Government's forest carbon accounting model (FullCAM) had not previously been calibrated for mallee eucalypt plantings and, in many circumstances, substantially under-estimated of biomass for these plantings. Our objective was to improve model applicability and reliability of estimates of carbon sequestration. To achieve this, we first collected and analysed above-ground biomass data from 257 mallee eucalypt plantings (or 744 observations, when including the multiple measurements made at some planting sites) to determine the key factors influencing growth. Plantings were categorised according to species, planting configuration (block or belt plantings) and stand density. Each category of planting had significantly different rates of growth, with the rates of sequestration of above-ground biomass carbon being relatively high when established in densely-stocked, two-row belts. These categories of plantings then provided the basis for calibration (estimation of appropriate modifiers) of FullCAM growth curves. Overall model efficiency was 63%, and there was no apparent bias when the model was applied to the various planting categories. Thus, modelled estimates of biomass accumulation will be reliable on average but at any particular location will be highly uncertain, with either substantial under- or over-prediction possible. For some categories of mallee eucalypt plantings, and for plantings with access to ground-water or established in non-productive soils, there were insufficient observations to provide confidence in new calibrations specific for these circumstances. Moreover, application of the calibrations provided here are applicable for prediction of sequestration of biomass carbon of relatively young (<15-year-old) stands, with more data needed for prediction of longer-term rates of growth.