Testing allometric equations for prediction of above-ground biomass of mallee eucalypts in southern Australia

In medium-low (250-850 mm year⁻¹) rainfall regions of southern Australia, reforestation with mallee eucalypts is promoted for biomass production for carbon sequestration and/or bioenergy. Cost-effective estimation of biomass is essential for assessing the economic viability of plantings. To explore this, we collated biomass data from 198 stands in southern Australia (N = 3384 individual trees, including 1065 trees re-grown as coppice) and developed allometric equations for non-destructive estimation of above-ground biomass based on either stem diameter at 10 cm height for uncut trees, or on crown volume index for coppiced trees. Three classes of allometric equations were developed. In order of decreasing specificity, these were; (i) site-and-specie specific, (ii) generic species-specific, and (iii) generic multi-species. Validation at the site level was realised by analysing the bias, precision and overall accuracy of allometry-predicted biomass when tested against directly-measured biomass harvested manually from whole-plots across six contrasting sites. Another 17 plantings were harvested with a mechanical harvester. A finer-scale analysis investigating the performance of these allometric equations at the individual-tree level across all stands was also undertaken. When predicting biomass at the site-level using either of the generalised equations, the percentage error of prediction was <±9%, but could be in the range of ±15% to ±21% at individual sites. Precision, and thus accuracy, increased slightly with the level of specificity of equations. Although allometry was statistically significantly influenced by climate, inclusion of the site-specific factor of average rainfall in generic equations increased efficiency of prediction of above-ground biomass by only 5%. We conclude: (i) site-and-species specific equations are more accurate than generic equations for predictions at the site-level, and (ii) generic equations, particularly species-specific relationships, can be confidently applied to provide regional, or estate-level, estimates of above-ground biomass across a range of mallee eucalypt plantings in the medium-low rainfall regions of southern Australia.