Accuracy of tree root biomass sampling methodologies for carbon mitigation projects

- Precision of biomass estimates improved by 10-15% if soil coring was focused in closer proximity to sample trees.
- Whole tree plot excavation improved the accuracy of tree root biomass estimates by up to 4% compared to root ball excavation.
- Tree age and root morphology are important factors governing sampling design and recovery of tree root biomass.

Tree root biomass contributes significantly to forest carbon pools but is difficult and costly to estimate. Accurate estimates of carbon fluxes from tree roots are required for specific mitigation projects, national accounts and global climate models. However, methodologies for the estimation of tree root biomass are varied and their effectiveness in terms of the precision of biomass and carbon estimates is difficult to evaluate. In this study tree root systems of 2 and 7 year old Eucalyptus globulus (Labill) trees were sampled volumetrically to a depth of 6 m using soil coring and excavation. These root mass data were used for Monte Carlo simulation of four different but typically used sampling strategies.The uncertainty of estimates increased with tree age as a result of increased heterogeneity of root mass due to the presence of large diameter roots. Coring had the largest sampling uncertainty when applied to estimate coarse root biomass which did not include the root bole. Bulk excavation was the simplest and potentially the most time efficient method to attain a sampling uncertainty of 10%. Excavation to a root diameter limit enables better association of root biomass to individual sample trees however, more effort is required if roots are directly related to the sample tree by tracing of roots to a small diameter. Although root ball methods required the least amount of excavation and soil coring to attain a sampling uncertainty of 10%, this method requires sieving of excavated soil through a small mesh size and is more time consuming and difficult to apply in heavier textured soils. The efficiency of coring and precision of root mass estimates can be improved if coring is concentrated in closer proximity of the sample tree or within the boundary of the larger diameter proximal roots.