Root biomass of carbon plantings in agricultural landscapes of southern Australia: Development and testing of allometrics

Root biomass may to contribute a substantial proportion of the carbon sequestered in new tree plantings, particularly in regions where rainfall and/or site quality is relatively low as this may result in relatively high allocation of plant biomass below-ground to source required water or nutrients. However, root biomass is often overlooked because of difficulty with measurement. In Australia, most carbon plantings are currently mixed-species environmental or mallee eucalypt plantings on agricultural land in regions with rainfall of 250-850 mm year−1. Here, we collated new and existing root biomass data from ca. 900 individual trees or shrubs to develop and test allometric equations for predicting root biomass based on stem diameter (of unharvested trees or shrubs) or height (of coppice harvested trees) in these plantings. Equations developed showed significant differences between groupings of species with differing growth habits or from different genera. Grouping species into categories of: (i) non-eucalypts, (ii) tree-form eucalypts, (iii) unharvested mallee eucalypts, and (iv) coppiced mallee eucalypts, provided equations with model efficiencies of 0.64-0.90. In the process of collating data across different studies, corrections were required for data consistency. Uncertainty analysis showed that although these corrections resulted in some uncertainty in the equations developed, measurement errors, particularly of stem diameter, were also important contributors to this uncertainty. We tested equations developed using data from 11 environmental and mallee planting sites where direct measurements of root biomass were made through whole-plot excavation. Site-level predictions of root biomass from individual tree allometry were effective, with an efficiency of prediction of 0.98. These results indicate that the generic allometric equations developed can be confidently applied across the Australian agricultural region with 250-850 mm year−1 rainfall to obtain accurate regional estimates of root biomass in the currently relatively young (<20 year old) environmental and mallee plantings.