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Overview: During the last two years, a major nationally-collaborative research program has been lead by CSIRO to improve the estimation of biomass accumulation by mixed-species environmental plantings and mallee eucalypt plantings. It has involved evaluation of the uncertainties associated with using alternative approaches to biomass estimation, and the collation and refinement of new and existing field inventories and biomass estimates for these plantings, growing in various configurations throughout the non-arid (>300 mm mean annual rainfall) regions of Australia. A large database on growth and biomass accumulation across a wide range of planting types has been developed, comprising 1,480 site-based observations, or 884 site-based observations not including repeated measures at the one site, 183,675 stem diameter measures (36% from new work in this project) and 8,288 measures of tree or shrub above- and below-ground biomass (40% from new work in this project). These data have been analysed to identify the key factors affecting the growth of plantings, resulting in 26 statistically-different categories of plantings. Modifiers that account for large variations in growth of these categories of plantings have been developed for use in FullCAM (which underpins the Reforestation Modelling Tool, RMT). Sampling error: Sampling error was found to be the main factor affecting the accuracy of biomass estimates. Unless a sufficient number of trees/shrubs are sampled in a manner that is representative of the planting, biomass estimates can have high coefficients of variation of >50%. Even within reasonably homogeneous plantings, a …