Accurate ground-based estimation of the carbon stored in terrestrial ecosystems is critical to quantifying the global carbon budget. Allometric models provide cost-effective methods for biomass prediction. But do such models vary with ecoregion or plant functional type? We compiled 15 054 measurements of individual tree or shrub biomass from across Australia to examine the generality of allometric models for above-ground biomass prediction. This provided a robust case study because Australia includes ecoregions ranging from arid shrublands to tropical rainforests, and has a rich history of biomass research, particularly in planted forests. Regardless of ecoregion, for five broad categories of plant functional type (shrubs; multistemmed trees; trees of the genus Eucalyptus and closely related genera; other trees of high wood density; and other trees of low wood density), relationships between biomass and stem diameter were generic. Simple power-law models explained 84-95% of the variation in biomass, with little improvement in model performance when other plant variables (height, bole wood density), or site characteristics (climate, age, management) were included. Predictions of stand-based biomass from allometric models of varying levels of generalization (species-specific, plant functional type) were validated using whole-plot harvest data from 17 contrasting stands (range: 9-356 Mg ha(-1) ). Losses in efficiency of prediction were <1% if generalized models were used in place of species-specific models. Furthermore, application of generalized multispecies models did not introduce significant bias in biomass prediction in 92% of the 53 species tested. Further, overall efficiency of stand-level biomass prediction was 99%, with a mean absolute prediction error of only 13%. Hence, for cost-effective prediction of biomass across a wide range of stands, we recommend use of generic allometric models based on plant functional types. Development of new species-specific models is only warranted when gains in accuracy of stand-based predictions are relatively high (e.g. high-value monocultures).

We quantified the extent and distribution of roots of four commonly planted tree species (Eucalyptus globulus Labill., Pinus radiata D. Don, P. pinaster Aiton and E. kochii Maiden & Blakely subsp. plenissima C.A. Gardner) in agricultural land adjacent to tree lines, and examined the effect of soil type and root pruning on root morphology. Root distribution in soil adjacent to tree lines was mapped by a trench profile method at 13 sites on the south coast of Western Australia. Soil samples were collected to determine water content and fertility. The lateral extent of tree roots ranged from 10 m for E. kochii to 44 m for P. pinaster. This equated to between 1.5 and 2.5 times tree height (H) for E. globulus and Pinus spp. to 4H for E. kochii. Root density declined logarithmically with distance from the trees and was greatest for P. pinaster and least for E. globulus (P < 0.001). The rate of decrease in root density with distance from the trees was greatest for the Pinus spp. and least for E. kochii (P < 0.05). Root density was generally greatest in the top 0.5 m of the soil profile and decreased with increasing depth. This decrease was relatively gradual in the deep sands, but abrupt in clay subsoil. Root dry mass in the sandy top soil beyond 0.5H ranged between 1.0 and 55.5 Mg km(treeline) (-1) for 6-year-old E. kochii and 50-year-old P. pinaster, respectively. Soil water content generally increased with distance from the trees (P < 0.001). There was no evidence of reduced soil fertility in the top 1.4 m of the soil profile adjacent to the trees. Two to four years after trees had been root pruned, both the lateral extent and vertical distribution of roots were similar for pruned and unpruned trees. The density of roots < 2 mm in diameter was greater for root-pruned trees than for unpruned trees (P < 0.05). We conclude that the study species can compete with agricultural crops based on the lateral extent of their roots and the occurrence of greatest root density within 0.5 m of the soil surface.

Accurate quantification of below-ground biomass (BGB) of woody vegetation is critical to understanding ecosystem function and potential for climate change mitigation from sequestration of biomass carbon. We compiled 2 054 measurements of individual tree and shrub biomass from across a broad range of ecoregions (arid shrublands to tropical rainforests) to develop allometric models for prediction of BGB. We found that the relationship between BGB and stem diameter was generic, with a simple power-law model having a BGB prediction efficiency of 72-93% for four broad plant functional types: (i) shrubs and Acacia trees, (ii) multi-stemmed mallee eucalypts, (iii) other trees of relatively high wood density, and; (iv) a species of relatively low wood density, Pinus radiata. There was little improvement in accuracy of model prediction by including variables (e.g. climatic characteristics, stand age or management) in addition to stem diameter alone. We further assessed the generality of the plant functional type models across 11 contrasting stands where data from whole-plot excavation of BGB were available. The efficiency of model prediction of stand-based BGB was 93%, with a mean absolute prediction error of only 6.5%, and with no improvements in validation results when species-specific models were applied. Given the high prediction performance of the generalised models, we suggest that additional costs associated with the development of new species-specific models for estimating BGB are only warranted when gains in accuracy of stand-based predictions are justifiable, such as for a high-biomass stand comprising only one or two dominant species. However, generic models based on plant functional type should not be applied where stands are dominated by species that are unusual in their morphology and unlikely to conform to the generalised plant functional group models.

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