Mallee eucalypts are hardy, woody perennials that are being developed as a short-rotation coppice crop in Australia for the production of eucalyptus oil, biofuels and other biomass products. The economic viability of this prospective crop is dependent on its ability to survive and regenerate following repeated harvesting of the above ground component. Here we report on survival and biomass yield of mallee belt plantings of Eucalyptus polybractea, E. loxophleba ssp lissophloia and E. kochii ssp plenissima, at 19 sites, under two harvest-frequencies (3-8 year cycles) and harvest seasons (autumn or spring) over a decade from 2006 to 2015. 16 sites had post-harvest mortality ranging from 1.0% to 12.2% while the remaining three sites with either shallow saline water tables or a silcrete hardpan failed. Average site dry biomass yield across treatments ranged from 2.2 to 32.8 Mg ha(-1) yr(-1). Higher yielding sites were generally characterised by pH between 3.8 and 8, ECe below 15.0 dS m(-1) and high soil fertility. Lower yielding Sites were generally near saline valley floors. After 7-years, biomass yield from unharvested treatments exceeded the average cumulative yield of harvest treatments at eight of the 16 sites, including all three E. kochii sites. For E. loxophleba, significant interactions were found between season and frequency of harvest with highest yields in long rotation spring treatments. There were also interactions between site and frequency of harvest, which were mainly driven by the variable performance of the uncut treatment. On average E. loxophleba yielded more biomass following spring harvests whereas E. kochii yielded more following autumn harvests. E. polybractea yield was unaffected by season or frequency of harvest; however, harvest treatments yielded more biomass than uncut treatments. After 10 years, at eight of the nine sites subjected to three 3-year cycles, no decline in biomass yield was observed. The site that declined in production was attributed to depletion of a sandplain aquifer by extensive mallee plantings. Overall, the results from this decadal study indicate that in warm-temperate semi-arid areas, such as the south-west of WA, mallees biomass can be harvested sustainably at most sites even in short (3-year) rotation cycles.

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).

The authors tested two harvesting systems especially designed for mallee agroforestry plantations on farmland. Both systems were based on versatile forest technology commonly used for conventional logging operations. They differed especially for the felling technology: small-scale drive-to-tree or industrial swing-to-tree equipment. Both systems were tested side-to-side on 12 experimental plots each. The resulting harvesting cost was 22 and 27 AU$ t−1, for the industrial and the small scale system, respectively. Chipping represented between 60 and 80 % of the overall harvesting cost, and offered much room for improvement. The industrial system always offered the lowest harvesting cost, regardless of annual usage, when equipment mobilisation costs were not considered. The productivity of conventional forestry equipment was strongly dependent on belt stocking and tree size. If the diameter at ankle height dropped below 10 cm, economic viability decreased very rapidly.

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 <+/- 19%, 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. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.

Mallee-based agroforestry has potential to provide farmers with new income sources derived from biofuels, biofeedstocks, and carbon sequestration. Although mallees are planted on >12 700 ha across the south-west of Western Australia, very little commercial harvesting of mallee has occurred to date. The development of biomass processing industries is constrained by lack of robust information regarding the productivity of integrated mallee and agricultural systems. This study addresses this constraint by quantifying the productivity and economics of agricultural crops and pastures growing in the competition zone adjacent to mallee belts at 15 sites across the Western Australian wheatbelt. The sites covered a range of climate and edaphic conditions, three mallee species (Eucalyptus polybractea R Baker, E. loxophleba ssp. lissophloia LAS Johnson and KD Hill, or E. kochii ssp. plenissima (CA Gardner) Brooker), various crop and pasture rotations, and various mallee harvest-management treatments. Mallee–crop competition was negatively correlated with rainfall and positively correlated with mallee age and size, and greater for crops than pasture. Consequently, extent and magnitude of competition were highly variable across sites and years. On average, mallee–crop competition extended 11.3 m from unharvested belts and reduced crop and pasture yields by 36% within 2–20 m of the mallee belts relative to open paddock yields. This is similar to what has been reported for taller tree species. Harvesting mallees reduced competition such that crop and pasture yield was reduced by 22 or 27% relative to open paddock yields for mallees harvested at 3- or 6+-year intervals, respectively. The economic cost of mallee–crop competition on agricultural enterprises was also highly variable between sites, and between years within individual sites. Averaged across all site-years, the opportunity cost of competition was equivalent to forgoing agricultural production for 14.4 m on each side of unharvested mallee belts, or 9–10 m on each side of harvested belts. Farmers with mallee agroforestry systems will need to manage the economic impacts of competition by reducing agricultural input costs in the competition zone, timing crop-grazing rotations with mallee harvests, ensuring that the width of alleys is at least 25 times the height of the mature trees, and possibly root-pruning mallees in unharvested or long harvest interval systems. This research has shown that mallee–crop competition presents a significant cost to farmers and must be considered when designing mallee agroforestry systems. The findings have relevance for the development of appropriate biomass and carbon sequestration pricing benchmarks for mallee plantings.

The removal of native vegetation and development of annual agricultural systems in the 250-650 mm rainfall zone of southern Australia has led to widespread dryland salinity and salinisation of waterways. Restoration of deep-rooted perennial vegetation can make a significant contribution to correcting this problem. The scale of perennial plant cover necessary to control salinity is very large and it is not feasible to rely solely on revegetation for biodiversity to restore hydrological equilibrium in the landscape. Consequently, development of a mosaic of land uses including tree crops driven by large-scale industrial markets, agricultural systems utilising annual and herbaceous perennial crops, and biodiversity resources is proposed. New agroforestry designs include short-cycle woody coppice and phase crops based on belts or plantations suited to local hydrological systems. These new designs can also be used to sequester carbon in an effort to combat greenhouse gasses. Carbon trading offers yet another opportunity to employ woody perennials as an alternate income stream for land managers. FloraSearch endeavours to select and develop woody perennial species suited to the concept of developing commercially viable industries that can also meet natural resource management goals. The FloraSearch project was initiated in 2002 to provide the national focus to the development of broad scale woody crops for the dryland wheat-sheep zone of southern Australia. Potential products are reviewed and taxa from southern Australia have been

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 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 ). 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 -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).