Energy Fuels 2010, 24, 225–231 Published on Web 08/28/2009
: DOI:10.1021/ef9005687
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Bioenergy Feedstock Potential from Short-Rotation Woody Crops in a Dryland Environment† R. J. Harper,*,‡,§, S. J. Sochacki,‡ K. R. J. Smettem,§ and N. Robinson‡ Forest Products Commission, Locked Bag 888, Perth Business Centre, Perth 6849, Western Australia, Australia, §Centre for Ecohydrology, School of Environmental Systems Engineering, The University of Western Australia, Nedlands 6907, Western Australia, Australia, and Murdoch University, South Street, Murdoch 6150, Western Australia, Australia )
‡
Received June 3, 2009. Revised Manuscript Received August 4, 2009
Producing biomass from plantations of short rotations (3-10 years) of fast growing woody crops that are alternated with agricultural production, in a system termed phase farming with trees (PFT), could offer a range of advantages compared to the use of permanent coppiced plantings. These include providing landholders flexibility in land use and increasing the sustainability of farming systems by lowering water tables, removing excess nutrients, and improving soil quality. Disadvantages from permanent belts and blocks, such as competition with adjacent agricultural crops are reduced. PFT thus offers a method of producing both food and fuel from the same land, while increasing the sustainability of current agricultural systems. This paper describes the development of the PFT system in the dryland Mediterranean climate of southwestern Australia. Dry biomass yields of high-density (4000 trees/ha) plantings of Eucalyptus occidentalis of up to 22 tons/ha were achieved after 3 years and up to 54 tons/ha of Pinus pinaster (2000 trees/ ha) after 7 years, in environments with only 300 mm of annual rainfall. Biomass yields of up to 31 tons/ha of E. occidentalis were achieved after 7 years on salinized soils, which had been effectively abandoned to agriculture. We describe the factors that affect yield in this water-limited environment, including the impact of initial planting density, rotation length, species, site selection (soils and landscape position), and fertilization and assess the impact of the system on sustainability in terms of removal of excess water and nutrients. with shallow-rooted annual plants. Rising saline groundwaters enter plant root zones or discharge on the ground surface. Australia-wide, it has been estimated that up to 17 million hectares are likely to be affected by salinity by 2050,3 although concerns related to salinity appear to be receding with the onset of a prolonged drought. Land use systems capable of reducing salinization and providing an adequate income are not currently available for large areas of Australia’s agricultural zone, although it is well-known that salinity control can be obtained by controlling groundwater recharge and that extensive tree plantings can achieve this.4 Trees are relatively cheap to establish and maintain, have lower offsite impacts compared to drains or diversion systems, and have the potential to produce saleable products, such as timber, pulp, or fuel wood, with the benefit of diversifying farm incomes.5 Although there have been increasing concerns about the negative impacts of extensive tree plantations on water availability,6 this does not
Introduction Climate change, energy security, water supply, and land degradation are all major issues that are increasingly impacting land management decision making. Climate change is ascribed to increasing concentrations of atmospheric greenhouse gases,1 such as carbon dioxide. Two approaches to reduce the net increase in atmospheric carbon dioxide concentrations are to replace fossil fuels with renewable biomass or sequester carbon in trees.2 Rising groundwaters and resultant salinity threaten agricultural land, conservation reserves, and water resources across Australia. This problem has been caused by increased groundwater recharge because of the widespread removal of deep-rooted native vegetation for farming and replacement † Presented at the 2009 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies. *To whom correspondence should be addressed. E-mail: r.harper@ murdoch.edu.au. (1) Adger, N.; Aggarwal, P.; Agrawala, S.; Alcamo, J.; Allali, A.; Anisimov, O.; Arnell, N.; Boko, M.; Canziani, O.; Carter, T.; Casassa, G.; Confalonieri, U.; Cruz, R. V.; Alcaraz, E.; Easterling, W.; Field, C.; Fischlin, A.; Fitzharris, B. B.; Garcı´ a, C. G.; Hanson, C.; Harasawa, H.; Hennessy, K.; Huq, S.; Jones, R.; Bogataj, L. K.; Karoly, D.; Klein, R.; Kundzewicz, Z.; Lal, M.; Lasco, R.; Love, G.; Lu, X.; Magrı´ n, G.; Mata, L. J.; McLean, R.; Menne, B.; Midgley, G.; Mimura, N.; Mirza, M. Q.; Moreno, J.; Mortsch, L.; Niang-Diop, I.; Nicholls, R.; Novaky, B.; Nurse, L.; Nyong, A.; Oppenheimer, M.; Palutikof, J.; Parry, M.; Patwardhan, A.; Lankao, P. R.; Rosenzweig, C.; Schneider, S.; Semenov, S.; Smith, J.; Stone, J.; van Ypersele, J.; Vaughan, D.; Vogel, C.; Wilbanks, T.; Wong, P. P.; Wu, S.; Yohe, G. Climate change impacts, adaptation and vulnerability. Intergovernmental Panel on Climate Change. Fourth Assessment Report. Summary for Policy Makers; Intergovernmental Panel on Climate Change: Geneva, Switzerland, 2007; p 23. (2) Canadell, J. G.; Raupach, M. R. Managing forests for climate change mitigation. Science 2008, 320 (5882), 1456–1457.
r 2009 American Chemical Society
(3) National Land and Water Resources Audit. Australian Dryland Salinity Assessment 2000. Extent, Impacts, Processes, Monitoring and Management Options; National Land and Water Resources Audit: Canberra, Australia, 2001; p 129. (4) George, R. J.; Nulsen, R. A.; Ferdowsian, R.; Raper, G. P. Interactions between trees and groundwaters in recharge and discharge areas;A survey of Western Australian sites. Agric. Water Manage. 1999, 39, 91–113. (5) Harper, R. J.; Beck, A. C.; Ritson, P.; Hill, M. J.; Mitchell, C. D.; Barrett, D. J.; Smettem, K. R. J.; Mann, S. S. The potential of greenhouse sinks to underwrite improved land management. Ecol. Eng. 2007, 29, 329–341. (6) van Dijk, A. I. J. M.; Hairsine, P. B.; Arancibia, J. P.; Dowling, T. I. Reforestation, water availability and stream salinity: A multi-scale analysis in the Murray-Darling Basin, Australia. For. Ecol. Manage. 2007, 251, 94–109.
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: DOI:10.1021/ef9005687
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apply to the lower rainfall agricultural landscapes considered in this paper, because they do not produce potable water. Given the scale and increase in salinity in Australia, extensive and rapid revegetation with trees is required to restore the hydrological balance over large areas. Although there has been widespread expansion of the plantation industry in higher rainfall areas of Australia,7 adoption of trees in lower rainfall areas has been limited. Apart from generally low initial rates of adoption of new farm technology, reasons could include (a) uncertainties about profits, (b) uncertainties about where to plant trees for best hydrological impact, (c) limitations on the effectiveness of trees, and (d) conflicts with agricultural activities.8 The policy response of the Australian government to increasing carbon dioxide concentrations has been to introduce an emissions trading scheme, termed the carbon pollution reduction scheme (CPRS),9 and to supplement this with other measures, such as a mandated 20% renewable energy target (RET). The CPRS includes provisions to include carbon sequestration in reforestation, whereas the RET allows for the use of bioenergy to produce renewable energy credits. Importantly, the two schemes will result in a market for products from reforestation that did not previously exist and may stimulate investment in lower rainfall reforestation. This, in turn, may thus help underwrite the extensive land treatment required to tackle salinity.8,10,11 Both the CPRS and RET require passage through the Australian parliament before they become law, and these debates will occur in late 2009. The major approach to reforestation in Australian dryland areas has been the use of strips of mallee eucalypts interspersed with agriculture.12 These systems have an energy productivity of around 206 GJ ha-1 year-1 and an energy ratio of 41.713 and will comprise permanent strips of mallees coppiced on a 3 year rotation producing a range of products, including biomass for bioenergy14 and carbon stored in the root system.12 Alternatively, unharvested mallees will be used as carbon sinks. In some landscapes, however, groundwater movement is slow and only localized drawdown around trees
may occur. Effective dewatering of landscapes may require trees to be planted at relatively close spacings, which may impede the use of farm machinery or result in competition with crops. Similarly, rates of groundwater recharge vary across the landscape, and there are practical difficulties in predicting where trees should be planted for the best effect. An alternative system to overcome dryland salinization of farming systems in medium to low (300-600 mm) rainfall areas of southern Australia was proposed by Harper et al.15 Phase farming with trees (PFT) was designed to use trees grown at high planting densities in very short-term rotations (3-5 years) to rapidly dewater agricultural catchments at risk of salinity. The system is designed to deplete soil-water reserves that have accumulated below the rootzone of annual crops while producing usable products, such as wood fiber, and biomass. Drying out of the soil profile over the depth of the tree roots creates a dry soil buffer that needs to be replenished before any further groundwater recharge can occur. The tree phase would be followed by an agricultural phase of a length defined by the time required to refill the dry soil buffer created by the trees. The system thus uses a resource (soil-water) that is contributing to environmental problems while building more sustainable agricultural systems. It is similar to the phase farming systems that use agricultural plants, such as lucerne (Medicago sativa), but differs in that trees have a larger leaf area than grazed pasture and are also more likely to have root systems that can exploit deep and often hostile subsoils. The system can be contrasted with mallee-based agroforestry systems; in that, competition for resources occurs over time rather than spatially. Potential benefits from phase farming with trees that were identified by Harper et al.15 include (a) the rapid restoration of local water balances and a subsequent reduction in salinity, (b) an impact on Australia’s net carbon dioxide emissions by providing both a carbon sink in the vegetation and a feedstock for bioelectricity, and (c) a disease and weed break and “biological ploughing” of subsoils. In this paper, we describe several Western Australian studies that have evaluated the concept, including developing an understanding of the factors required to maximize yield in a water-limited environment and the impact of the system on sustainability in terms of removal of excess water and nutrients. The paper also considers unresolved issues related to this system, including cost-effective establishment and harvesting methods and the development of a market for environmental services provided by the plantings.
(7) Parsons, M.; Gavran, M. National Plantation Inventory Australia;2007 Update; Department of Agriculture, Fisheries and Forestry: Canberra, Australia, 2007; p 8. (8) Harper, R. J.; Smettem, K. R. J.; Townsend, P. V.; Bartle, J. R.; McGrath, J. F. Broad-scale restoration of landscape function with timber, carbon and water investment. In Forest Landscape Restoration: Integrating Social and Natural Sciences; Stanturf, J. A., Lamb, D., Madsen, P., Eds.; Springer: New York, in review. (9) Australian Government. Carbon Pollution Reduction Scheme: Australia’s Low Pollution Future. White Paper Summary Report; Australian Government: Canberra, Australia, December 2008; p 77. (10) Consortium. The Contribution of Mid to Low Rainfall Forestry and Agroforestry to Greenhouse and Natural Resource Management Outcomes: Overview and Analysis of Opportunities; Australian Greenhouse Office/Murray-Darling Basin Commission: Canberra, Australia, October 2001; p 72. (11) Harper, R. J.; Smettem, K. R. J.; Reid, R. F.; Callister, A.; McGrath, J. F.; Brennan, P. B. Pulpwood crops. In Agroforestry for Natural Resource Management; Reid, R. F., Nuberg, I., Eds.; CSIRO Publishing: Melbourne, Australia, 2009; pp 199-218. (12) URS Australia. Oil Mallee Industry Development Plan for Western Australia; Forest Products Commission, Western Australia: Perth, Australia, 2008; p 102. (13) Wu, H.; Fu, Q.; Giles, R.; Bartle, J. Production of mallee biomass in Western Australia: Energy balance analysis. Energy Fuels 2008, 22, 190–198. (14) Yu, Y.; Bartle, J.; Li, C.; Wu, H. Mallee biomass as key bioenergy source in Western Australia: Importance of biomass supply chain. Energy Fuels 2009, 23, 3290–3299.
Modeling 15,16
A study was undertaken using the hydrological model WAVES17 to determine the feasibility of the PFT concept in the medium to low (300-600 mm) rainfall areas of southern (15) Harper, R. J.; Hatton, T. J.; Crombie, D. S.; Dawes, W. R.; Abbott, L. K.; Challen, R. P.; House, C. Phase Farming with Trees: A Scoping Study of Its Potential for Salinity Control, Soil Quality Enhancement and Farm Income Improvement in Dryland Areas of Southern Australia; Rural Industries Research and Development Corporation (RIRDC): Canberra, Australia, 2480; RIRDC publication number 48/48, p 53. (16) Hatton, T. J.; Dawes, W.; Harper, R. J., Woodlots in rotation with agriculture. In Trees, Water and Salt;An Australian Guide to Using Trees for Healthy Catchments and Productive Farms; Stirzaker, R., Vertessy, R., Sarre, A., Eds.; Rural Industries Research and Development Corporation (RIRDC): Canberra, Australia, 2002; pp 43-55. (17) Zhang, L.; Dawes, W. R. WAVES: An Integrated Energy and Water Balance Model; CSIRO Land and Water: Canberra, Australia, 1998; 31/98.
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Australia. This study examined the potential biophysical performance of the system in both Western Australia (Merredin; mean rainfall, 320 mm; potential evapotranspiration, 1800 mm) and the Murray-Darling Basin [Walpeup, Victoria (mean rainfall, 340 mm; potential evapotranspiration, 1750 mm) and Hillston, New South Wales (mean rainfall, 581 mm; potential evapotranspiration, 1750 mm)]. Several scenarios were examined, with these suggesting broad differences in likely response to the PFT system. (1) 20 m deep sandy soils. Here, PFT depleted soil-water storage and stopped groundwater recharge under agriculture (100 mm/year) within 2-3 years of planting. The high recharge rates resumed 3 years into the next agricultural phase. PFT was thus considered unsuitable for such soils, with the best strategy for salinity control considered to be permanent blocks of trees. These soils are also beset by low agricultural productivity and are highly susceptible to wind erosion.18 (2) Soils with 1 m of sand overlying 2 m of clay, with and without a fresh or saline water table. Here, recharge rates returned to a maximum within 1-5 years of clearing the trees. It is likely that soil salinity will accumulate under the trees grown over a saline water table,19 with this requiring leaching before another rotation is possible. Permanent plantations of salttolerant perennials are recommended in these situations. (3) Soils with a clay surface horizon. Here, the rates of recharge are very low (1 mm/year) under agriculture, and trees may only be required at very long rotation intervals (∼decades). (4) Soils with 1 m of sand overlying 9 m of clay. Here, PFT depleted soil-water storage and stopped recharge within 3-4 years of tree planting. In contrast to the other sites, it took 17 years for recharge to return to a peak rate of 67 mm/year following a return to agriculture. It may be possible to obtain zero recharge with a system with 5 years of trees and 10 years of crop. Soils with these broad properties are widespread across southwestern Australia.20 Other components of the scoping study examined the likely impact on soil biology and the economics of the system. One major issue identified was a lack of adequate accounting for environmental benefits when assessing the profitability of the technique. Thus, the PFT technique was evaluated on a cash-flow basis, whereas the potential private and public environmental benefits were not valued. This remains a general problem with the valuation of land conservation treatments.
Figure 1. Soil-water content (g/g) under 7-year-old strips of mallees at (a) Narrogin, (b) Wickepin, and (c) Newdegate. This was measured at the end of the summer and clearly shows the depletion in soil moisture beneath the trees to depths of more than 10 m and into the adjacent paddock.21
of soils, with only silcrete pans and free water restricting root growth. The lateral extent of the tree roots varied with site and soil type, with a range of 9-21 m. This suggests that the premise of phase farming with trees, which is the rapid depletion of soil-water to depths of several meters to provide a dry soil buffer for the subsequent crop phase, is feasible. Testing the Concept in a Cropping System Site 1: Corrigin. Building on the recommendations of the scoping study,15 the PFT concept was tested in a field experiment on a farm near Corrigin, Western Australia (300 mm/year annual rainfall) (Figure 2). This region has a semi-arid Mediterranean climate, with a seasonal dry period from November to April, with farming comprising crops of cereals, such as wheat, in rotation with annual pastures. Leaf area was manipulated through variations in tree species (Eucalyptus globulus, Eucalyptus occidentalis, Acacia celastrifolia, Pinus radiata, and Allocasuarina huegeliana), planting density (500, 1000, 2000, and 4000 trees/ha), and soil fertility to determine (a) if the premise of soil-water depletion to depths of several meters in 3-4 years was justified and (b) if it is feasible to accelerate the rate of water depletion by intensifying stocking rates and hence decrease the duration of the forestry phase. The results from the phase farming with trees experiment were very promising, with significant soil-water depletion of 440-780 mm occurring beneath high density (4000 trees/ha) plantings of E. occidentalis within 3 years of planting. Assuming a recharge rate of 40 mm/year under annual cropping, this could suggest that an optimal rotation for controlling recharge comprises 3 years of trees and 11-20 years of agriculture.22
Field Examination of Mallee Belts The spatial patterns of soil-water depletion by various eucalypts were assessed by Robinson et al.21 to determine the potential of both tree belts and PFT for groundwater recharge reduction and salinity control. Soils were sampled to depths of up to 10 m in transects perpendicular to 4-7-year-old mallee eucalypt belts (Eucalyptus horistes, Eucalyptus kochii ssp. plenissima, Eucalyptus loxophleba ssp. lissophloia, and Eucalyptus polybractea) and in a 4-year-old block of Eucalyptus astringens. Results indicated that eucalypt species can exploit soil-water to depths of at least 8-10 m within 7 years of planting (Figure 1). The root systems of these eucalypts were also able to penetrate clay subsoils with bulk densities of up to 2.0 g/cm3, probably by exploiting old root channels. Results were similar across a range (18) Harper, R. J.; Gilkes, R. J.; Hill, M. J. Wind erosion and soil carbon dynamics in south-western Australia. Aeolian Res., in review. (19) Archibald, R. D.; Harper, R. J.; Fox, J. E. D.; Silberstein, R. P. Tree performance and root-zone salt accumulation in three dryland Australian plantations. Agrofor. Syst. 2006, 66, 191–204. (20) McArthur, W. M. Reference Soils of South-western Australia; Australian Society of Soil Science, Inc. (WA Branch): Perth, Australia, 1991; p 265. (21) Robinson, N.; Harper, R. J.; Smettem, K. R. J. Soil water depletion by Eucalyptus spp. integrated into dryland agricultural systems. Plant Soil 2006, 286, 141–151.
(22) Harper, R.; Robinson, N.; Smettem, K.; Sochacki, S.; Pitman, L. Phase Farming with Trees: Field Validation of the Tree Phase; Rural Industries Research and Development Corporation (RIRDC): Canberra, Australia, 2008; RIRDC publication number 08/002, p 26.
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Figure 2. Corrigin experimental site when the trees were 33 months old. The site has subdued relief (
