Estimating potential harvestable biomass for bioenergy from sustainably managed private native forests in Southeast Queensland, Australia

Increasing dependency on fossil fuels (coal, natural gas, and oil), high costs, security of energy supply and the need to cut carbon dioxide emissions have driven a global quest for the substitution of fossil fuels with renewable energy alternatives. The use of renewable biomass for bioenergy production for heating, electricity production and liquid fuels is now subject to considerable interest and activity world-wide (Raison 2006; Janowiak and Webster 2010; Farine et al. 2012; Booth et al. 2014; Rothe et al. 2015; Murphy et al. 2015; Hayward et al. 2015; Crawford et al. 2016).

Currently, bioenergy only provides 0.9% of Australia’s electricity generation. However, projections suggest it has the potential to provide between 19.8% and 30.7% of Australia’s electricity requirement by 2050 (RIRDC 2010). National assessments of potential bioenergy resources in Australia including agricultural crops and residues, native grasses, forestry (plantations, short-rotation tree crops and native forests) and organic municipal solid wastes based on statistics published by Australian Bureau of Agricultural and Resource Economics (ABARE) (Farine et al. 2012; Crawford et al. 2016; AREMI 2017). Among these bioenergy resources, the specific role of forests as a renewable energy source has received widespread global attention (Fung et al. 2002; Janowiak and Webster 2010; Davis et al. 2012; Klockow et al. 2013; Meadows et al. 2014; Rothe et al. 2015). However there has been less attention in Australia where energy needs have largely been met by cheap coal and gas fossil fuels (Raison 2006). Action against climate change, rising energy costs and job creation are key driving forces towards the transition of the Australia economy from dependence on fossil fuels to renewable energy future. Consequently, the need for reliable localised information on the potential role of the Australia’s expansive native forest resource in energy generation is overdue (Raison 2006; Rothe et al. 2015).

Australia is the seventh most forested country in the world with 125 million hectares of forest, equivalent to 16% of Australia’s land area (National Forest Inventory 2013). The total area of Australia’s native forest suitable and available for commercial wood production in 2010–2011 was 36.6 million hectares (DAWR 2015). This includes 29.1 million hectares of native forests under leasehold and private ownership (DAWR 2015), a resource that is often minimally managed (Clinnick et al. 2008). Improved silviculture involving thinning from below to release suppressed growth has been mooted to rejuvenate the growth of these forests and to restore their productivity and ecological values (Ryan and Taylor 2006).

Silvicultural management of private native forests in Queensland is crucial for ecological sustainability and provision of wood products in perpetuity (DNRM 2014). The commercial value of forest products currently harvested using selective method from native forests, namely sawlog, poles, girders, fencing material and piles, does not provide sufficient economic incentive for land owners to undertake non-commercial silvicultural improvement treatments. It is anticipated, that the emerging renewable energy market will provide an opportunity to utilize biomass from silvicultural thinning of forests, for bioenergy production. However little is known about the currently available stock of biomass in private native forests that can support the bioenergy industry in addition to their crucial role of environmental protection and the traditional role of timber production.

The proposition of biomass harvesting for bioenergy is often met with the apprehension that increasing demand for bioenergy would lead to over-utilisation of the relatively slow growing native forests (Hall 2002; Davis et al. 2012). In particular, there is concern that biomass harvesting has the potential to extract a greater proportion of a tree, and thereby jeopardise long-term soil productivity, water quality, and site-level biodiversity values (Benjamin 2010; Briedis et al. 2011; Klockow et al. 2013), including coarse wood materials crucial for fauna habitat (West et al. 2008; Briedis et al. 2011). On the contrary, current native hardwood forest harvest in Australia usually removes only a portion of the live tree from the site. Based on forest harvesting studies in the adjacent state of New South Wales (NSW) in Australia, it is estimated that a commercial timber harvest in native hardwood forests, extracts between 44% to 70% of whole tree biomass mainly for sawlog, veneer and pulp, and retains the remaining 30% to 55% of the tree on the forest floor in the form of bark, stump and crown (Ximenes et al. 2005; Ximenes et al. 2006; Ximenes et al. 2008). In Queensland, timber harvest of private native forests (excluding poles, girders, fencing material and piles) is mainly done for sawlog (does not include pulp and veneer) and only extracts high quality logs while retaining low-quality-log sections on the forest floor. This harvesting is regulated through a self-assessable vegetation clearing code of forest practice (DNRM 2014), to ensure the protection and maintenance of biodiversity values such as old-growth habitat trees, and soil and water resources. Hence, it is anticipated that biomass for bioenergy harvesting would be subject to this government regulation.

The main aim of this study was to provide an estimate of current harvestable biomass for bioenergy in living trees from private native forests within the Southeast Queensland bioregion. This is in recognition that in addition to biomass residual from wood processing (ABBA Qld 2016), residual biomass from sawlog harvesting and silvicultural thinning of poorly formed trees and those suppressing growth of timber trees (i.e., normal silvicultural treatment) has potential bioenergy value. Specific objectives were to: (1) map the extent and location of harvestable forest types within private native forests in the Southeast Queensland bioregion; (2) determine surplus recoverable biomass for bioenergy following conventional sawlog harvest; (3) determine and provide baseline biomass for bioenergy yield estimates from integrated sawlog and non-merchantable silvicultural treatment harvest; and (4) estimate the proportion of biomass retained on-site following a biomass for bioenergy harvest to address protection of soil productivity, water quality, and site-level biodiversity values.

Past attempts to estimate timber resources in private native forests have been limited by lack of inventory data and long-term monitoring programs in contrast to most state owned native forests in Australia (MbaC 2003; Raison 2006; Ngugi et al. 2015). The methodology used in this study to map the extent of privately owned native forests provides a consistent approach for mapping the harvestable forest resource that can be applied elsewhere in Queensland. This approach utilized the regional ecosystem spatial mapping dataset for Queensland to stratify the entire forest estate into broad vegetation groups (Table 1) and utilized inventory data collected from 541 plots on native forests. This study however did not exclude buffer zones along watercourses because of the prohibitive computing resources necessary to handle drainage dataset for the entire bioregion. However, while exclusion of buffer zones would result in some reduction of harvestable area, the majority of watercourses in these mainly dry sclerophyll forests are mapped as stream type of order 1 to 4, and have a buffer zone requirement of between 0 and 5 m stipulated by the Code of Practice (DNRM 2014). Future detailed resource maps targeting local government areas of interest should exclude waterway buffer zone areas from maps of harvestable forest areas.

Sawlogs are the primary forest product harvested in Queensland’s private native forests (Ryan and Taylor 2006). The standards for sawlog are high and not many individual trees meet them – they need to be straight, have a round cross-section, small branches, minimal defects such as knots or insect damage, and have a sufficient girth (>25 cm) and length (>2.4 m) (James 2001). These high quality requirements combined with the high degree of defect and the characteristic branching form of species harvested in Queensland explain the low mean sawlog recovery of 41.4% found in this study. It also highlights that there is an opportunity to utilize for bioenergy those portions of felled trees that are defective, are too small in diameter and/or too short in length to be utilized as sawlog and hence would otherwise be left on the forest floor. In NSW, Australia, where the estimated mean proportion of aboveground tree biomass utilized as commercial log can accounts for 64%, the portions that do not meet high value veneer or sawlog quality have in the past been harvested for low value uses such as pulp and posts (Ximenes et al. 2008). In Queensland markets for these lower grade components are currently limited or do not exist and about 22.6% of whole tree biomass is therefore potentially available for bioenergy or for other emerging markets such as veneer processing using a spindleless lathe (McGavin et al. 2014).

The proportion of whole tree biomass able to be harvested as commercial log when pulpwood is recovered has been reported to be as high as 70% for Eucalyptus obliqua in NSW (Ximenes et al. 2008). This suggests that more biomass for bioenergy can be sourced from forests in Queensland if such a proportion of tree biomass was harvested. Species with comparable growth habits do occur in Queensland, such as Eucalyptus grandis and Eucalyptus saligna (BVG 8a), and are likely to yield a similar high proportion of commercial log. However, with such species, the top section of the stem is likely to be straight and comparatively free of knots and thus to still meet specifications for veneer, a more valuable product than biomass for bioenergy (McGavin et al. 2014).

The future biomass production of sustainably managed private native forests is anticipated to increase. This potential is evident when we compare current productivity of these forests with similar native forests found on state owned forests in Queensland that have in the past been sustainably managed for timber production (Ngugi et al. 2014). Using the total aboveground biomass recorded before harvest in this study, the estimated biomass values from similar BVG in state managed forests (Ngugi et al. 2014) are between 2 to 7 times higher. For example, the mean stand biomass of spotted gum forests (BVG 10b) in state managed forests is twice as much, that of moist open forests dominated by Eucalyptus pilularis (8b) is seven times higher and that of wet tall open forests (8a) is five times higher than that reported for equivalent BVG in private native forests in this study. This difference is largely the result of past disturbance such as fires and vegetation management history such as “high grading” forest harvesting and clearing for grazing purposes in private land ownership (Ryan and Taylor 2006). Ryan and Taylor (2006), have demonstrated that application of silvicultural treatment and sustainable wood harvesting of these forests has the potential to enhance rates of biomass accumulation by concentrating biomass growth on larger trees instead of many small trees. Additionally, this practice enabled development of a ground layer of grass cover needed for soil protection and grazing, ensuring that these forests continue to provide environmental benefits and economic returns far into the future.

This study has demonstrated that private native forest management in SEQ has the potential to produce a very large amount of biomass for bioenergy (13,575,000 t). Based on biomass distribution mapping shown in Fig. 2, over 8 million tonnes of biomass are currently found in local government areas in the northern part of the bioregion around the towns of Bundaberg, Miriam Vale, Maryborough and Gympie. This biomass could make a significant ongoing contribution to reducing Australia’s greenhouse gas emissions by displacing the use of coal in the generation of electricity for plants that use technology which is compatible with co-firing woody biomass (McEvilly et al. 2011). In general terms, the available biomass for bioenergy in SEQ is within 200 km to the nearest major coal-fired electricity generation facility which include Tarong (near Kingaroy), Callide (near Biloela), Millmerran, Gladstone and Stanwell (near Rockhampton) (McEvilly et al. 2011). Put into perspective, the biomass resource in the bioregion has the capacity to produce 13,575,000 MW hours of heat energy (using conversion of one tonne of wood biomass for one megawatt hour of heat (Shelly 2010)). This energy can supply electricity to a city of 50,000 households such as Gladstone (average household electricity consumption in Australia in 2014 was 5817 kWh∙yr.⁻¹ (Acil Allen Consulting 2015)) for approximately 47 years.

Alternatively, woody biomass resource could provide a valuable feedstock for producing biofuels such as aviation fuels (Booth et al. 2014; Murphy et al. 2015) and to potential biofuel generators within SEQ such as the Northern Oil at Gladstone. Additionally, the resource could supply wood pellet for export to distributors such as Altus Renewables at Tuan in Queensland. The demand for biomass is also set to increase in order to meet the “biobased” diesel mandate that requires that by July 2018, 0.5% of all diesel fuel sold in Queensland should be “biobased” (Advance Queensland 2016). The emerging biomass for bioenergy industry creates an additional revenue stream offering the economic incentive necessary for private native forest landowners to undertake silvicultural treatments. In addition, a rejuvenated forest industry is likely to also benefit the community by providing employment opportunities and income, displacing energy intense and non-renewable building materials (such as steel and concrete) (Moroni 2012; Smyth et al. 2017) and by improving the condition of the forests for other environmental outcomes (Commonwealth of Australia, 2015a, b).

Community concerns about potential negative environmental effects are possibly the largest barrier to establishing a native forest-based biomass harvesting industry in Australia (Rothe et al. 2015). Harvesting timber from native forests, in particular harvesting of low grade residues, has been a point of contention within parts of the Australian community for many decades, for example in Tasmania (Routley and Plumwood 1986; Ulrik 2012; Rothe et al. 2015). Instances of questionable practices where tree harvesting has been particularly intense and the impacts (particularly of animal habitat, water quality and aesthetics) obvious and lasting have contributed to this concern (Routley and Plumwood 1986). Concern about the negative environmental impacts of native forest logging has been strongest where forests are in public ownership, have limited history of previous logging, and where residues are being exported for low value use (such as paper manufacture), however it has influenced political and community attitudes to management and land clearing of native forests more generally (Routley and Plumwood 1986; Ulrik 2012; Bradshaw 2012; Macintosh 2013).

There are a number of factors mitigating against the risk of environmental harm in relation to biomass harvesting from private native forests in Queensland. First, estimates from biomass proportion studies discussed in this study (Ximenes et al. 2008) suggest that approximately 36% of felled tree biomass would be retained on-site and on the forest floor for enhancing long-term soil productivity, water quality, and site-level biodiversity values. This proportion suggest a higher on-site biomass retention compared to the estimated mean proportion of total aboveground biomass as forest floor coarse wood debris in Australian native forests that ranges from 16% in tall open forests to 18% in open forests (Woldendorp and Keenan 2005).

Secondly, tree harvesting practices within private native forests are strictly regulated by the Code of practice (DNRM 2014), to protect habitat, soil and water values under Queensland’s Vegetation Management Act (QLD VM Act 1999). Thirdly, for an energy producer to gain credits under Australia’s Federal Renewable Energy Legislation (Commonwealth of Australia, 2015a, b) for substituting fossil fuels with native forest biomass, they must demonstrate that the biomass is from an operation that is: primarily geared to produce higher-value products such as sawlogs (including thinnings and coppice) and is carried out in accordance with ecologically sustainable forest management principles approved and certified under relevant Commonwealth, State or Territory planning and approval processes. This requirement is generally considered to be as a minimum the Australian Forestry Standard (AFS) or the Forestry Stewardship Council (FSC) certification (Department of Agriculture 2015).

Nevertheless, the removal of poorly formed individual trees for biomass production, and removal of 64% of the bole section of trees is likely to affect long-term availability of coarse wood material on the forest floor as observed in forests harvested for bioenergy supply for a period of over a decade in the United States of America (Briedis et al. 2011; Klockow et al. 2013). Under the current forest practice in Queensland, Australia, poorly formed individual trees not suitable for sawlog harvest, rotten heartwood or knotted or damaged large diameter sized sections of felled trees and the crown section deemed unsuitable for sawlog, provide necessary food and habitat for fauna species (Eyre et al. 2015) and fuelwood (West et al. 2008). Normally, when these poorly formed standing trees fall down they become part of the large coarse woody material on the forest floor (Eyre et al. 2015). Although the present code (DNRM 2014) only prescribes the retention of standing habitat trees, if an industry developed around the harvesting and use of poorly formed trees, the Code may need to be extended to provide guidelines for the retention of snags (standing dead trees) and coarse wood material on the forest floor for fauna habitat functions (Woldendorp and Keenan 2005).

This study has provided a point-in-time stock-take of potentially available biomass for bioenergy in privately owned native forests in the Southeast Queensland bioregion estimated at 13,575,000 t. The study has also provided an estimation methodology and maps showing the 2014 extent and location of these forests and estimates of the amount of biomass that can be sourced from each local government area within the bioregion. These estimates indicate the potential capacity of private native forest and are useful for informing policy and potential development of a wood based renewable bioenergy industry. Developing a bioenergy market for biomass from these forests could provide the motivation and financial means for landowners to adopt silvicultural management necessary to promote growth of the retained forests and hence ensure sustainable supply of ecological and economic benefits into the future. However, more detailed research on the long-term availability of ‘sustainable’ biomass over coming decades to support investment in bioenergy (Booth et al. 2014; Murphy et al. 2015), economic viability of a biomass for bioenergy industry (Hayward et al. 2015), sustainable biomass retention proportion for environmental protection including coarse wood material for fauna habitat as well as energy generated from burning different types of tree species is needed.