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Benefits of tree mixes in carbon plantings

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Benefits of tree mixes in carbon plantings
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  Co-benefits of planting speciesmixes in carbon projects By Rachel J. Standish and Kristin B. Hulvey Rachel J. Standish  is a Research Assistant Pro-  fessor in the School of Plant Biology, The Univer-  sity of Western Australia (35 Stirling Hwy,Crawley, Western Australia 6009, Australia; Tel: + 61 8 6488 1073; Email: rachel.standish@uwa.edu.au).  Kristin B. Hulvey   is an Honorary Research Associate in the School of Plant Biology,The University of Western Australia (35 Stirling Hwy, Crawley, Western Australia 6009, Austra- lia; Tel:  + 61 8 6488 1073; Email: khulvey@ucdavis.edu). Summary  The carbon market offers a unique opportunity to achieve large-scale eco-logical restoration of degraded agricultural landscapes. Here, we outline some of thebenefits of planting mixes of native species rather than monocultures in carbon plantingsas a step towards creating biodiverse carbon-rich forests and woodlands in Australia. Wehighlight the gaps in our knowledge and emphasise the importance of setting benchmarksfor carbon projects to maximise their potential to deliver co-benefits such as habitat provi-sion for wildlife. On the whole, we are optimistic that ongoing refinement of joined biodiver-sity conservation and carbon credit initiatives will help to develop a carbon market that candrive ecological restoration of Australian agricultural landscapes.Key words:  biodiverse carbon plantings, Biodiversity Fund, carbon credits, Carbon FarmingInitiative, Direct Action Plan, ecosystem services. T he Australian Government’s Biodiver-sity Fund and Carbon Farming Initiative(CFI) (Australian Government 2011a and2012, respectively) offered an unprece-dented opportunity to expand ecologicalrestoration of Australia’s degraded agricul-tural landscapes, building on a substantivelegacy of state and federal programmesthat have supported biodiversity conserva-tion and the environment. Instigated in2012, both initiatives formed part of theClean Energy Future plan (Australian Gov-ernment 2011b) and provided support toland managers to reduce carbon pollutionand plant woody species for carbon cred-its. As such, there were some synergiesbetween the initiatives. For example,applicants who had received support fromthe Biodiversity Fund could later claim thecredits for the carbon that was seques-tered and stored by woody species if they had been planted using a methodology approved by the CFI. In this manner,applicants were encouraged to plant amix of native woody species to achieveboth biodiversity outcomes and the pro-duction of carbon credits.Shifts in Australian politics saw thereplacement of the carbon tax with aDirect Action Plan in 2013 (AustralianGovernment 2013). The shift away froma carbon tax and associated policy changes adds to the ongoing uncertainty in the state of the emerging carbon mar-ket. However, bipartisan support for aprice on carbon (Muthuswamy 2013)continues to provide an opportunity toconsider co-benefits alongside carboncredits as the policy guidelines for carbonprojects evolve. Moreover, as the worldmoves towards a low-carbon economy,the voluntary carbon market may continueto develop regardless of the actions of thenew and future governments. For exam-ple, on the voluntary market, companiesare able to sell the carbon sequestered indiverse plantings at a higher price thancarbon sequestered in monocultures by offering a ‘biodiversity benefit’ alongsidethe carbon (The Carbon Neutral Company 2013). Finally, despite policy changes, theDirect Action Plan will ‘build on the Car-bon Farming Initiative’ (Australian Govern-ment 2013), providing some hope thatthere may still be possibilities for syner-gies between the CFI and the Biodiversity Fund. As it stands, the CFI supports the devel-opment of carbon projects comprisingeither a mix of species or single-speciesmonocultures (Australian Government2012). Specifically, the guidelines statethat carbon plantings ‘may be a mix of tree and understorey species or one singlespecies, where a monoculture occursnaturally in the region’ (Australian Govern-ment 2012). Additional methodologiesfocus on ‘reforestation and afforestation’of cleared lands or lands where forestshave not occurred in the past. These plant-ings may be either mixtures or monocul-tures regardless of the diversity of thehistorical forest or woodland and indeed,may be non-native species. Here, we high-light some benefits of planting mixes of native species rather than monoculturesin carbon plantings where diverse forestsor woodlands existed in the past. We areparticularly interested in the developmentof joined carbon credit and biodiversity conservation initiatives in Australiabecause this is where we live and work,but acknowledge the existence of global-scale initiatives that link the two ecosys-tem services (e.g. reducing emissions fromdeforestation and forest degradation) andcite the ever-increasing international liter-ature on the topic where relevant.One of the reasons that land managersopt to plant monocultures rather thanspecies mixes is the assumption thatmonocultures of fast-growing species willmaximise carbon sequestration and stor- 26  ECOLOGICAL MANAGEMENT & RESTORATION VOL 15 NO 1 JANUARY 2014  ª 2014 Ecological Society of Australia doi: 10.1111/emr.12084 C O M M E N T EcologicalSociety of  Australia  age (Erskine  et al.  2006; Kanowski & Cat-terall 2010; Phelps  et al.  2012). A recentmeta-analysis of global carbon plantingshas questioned this assumption. Usingabove-ground tree biomass as a proxy for C storage, the authors found that treemixes stored at least as much carbon as,and sometimes more than, monoculturesof the most productive species in themixed-species plantings (Hulvey   et al. 2013). A study of plantations in tropicalnorth-eastern Australia also found thatmixed-species plantations were more pro-ductive than monocultures as measuredby stand basal area or mean individual treebasal area (i.e. to account for variableplanting densities; Erskine  et al.  2006). A more recent study, also completed in trop-ical north-eastern Australia, measured62    4.2 (SE) and 83    12 (SE) t carbonper ha in the above-ground biomass of monoculture and mixed-species planta-tions, respectively, although these differ-ences were not statistically significant(Kanowsi & Catterall 2010). While thereis increasing evidence of the benefits of planting tree mixes (refer also to Piotto2008), further research is needed to iden-tify mixes of species that result in high lev-els of carbon sequestration and storage for particular climates and soil types (Pichan-court  et al.  2013). This is particularly truefor plantings of more than 2  –  10 species,for mature aged (  > 15 years) stands andfor plantings that include shrubs as wellas trees, because there are few studies thatexamine these factors. A source of uncertainty for partici-pants and observers of the Australian car-bon market is the contribution of soilcarbon to overall carbon budgets and,additionally, the contribution of mixedspecies to soil carbon compared with thatcontributed by monocultures. In Western Australia, soil-stored carbon can be as sig-nificant as that stored in above-groundbiomass (i.e. up to 59% of total carbonstored in above-ground biomass, leaf litter and soil of single-species eucalypt stands26 years after planting; Harper   et al. 2012). If accounted for, carbon seques-tered in soil (Harper   et al.  2012) may sig-nificantly add to the carbon credits talliedin the above-ground biomass. Interest-ingly, however, the amounts of soilorganic carbon under the stands of planted trees were similar to the amountsheld in adjacent farmland soils despite26 years of eucalypt growth (Harper  et al.  2012). In this case, it is an openquestion as to whether significant stocksof soil carbon would develop within the100-year lifetime of a carbon project. Another study of biodiverse plantings innorth-central Victoria found increased soilorganic carbon under   Acacia mearnsii  and no differences in soil organic carbonunder   Acacia implexa ,  Allocasuarinaverticillata  and  Eucalyptus melliodora ,compared with unplanted soils (Kasel et al.  2011). The results of these twostudies support the idea that storage of organic carbon in soil depends on a com-bination of factors including tree species,land-use history, soil type, site distur-bance, planting age and climate (Guo & Gifford 2002). We need more data onthe relative importance of these factorsfor Australian soils and climates before we can assess the benefit of accountingfor soil carbon beneath mixed-speciesplantings given the likely costs involvedin developing cheap and reliable methodsto measure it (Paul  et al.  2003).Despite possible benefits of speciesmixes for carbon storage, biodiverse car-bon plantings may fail to deliver signifi-cant co-benefits for biodiversity conservation unless the principles of eco-logical restoration, such as the use of refer-ence ecosystems to guide habitatrestoration and the presence of key func-tional groups as benchmarks of restorationsuccess, are used to guide their design andimplementation (Society of Ecological Res-toration International Science & Policy  Working Group 2004; Shackelford  et al. 2013). Achieving biodiversity outcomesfrom mixed-species carbon plantings willrequire an understanding of the ecologicalfactors that facilitate the return of nativespecies to restored landscapes. For exam-ple, in temperate Australia, woodlandbirds tend to respond positively to themore complex vegetation structureoffered by mixed-species plantings com-pared to that offered by plantings consist-ing of single-tree species (Lindenmayer  et al.  2010; Munro  et al.  2011). However,in tropical Australia, planting more speciesis not sufficient to provide habitat for rain-forest birds if the mixed-species plantingscomprise widely spaced stems with an‘open’ rather than a ‘rainforest-like’ struc-ture (Catterall  et al.  2004, 2012). Further-more, the biodiversity value of mixed-species plantings can be significantly lower than the biodiversity value of rem-nant vegetation in agricultural landscapes,suggesting that some values (e.g. particu-lar species, functions) may take decadesor longer to develop in carbon plantings(Catterall  et al.  2004; Munro  et al.  2007;Smith 2009). Overall, maximising biodi- versity outcomes from carbon plantingsrequires more than just a push to steer the industry away from the widespreadadoption of monocultures.Indeed, having some clarity around thebenchmarks for such plantings is a firststep to maximising their value to biodiver-sity conservation. There is scope to raisethe bar so that biodiverse carbon plantingsare not only appropriate to the region(Australian Government 2011a; Bradshaw  et al.  2013) but contain a representativenumber of species from the native ecosys-tem that existed at the site prior to itsbeing cleared for agriculture. Knowing which plant and animal species are likely to recruit into carbon plantings withoutassistance and where this is likely to occur (e.g. Keenan  et al.  1997; Standish   et al. 2007; Munro  et al.  2009; Smith 2009;Catterall  et al.  2012) would help designcost-effective joined carbon credit andbiodiversity conservation projects. Sincebeginning their mining operations in1963, Alcoa of Australia has developedtheir postmining restoration practice torestore between 80 and 100% of the spe-cies found in representative (undisturbed) jarrah forest (Koch 2007). Similar bench-marks could be set for carbon projectsusing nearby remnants of native woodlandor forest. Recent efforts to improve theagronomy of large-scale restoration of agri-cultural lands will pave the way to achievesuch outcomes from carbon plantings(Jonson 2010). While accounting for thecarbon in multiple species is necessarily more complicated than accounting (andprojecting) carbon sequestration for justone species, ongoing efforts to estimate al-lometric relationships for a wide variety of  ª 2014 Ecological Society of Australia ECOLOGICAL MANAGEMENT & RESTORATION VOL 15 NO 1 JANUARY 2014  27 CO MM EN T  native woody species will facilitate their inclusion in carbon accounting models(Paul  et al.  2013).Deriving ecosystem services from car-bon plantings that are additional to theservices supported by the Carbon Farm-ing Initiative and the Biodiversity Fund isa creditable goal. These might includepollination services, climate regulationand the provision of recreational andspiritual opportunities for people. Yet we have limited understanding of thetrade-offs that might prevent some of these services being delivered (Phelps et al.  2012). It might be possible to max-imise the delivery of certain sets of eco-system services such as carbon storageand habitat provision (Bekessy & Wintle2008). On the other hand, maximisinghabitat provision for native species requir-ing vegetation structure by plantingshrubs alongside the trees may reducecarbon sequestration compared with carbon plantings consisting of just trees(Paul  et al.  2013). More research isneeded to identify potential trade-offsamong ecosystem services. There areexperiments underway to understandpotential trade-offs from an ecologicalperspective (e.g. Greening Australia2010; Preece & Mayfield 2011; Perring et al.  2012). Ultimately, such analyses willneed to include socio-economic perspec-tives as well (Pichancourt  et al.  2013),for example how farmers might allocateland use so as to maximise their paymentsfor ecosystem services without affectingtheir more traditional income from cropsand livestock. Awareness of regional-scaleactivities associated with land use andbiodiversity conservation would help toguide land-use decisions for individualfarms (van Oosterzee 2012). Under such a framework, the biodiversity and produc-tion values of a range of land-use options would be considered for their potentialimplementation alongside carbon projects(e.g. native perennial pastures, planta-tions, alley crops, managed native regen-eration). Australia’s progress towards a low-car-bon economy is encouraging and givesus reason to be optimistic that a carbonmarket can help to pay for the restora-tion of our degraded agricultural land-scapes at the scale required to achievesignificant outcomes for biodiversity con-servation (Climate Works Australia 2013).That said, achieving ecological restora-tion to the standard required to restorebiological diversity may require morefunding and effort than a carbon marketis able to supply. Thus, bundling carbon with other ecosystem services could becritical to achieving real gains for conser- vation and restoration. Ecological restora-tion of individual sites is often achieved via a single (one-time) effort. Therefore,it is vital that each effort is executed tothe highest standard we can practically achieve. Australia’s unique biologicaldiversity stands to be richly rewardedby such efforts. Acknowledgements Rachel acknowledges support from theNational Environmental Research Programand the ARC Centre of Excellence for Environmental Decisions. Kris acknowl-edges the support of an ARC LaureateFellowship awarded to Richard Hobbs. We thank Justin Jonson for sharing hisexperiences with us and Tein McDonaldand two anonymous reviewers for their constructive feedback on a draft manu-script. References Australian Government (2011a) Fact sheet  –  Biodiversity Fund Round 1 2011  –  12.[Accessed 4 April 2013]. 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