To what degree can restoration of rangelands in Australia be utilized for Australia’s greenhouse gas emissions mitigation? A literature review of evidence
Carbon dioxide (CO2) emissions, which is the primary greenhouse gas (GHG), have increased by around 40% over the last two decades from 1994 to 2009 and each year an additional carbon of around 2 Gt is released to atmosphere mainly due to land use change (Sommer and Bossio, 2014). Soil carbon (C) sequestration is considered as the most potential way for mitigating GHG emissions, mostly depending on the amount of carbon stored and the land cover of soils. Australia’s rangelands, covering 80% of Australia’s landmass, play a vital role in achieving a significant carbon sink, though carbon storage rates per hectare are low in some types of rangelands such as arid and semi-arid areas (Garry et al., 2010). It is reported that Australia’s rangelands would have a potential C sequestration of approximately 48 Gt to a one-meter depth (Squires and Glenn, 1995). However, overgrazing, deforestation and fire have caused a marked increase in soil C losses in the rangelands. For instance, Garry et al. (2010) document that soil C store after clearing a woodland in Queensland for grazing has reduced by around 8% to 0.3m depth, which is equivalent to nearly 260 Mt CO2 equivalent (CO2-e) emissions. Methods and management related to restoration of rangelands are in great need for mitigating GHG emissions. This literature review will first explore why the restoration of rangelands can be considered as a mitigation option. The proposed mitigation potential of such restoration will then be analysed, followed by a comparison with potentials in other countries. These potentials are extensive and variable, but when environmental, research and financial limitations are involved, how the realistic mitigation outcomes present and whether this land-based option is feasible and achievable will be discussed finally.
Why restoration of rangelands can be considered as a type of mitigation options?
Rangelands, including grasslands, shrublands, woodlands and the tropical savannas, cover more than 50% of the global land area and are estimated to store up to 30% of the global soil organic carbon (Henderson et al., 2015, Schuman et al., 2002). In Australia, rangelands occupy over 80% of the landmass (see Fig. 1). It means that the amount of soil carbon storage is low per unit compared with forests, but due to the large expanse of the rangelands the total carbon store is considerable (Allen et al., 2014). In order to evaluate the potential C sequestration in Australian rangelands and related management practices for increasing C storage to achieve the goal of GHG mitigation, rangelands are often considered as perpetual grazing lands, mostly used for grazing of sheep and cattle (Garry et al., 2010, Allen et al., 2014, Witt et al., 2011, Eady et al., 2009), though large areas in arid and semi-arid rangelands are relatively intact and not grazed such as the mulga (Acacia aneura) rangelands (Eady et al., 2009, Garnaut, 2008)
Fig. 1. The rangelands in Australia (Natural Resource Management Ministerial Council, 2010).
Proposed mitigation potential
It is more difficult to estimate the potential of C sequestration in rangelands than forests and croplands (Schuman et al., 2002). Not only because of the various definitions of the term “rangelands” mentioned above, but there are also vast uncertainties in estimates and calculation of mitigation potentials. Witt et al. (2011) report that the potential of Australian arid and semi-arid areas to sequester C has been estimated to be 20-250 Mt CO2-e/year. The large-scale data shows a fact that there are significant variations in predicting mitigation potentials of rangelands. This is primarily due to the differences in vegetation communities, climatic conditions and soils in rangelands. Additionally, land change and management practices such as grazing and savanna burning have led to more complex ecosystems. The mitigation models providing data for researchers are variable as well (Schuman et al., 2002, Garry et al., 2010).
Table 1 demonstrates the mitigation potentials of Australian rangelands and one United States rangeland, categorized by different literatures and research sites. Grazing land use and degradation of such grazing lands have resulted in significant GHG emissions globally (Garry et al., 2010). It is not surprising that the majority of research studies focus on grazed and degraded areas when it comes to restoration of rangelands. When looking at the data, the mitigation potentials vary widely, ranging from Witt et al. (2011) about 1 t CO2-e per year in a 25.4 million-ha mulga grazed land and Conant and Paustian (2002) 16 Mt CO2-e per year in overgrazed lands (49 million ha), to Garnaut (2008) 286 Mt CO2-e per annum in the whole grazed rangelands in Australia (358 million ha). Most of the difference in these potentials is due to variations in area (some may even overlap), which makes such predictions dramatically difficult.
Table 1. Rangeland area and related mitigation potential
The mitigation potential of Australian rangelands is lower than that of US rangelands (286 Mt CO2-e/year versus 388 Mt CO2-e/year). However, the potential of Australian rangelands (roughly equivalent to Oceania) ranks highly when compared with similar mitigation options internationally (see Table 2). Overall, the total global C sequestration potential in rangelands is documented by Henderson et al. (2015) as 110.1 Tg CO2 per year. The potential C sequestration between regions is variable as well. The highest potentials are in Central & South America (26.7 Tg CO2/year), Sub-Saharan Africa (24.3 Tg CO2/year), and Oceania (15.6 Tg CO2/year). Although the comparison with different countries remains substantial uncertainties because of highly various assumptions, methodologies and definitions, some key trends can be found in worldwide mitigation potentials of rangelands. Generally, on the global scale, the largest potential is more likely to be in the humid rangelands, which is followed by temperate and arid rangelands. However, since arid and semi-arid areas cover more than half of the overall rangelands, the worldwide trend of C sequestration potential is displayed on the regional scale, where Central & South America, Sub-Saharan Africa and Oceania have the highest potentials, consistent with humid and arid areas occupying the largest part of rangelands in these regions.
Table 2. Carbon sequestration potentials in different regions under altered grazing management (Henderson et al., 2015).
Obstacles for achieving the mitigation potential
Rangelands are characterized by patchy and complicated patterns of plant communities and soils (Schuman et al., 2002) with variable climatic conditions, especially temperature and precipitation, on both spatial and temporal scales (Eady et al., 2009). More research studies are needed to tackle rangeland ecological dynamics, in particular, soil C dynamics. What has been known in terms of the rangelands’ ecological behavior is still unclear and how it is influenced and altered by environment and management remains uncertain (Booker et al., 2013). The residence time of carbon stored in the soil is also changing due to the variable characteristics of rangelands. These variations make it difficult to assess the soil C storage, and therefore, cause limitations for predicting mitigation potentials.
Booker et al. (2013) claim that vastly insufficient information has resulted in overgeneralization of ecological knowledge of rangelands which should have been specific and is associated with the locally diverse environmental conditions. Such overgeneralized knowledge can cause erroneous assumptions, definitions and methods, and thus improper estimates of mitigation potential and management. For instance, defining the word “rangeland” varies in different literatures, which can lead to variations in the rangeland area, and hence, widely ranged potentials of C sequestration (Garnaut, 2011).
A huge constraint on estimates of mitigation potential of rangeland ecosystem is lacking sufficient quantitative data. Eady et al. (2009) report that it is more difficult to quantify the uncertainties due to the scarcity of data on C storage and fluxes in Australia. In addition, the impact of natural disturbances such as fire and flood on stored carbon seems to be unpredictable. Methodologies of rangelands are limited by the lack of specific data as well (Sudmeyer et al., 2014). For example, monitoring the horizontal transfers of carbon in the soil caused by soil and wind erosion is likely to be infeasible due to the shortage of methodologies (Eady et al., 2009). It still remains unclear in terms of the effectiveness of management practices that can gain positive C accumulation and environmental benefits (Witt et al., 2011). Modelling of the influences of management is also problematic since data for long period (> 20 years) are constrained (Allen et al., 2014). There also appears to confusions as to whether a management does actually alter C sequestration (Eady et al., 2009), because some additional C sequestered after the implementation of a policy seems not because of the altered management, and is more likely to be continuously sequestered even after the policy is terminated (Booker et al., 2013).
The environmental and research limitations of rangelands are considerable, however, the financial constraints on achieving the mitigation potential are substantial. Witt et al. (2011) mention that though restoration of rangelands is necessary, it is way too expensive when compared with its economic potential gained from the landscape. Since the potential C sequestration is vastly variable in the rangelands and mostly lower than forests, only little returns can be obtained from carbon offsets, which is likely to be no more than fourteen dollars to 2020 (Sudmeyer et al., 2014). It means that all costs related to restoration management and projects, including operational costs, research and development costs and administration costs, are required to be minimal. Eady et al. (2009) suggest that there is a conflict between maintaining production and GHG abatement. A low income the pastoralists obtain from livestock production would be permanent until the cost of GHG mitigation projects is under the income from grazing. In terms of the scientific research costs, as C stocks in rangelands are not constant, repetitive monitoring and testing the growth of carbon accumulation every 10-20 years in every ecological site is indispensable. The related costs are tremendously high due to great variations in environmental characteristics. Although many models are tried to replace the direct measurement, they cannot guarantee the accuracy. Also, the large amount of carbon stored below ground makes it hard to measure by using relatively cost-effective methods (e.g. remote sensing techniques) (Booker et al., 2013).
Whether this mitigation option is achievable, realistic and feasible?
Schuman et al. (2002) and Booker et al. (2013) state that restoration of rangelands has the potential to increase C sequestration, thereby mitigation GHG emissions. However, most research associated with revegetation is on the basis of conversion from agricultural lands such as croplands to rangelands (e.g. grasslands) by replanting grass, instead of rehabilitation from previously degraded rangelands. Estimating the mitigation potentials is more likely to be used to emphasize the potential value of rangelands for C sequestration rather than the quantitative data itself (Schuman et al., 2002). These estimates show how significant the rangeland it is as a sink of carbon, and what the potential of C storage can be increased by changing management and learning more about rangeland ecological dynamics, especially, soil C dynamics. The more related research there is, the more correct it will be to estimate the potential C sequestration, and the more the GHG emissions can be mitigated. Additionally, Booker et al. (2013) state that, instead of C fluxes, more attention should be paid to C stocks since improper policy and management is more likely to convert the rangelands from a net C sink to a net C source. Similarly, Garry et al. (2010) suggest that the restoration of degraded rangelands would have ecological value as C stocks increase and land productivity is improved. Garnaut (2008) also report that changed management towards Australian grazing lands is predicted to potentially reduce the GHG emissions, despite some extremely degraded areas where management has less capacity to rehabilitate.
Mitigation potentials of restoration of rangelands largely rely on land area. Globally, the potential of Australian rangelands ranks highly. However, extensive obstacles discourage the achievement of these potentials, including the environmental and research limitations such as the variable environmental characteristics that are hard to measure both spatially and temporally, the consequently ever-changing residence time of soil C stocks, the overgeneralization of ecological knowledge of rangelands, and the vast lack of quantitative data that causes unpredictable influences on natural disturbances, limited monitoring methodologies, unclear effectiveness of management practices and confusions about whether a management does actually change C sequestration; and the financial constraints including the high costs such as the scientific research costs, low returns from carbon offsets and difficulty in creating alternative cost-effective methods to substitute for direct measurement. Having said that, regardless of those uncertainties, the estimate of mitigation potentials is more likely to be the emphasis on the potential value of rangelands for C sequestration rather than the data value. The significance of the rangeland as a carbon sink and its potential C storage increase via improved management have been broadly explored. The more related research there is, the more correct it will be to estimate the potential C sequestration, and the more the GHG emissions can be mitigated.
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