A global estimate of carbon stored in the world's mountain grasslands and shrublands, and the implications for climate policy

Highlights

• This paper provides the first global estimate of C stored in mountain grasslands and shrublands.

• The paper estimates this amount to be between 60.5 Pg C and 82.8 Pg C. To put this in perspective tropical peatlands are estimated to contain 88.6 Pg C.

• This C is accounted for in a robust manner when considering the UNFCCC's mitigation objectives.

Abstract

Carbon market and climate finance schemes (e.g. the CDM, REDD+ and the Green Climate Fund) are being investigated for their ability to achieve enhanced sustainability outcomes in terrestrial forests, lowland grasslands and marine ecosystems, all which store large amounts of carbon (C). To date however climate policy discourse has largely overlooked the conservation of existing C stored in mountain grasslands and shrublands. These ecosystems provide critical ecological goods and services to humanity yet are increasingly at risk from anthropogenic stressors including agricultural intensification, mining and climate change. The absence of a global estimate for these C stocks is likely to be one reason for their exclusion from climate change policy discussions, both on a political and scientific basis. This represents a missed opportunity in two respects: firstly, by conserving and restoring existing C stocks the impacts of climate change can be lessened; and secondly, carbon finance and climate finance might provide the necessary financial support to address the aforementioned stressors. In this paper we use spatial analysis and estimate there to be between 60.5 Pg C and 82.8 Pg of C contained within biomass and soils of the world's mountain grasslands and shrublands. To put this in perspective, globally tropical Savannas and grasslands, temperate forests and tropical peatlands are estimated to contain 326–330 Pg C, 159–292 Pg C and 88.6 Pg C respectively. Our review of existing empirical studies and of United Nations Framework Convention on Climate Change (UNFCCC) national greenhouse accounts suggests that this C is not reliably accounted for in international carbon budgets. Our estimate is the first to provide a global point of reference, useful in developing future research and in climate policy discussions. We conclude by briefly discussing how climate finance might be leveraged to support the sustainable management of these C stocks, and in so doing uphold the other important socioeconomic benefits provided to humanity.

Introduction

Nearly one quarter of the Earth's landmass is covered by mountains which provide clean water to over 50% of the world's population, shelter almost half of the world's biodiversity ‘hot spots’, and afford important ecologically derived goods and services (Dixon et al., 1994, Kapos et al., 2000). Like mountain forests, healthy and well-functioning mountain grasslands and shrublands (Fig. 1, Fig. 2) provide important benefits to humanity that are often unique due to the presence of steep slopes, extreme weather, and soil types. These services include downslope safety and stable arable terrain, high-quality water for drinking and energy generation, pasture for grazing, recreational opportunities, medicinal plants and a buffer against the spread of bushfires (Hassan et al., 2005, Worboys and Good, 2011). Critically, these ecological goods and services are relied upon by some of the most impoverished people in the world who are often marginalized due to cultural factors and remoteness (Gerlitz et al., 2012). They are also incredibly important in regions that rely on the ‘Alpine Economy’ (Ariza et al., 2013).

One discretely important ecological service performed by these ecosystems is climate regulation. Carbon dioxide (CO₂) is removed from the atmosphere and sequestered in the biomass of dwarf shrubs, heaths, alpine meadows, sages and other vegetation during a short but highly productive growing season, before that C is slowly stored in soil (Djukic et al., 2010). Though this process is relative slow due to tough growing conditions, over time significant amounts of C has accumulated in both the shallow (e.g. Leptosols) and deep soils (e.g. Histosols) of the world's mountain ranges. While similar to soils located in lowland and boreal regions, mountain soils are subject to a number of additional factors including: historical intensive use by humans; especially in Central Europe; higher rainfall, thicker snow cover (insulation) and thus warmer winter ground temperatures; the prevalence of steep well-drained slopes inhibiting widespread peatlands formation, and natural disturbances such as soil erosion, rock fall, spring snow thaw and avalanches (Hagedorn et al., 2010). The perception that mountains are remote, inaccessible and thus not under threat has contributed to government neglect and a lack of national and international conservation action (Ariza et al., 2013). In particular, the potential for more severe erosion make mountain soils more vulnerable to anthropogenic stressors (e.g. grazing) than their lowland and boreal equivalents. Moreover, studies have shown these soils, and the C contained within, are most effectively protected from erosion when there is overlying natural shrubland and grassland vegetation (Luz et al., 2002).

Natural (and semi-natural) mountain grassland and shrubland vegetation is under increasing threat, mainly in developing countries where rapid population growth is contributing to the intensification of agriculture (e.g. potatoes and maize), the expansion of grazing, growth in tourism and expanded high-altitude mining (Körner et al., 2005). While many of these stressors are declining in wealthier nations, others, for instance the infiltration of exotic plant and animal species and the expansion of tourism infrastructure, are having a substantial impact on natural vegetation within protected areas (Booth and Cullen, 2001, Körner et al., 2005, Worboys and Good, 2011). These invasions are not disconnected from the greatest of pressures, climate change, which is projected to have a disproportionate influence at higher latitudes and altitudes (Schröter et al., 2005, Nogués-Bravo et al., 2007). Many of the invading species are range extending, pushing up into alpine areas as lower levels become warmer and less hospitable due to global warming (Benitson, 1994). While one might immediately ponder the potential C gains from a change in vegetation type (i.e. trees), counterintuitively, there is a growing body of evidence to suggest that the expansion of trees into mountain grassland and shrublands may actually result in a net loss of C due to a lower capacity of forests to support soil organic carbon (SOC) compared to tundra (Hartley et al., 2012, Qian et al., 2010). The complex interaction of other factors, such as the suppression of indigenous burning regimes and introduction of grazing, can also lessen the extent of mountain grassland and shrubland vegetation, resulting in higher biomass loads, more frequent and intense fires, and the subsequent loss of additional SOC due to the erosion of exposed soils (Gross and Coppoletta, 2013).

Mountain vegetation plays a critical role in the control of soil erosion. When this vegetation is degraded SOC becomes particularly vulnerable to erosive forces (such as rain, runoff, wind and gravity), risking eventual decomposition and therefore a strong negative impact on the global carbon cycle (Lal, 2003). Without stabilization this process is likely to accelerate and lead to a positive feedback relationship whereby heightened erosion causes loss of more vegetation, which in turn causes more erosion, and so on (Juying et al., 2009). The environmental conditions prevalent at altitude discussed above mean that erosion rates in the mountains can be at least three times that of the lowlands (Ariza et al., 2013). This powerful force degrades biodiversity values, ecological goods and services (e.g. water quality, slope stability and provision of medicinal plants) and local upland communities. Furthermore, erosion has been linked with severe socio-economic and political disturbance in downstream low-land communities (Egziabher, 1991, Lal, 2003).

Damage to mountain vegetation and soils is often irreversible (Jansky et al., 2002) and there needs to be polices in place to encourage the conservation and restoration of the existing natural resource stock by ensuring mountain ecosystems remain healthy and more resilient to climate change and other stressors (Ariza et al., 2013). The establishment of protected areas is a widespread conservation strategy, but when implemented as a standalone policy it has been shown to be largely ineffective in managing these threats through creating economic disadvantage amongst local communities (Gaston et al., 2008). For mountains, trans-boundary and biophysical complexities provide further challenges for this approach. Moreover, the increasing shortage of available financial resources is repeatedly cited as one of the major constraints to arresting biodiversity loss (Waldron et al., 2013) and engaging in sustainable natural resource management (Worboys and Good, 2011). Therefore, the question here is ‘are there other non-traditional sources of funding that could support the sustainable management of mountain grassland and shrublands’?

It is well known that other ecosystems such as terrestrial forests, peatlands, lowland grasslands and shrublands, mangroves, and seagrass meadows store large amounts of C within biomass and soils (Dixon et al., 1994, Beer et al., 2010, Donato et al., 2011, Fourqurean et al., 2012, Scurlock and Hall, 1998, Siikamäki et al., 2012). Carbon markets and climate finance schemes (such as the Clean Development Mechanism, REDD+, and the emerging US$100 billion Green Climate Fund) are being investigated for their role in protecting and replenishing C stocks while achieving enhanced sustainability outcomes (Alongi, 2011, Gibbs et al., 2007, Pendleton et al., 2012, Smith, 2010, Ullman et al., 2013). We propose that the same action be taken for mountain grasslands and shrublands.

Efforts to investigate how international carbon markets and climate finance may support marine ecosystems, existing forests and degraded lands are underpinned by studies estimating C stocks at a global level (Donato et al., 2011, Fourqurean et al., 2012, Siikamäki et al., 2012). By contrast the distribution, extent, volume and density of mountain grassland and shrubland C pools remain largely unknown on a global scale. This presents a number of issues: one, it raises the question as to whether these C pools are adequately accounted for in the IPCC's global carbon budget; two, without an accurate baseline of these C pools it is difficult to track and assess the potential ‘loss and damage’ arising from CO₂ emissions due to anthropogenic stressors; and three, without a global perspective we cannot evaluate where and how carbon markets and climate finance might be used most effectively to address the aforementioned stressors, improve ecosystem resilience and arrest emissions from mountain grasslands and shrublands. In meeting the UNFCCC's ultimate objective of avoiding dangerous climate change, international climate policy will need to “cover all relevant sources, sinks and reservoirs for greenhouse gases” (UNFCCC, 1992). Britton et al. (2011, p. 287) recognize this “urgent need” to “quantify the stocks of C” held in alpine ecosystems.

Here we assemble the first global estimate of C stored in mountain grasslands and shrublands. We present our review of existing literature, conduct a comprehensive Geographical Information System (GIS) analysis, and show that C stocks in these ecosystems (ecoregions) are neither accurately nor adequately accounted for in Annex I country UNFCCC national accounts. Finally, we briefly discuss ways in which carbon markets and climate finance might be leveraged to support conservation while considering sustainable development outcomes for some of the world's poorest people. Ultimately our aim is to bridge the nexus between climate policy and mountain grasslands and shrublands by providing a sound reference point that will serve to underpin future scientific and economic studies, and encourage climate policy discourse – a precursor to leveraging carbon markets and climate finance.