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Drylands and Climate Change Synthesis Paper Drylands and Climate Change Synthesis Paper. Chris Magero, 1/7/2019 1. Introduction Drylands are characterized by variable precipitation, climate variability and water scarcity and are therefore defined by aridity index (AI), which is expressed as a function of average annual precipitation, temperature and potential evapotranspiration (PET) (N. Middleton and Thomas 1997). The aridity index is therefore used to divide the drylands into four subtypes ‐ dry subhumid, semi‐arid, arid, and hyper‐arid . Table 1 Land Classification According to Aridity Index (AI) (N. Middleton and Thomas 1997) CLIMATE TYPE ARIDITY INDEX Dryland subtypes Hyper‐arid AI < 0.05 Arid 0.05 ≤ AI ≤ 0.2 Semi‐arid 0.2 ≤ AI ≤ 0.5 Dry Subhumid 0.5 ≤ AI ≤ 0.65 Non‐Dryland Humid AI ≥ 0.65 Cold PET < 400mm Drylands are an important biome, occupying more than 41% of the global land area and comprising grasslands, agricultural lands, forests and urban areas. Drylands provides much of the world’s food and fiber while maintaining habitats that supports biodiversity and provide ecosystem services (UNCCD 2017). They are home to over 2 billion people across the planet, and found in more than 100 countries globally, majority of which are in developing nations of Africa and Asia Error! Reference source not found.. Table 2 Regional Extent of Drylands Arid Semi‐arid Dry‐subhumid All drylands Region (1000 Km2) (%) (1000 Km2) (%) (1000 Km2) (%) (1000 Km2) (%) Asia (incl. Russia) 6164 13 7649 16 4588 9 18401 39 Africa 5052 17 5073 17 2808 9 12933 43 Oceania 3488 39 3532 39 996 11 8016 89 North America 379 2 3436 16 2081 10 5896 28 South America 401 2 2980 17 2223 13 5614 32 Central America and 421 18 696 30 242 10 1359 58 Caribbean Europe 5 0 373 7 961 17 1359 24 1 World Total 15910 12 23739 18 13909 10 53558 40 Adapted from (Koohafkan and Stewart 2008) The predicted rise of temperature due to climate change is expected to amplify existing risks in natural dryland systems including water scarcity, droughts and floods (IPCC 2015; Koohafkan and Stewart 2008). The likelihood of severe, pervasive and irreversible impacts on species, loss of agricultural productivity is expected to increase with the impacts weighing disproportionately on vulnerable populations (Davies et al. 2012; IPCC 2015). These impacts will have adverse consequences on the ability of dryland people and ecosystems to effectively respond to shocks as system unpredictability increases and as ecosystem services are degraded, leading to declining land productivity and biodiversity loss (UNEP 2009). 2. Dryland Ecology Like many ecosystems, drylands are under constant threat on multiple fronts owing to their complex nature and the interwoven biophysical and socioeconomic interactions which includes high temporal and spatial rainfall variability. These annual and interannual rainfall variations determine the distribution of resources including water and nutrients and consequently the distribution and abundance of vegetation. Species living under these conditions have developed very unique physiological and behavioral strategies to cope with the dramatic variations in rainfall and temperature, enabling them capitalizing on the amorphous distribution of water and nutrients across dryland landscapes. 2.1. Dryland Soils Like many ecosystems, the structure and function of drylands can be traced down to the basic processes that underpin its ecosystems services that provides benefits to people. Nutrient cycling is one of the key underpinning processes in drylands as it compiles the annual nutrient budgets for the ecosystem (Hartley et al. 2007). Nutrient accumulation in dryland ecosystem is however constrained by the extreme variations in rainfall, as well as the overall water deficiency, which ultimately drives nutrient cycling (ibid). Soil has one of the most notable functions in nutrient cycling. It also plays a central role in food production, water storage and climate change mitigation in drylands (Laban, Graciela Metternicht, and Davies 2018; Safriel 2017). Through the accumulation of organic matter, soils provide a habitat for soil organisms which play a key role in nutrient cycling contributing to its fertility and productivity. Above and below ground organisms including bacteria, fungi, protozoa, insects, worms, other invertebrates and vertebrates (Laban, Graciela Metternicht, et al. 2018) play an important role in the carbon and water cycles contributing to the net primary productivity by aiding decomposition processes that convert organically stored nutrient in plants like nitrogen to usable forms by living plant and also determining the soil structure (ibid). The texture and structure of soil including the degree to which soil particles are bound together by organic matter, water holding properties, dissolved minerals and oxygen contained in between the spaces are rudimentary determinant of soil fertility (Safriel 2017). Dryland soils play an extremely important role in mitigation of climate change. They store at least one third of the worlds carbon and they also occupy 42% of the earths terrestrial surface (Laban, Graciela Metternicht, et al. 2018). Soil organic carbon is important because it enhances soil properties including structure and stability, water retention capacity and porosity which are essential for improving soil fertility and productivity and supporting primary production. Dryland soils typically have low organic carbon 2 content due low primary productivity constrained by water scarcity, which affect the accumulation of soil organic matter and soil organic content. Nevertheless, dryland ecosystems quite significant for carbon sequestration and mitigation in comparison to other soils because they have little soil moisture which means that carbon has a longer residence time in the soil (Parton et al. 1995). 2.2. Biodiversity in drylands Water scarcity primarily influences the occurrence and distribution of biodiversity in drylands but as ecosystems they are shaped by underlying factors including geology, topography, rainfall, fires, herbivores and human management over millennia. Climate and latitudinal distribution have a strong influence on drylands biodiversity (Davies et al. 2012) interacting with other factors to form several distinct patchy habitats with unique features across the landscapes ranging from oasis, grasslands, mixed wood grasslands and evolving alongside them species that are equally unique that colonize the varied habitats (ibid). The variety of habitats created by the kaleidoscope of resources spreads across the landscape and supports a wide array of species diversity including unique endemic populations that occur in these patchy specialized niches. They also support an array of ecosystem services essential for the survival of these species that also contribute to local livelihoods (ibid). In drylands, the loss of biodiversity is caused by inter‐connected factors that include land use conversion primarily to crop farming, growth of urban settlements, invasive species and habitat fragmentation that alter migration routes of species and limits their access to previously utilizable land resources (ibid). Biodiversity loss accelerates land degradation, increases vulnerability to climate change and contributes to poverty. 2.3. Grasslands Grasslands are important drylands ecosystems contributing to more than 800 million livelihoods and supporting essential ecosystem services including forage production, carbon storage, soil protection and biodiversity conservation (NatureServe 2010). Grasslands are globally distributed and they can be found in Europe, Asia, America, South America, Australia and Africa and they play an important role as grazing land for herbivore. They are among the largest ecosystems in the world covering 52.5 million square kilometres which is about 40.5 percent of terrestrial area, excluding Greenland and Antarctica (FAO 2005). Grasslands are the source of a substantial amount of milk (27%) and meat (23%) production in the world and supports about 1 billion livelihoods who depend on the livestock industry (ibid). Grasslands are major vegetation types covering about one fifth of the world’s surfaces and holding more than 10% of the world’s terrestrial carbon (Parton et al. 1995). Grasslands can store up to 70 tonnes/ha soil carbon, values that are comparable with carbon stored in forest soils, but their potential has been underestimated in the past (Chuluun and Ojima 2002; Laban, Graciella Metternicht, and Davies 2018; Parton et al. 1995; UNEP 2009). They are also an important habitat for wildlife and people and provide forage for domestic livestock. 3 Globally, grasslands are under a lot of pressure of being converted to cropland and used for intensive livestock (FAO 2010). Grasslands are generally undergoing degradation as a result of grassland cultivation leading to huge releases of soil carbon into the atmosphere (FAO 2010). The rate of carbon accumulation in grasslands varies from year to year based on climatic variability. The amount and distribution of rainfall in drylands is directly correlated to annual biomass production and Net Primary Production will therefore vary with the differences in rainfall, which can be dramatic (IPCC 2006). Annual biomass production in grasslands is substantial but it is often masked by the short growth period of grasses and annual senescence of herbaceous vegetation which quickly leave the ground bear. Additional losses of vegetation through grazing, fire, natural disturbance and harvesting essentially leaves standing biomass
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