Plants harness the energy of the sun and accumulate carbon from the atmosphere to produce biomass on which the rest of the biota depend. The great innovation of agriculture 10,000 years ago was to manage the photosynthesis of plants and ecosystems so as to dependably increase yields. With 5 billion hectares of Earth's surface used for agriculture (69 percent under pasture and 28 percent in crops) in 2002, and with half a billion more hectares expected by 2020, agricultural production systems and landscapes have to not only deliver food and fiber but also support biodiversity and important ecosystem services, including climate change mitigation. A major strategy for achieving this is to increase the role of perennial crops, shrubs, trees, and palms, so that carbon is absorbed and stored in the biomass of roots, trunks, and branches while crops are being produced. Tree crops and agroforestry maintain significantly higher biomass than clear-weeded, annually tilled crops.20
Although more than 3,000 edible plant species have been identified, 80 percent of world cropland is dominated by just 10 annual cereal grains, legumes, and oilseeds. Wheat, rice, and maize cover half of the world's cropland. Since annual crops need to be replanted every year and since the major grains are sensitive to shade, farmers in much of the world have gradually removed other vegetation from their fields.
But achieving a high-carbon cropping system, as well as the year-round vegetative cover required to sustain soils, watersheds, and habitats, will require diversification and the incorporation of a far greater share of perennial plants.21
Perennial grains. Currently two thirds of all arable land is used to grow annual grains. This production depends on tilling, preparing seed beds, and applying chemical inputs. Every year the process starts over again from scratch. This makes production more dependent on chemical inputs, which also require a lot of fossil fuels to produce. Furthermore, excessive application of nitrogen fertilizer is a major source of nitrous oxide emissions, as noted earlier.22
In contrast, perennial grasses retain a strong root network between growing seasons. Hence, a good amount of the living biomass remains in the soil instead of being released as greenhouse gases. And they help hold soil organic matter and water together, reducing soil erosion and GHG emissions. Finally, the perennial nature of these grasses does away with the need for annual tilling that releases GHGs and causes soil erosion, and it also makes the grasses more conservative in the use of nutrients. In one U.S. case, for example, harvested native hay meadows retained 179 tons of carbon and 12.5 tons of nitrogen in a hectare of soil, while annual wheat fields only retained 127 tons of carbon and 9.6 tons of nitrogen. This was despite the fact that the annual wheat fields had received 70 kilograms of nitrogen fertilizer per hectare annually for years.23
Researchers have already developed perennial relatives of cereals (rice, sorghum, and wheat), forages (intermediate wheatgrass, rye), and oilseeds (sunflower). In Washington state, some wheat varieties that have already been bred yield over 70 percent as much as commercial wheat. Domestication
Farming and Land Use to Cool the Planet work is under way for a number of lesser known perennial native grasses, and many more perennials offer unique and exciting opportunities.24
Shifting production systems from annual to perennial grains should be an important research priority for agriculture and crop breeding, but significant research challenges remain. Breeding perennial crops takes longer than annuals due to longer generation times. Since annuals live for one season only, they give priority to seeds over vegetative growth, making yield improvement in annuals easier than in perennials that have to allocate more resources to vegetative parts like roots in order to ensure survival through the winter. But in the quest for high-carbon agricultural systems, plants that produce more biomass are a plus. Through breeding, it may also be possible to redirect increased biomass content to seed production.
A Billion Tree Campaign launched in 2006 shattered initial expectations and mobilized the planting of 2 billion trees in more than 150 countries.
Agroforestry intercrops. Another method of increasing carbon in agriculture is agro-forestry, in which productive trees are planted in and around crop fields and pastures. The tree species may provide products (fruits, nuts, medicines, fuel, timber, and so on), farm production benefits (such as nitrogen fixation for crop fertility, wind protection for crops or animals, and fodder for animals), and ecosystem services (habitat for wild pollinators of crops, for example, or micro-climate improvement). The trees or other perennials in agroforestry systems sequester and store carbon, improving the carbon content of the agricultural landscape.
Agroforestry was common traditionally in agricultural systems in forest ecosystems and is being newly introduced into present-day subsistence and commercial systems. The highest carbon storage results are found in "multistory" agroforestry systems that have many diverse species using ecological "niches" from the high canopy to bottomstory shade-tolerant crops. Examples are shade-grown coffee and cocoa plantations, where cash crops are grown under a canopy of trees that sequester carbon and provide habitats for wildlife. Simple intercrops are used where tree-crop competition is minimal or where the value of tree crops is greater than the value of the intercropped annuals or grazing areas, or as a means to reduce market risks. Where crops are adversely affected by competition for light or water, trees may be grown in small plots in mosaics with crops. Research is also under way to develop low-light-tolerant crop varieties. And in the Sahel, some native trees and crops have complementary growth patterns, avoiding light competition all together.25
While agroforestry systems have a lower carbon storage potential per hectare than standing forests do, they can potentially be adopted on hundreds of millions of hectares. And because of the diverse benefits they offer, it is often more economical for farmers to establish and retain them. A Billion Tree Campaign to promote agroforestry was launched at the U.N. climate convention meeting in Nairobi in 2006. Within a year and a half the program had shattered initial expectations and mobilized the planting of 2 billion trees in more than 150 countries. Half the plantings occurred in Africa, with 700 million in Ethiopia alone. By taking the lead from farmers and communities on the choice of species, planting location, and management, and by providing adequate technical support to ensure high-quality planting materials and methods, these initiatives can ensure
Farming and Land Use to Cool the Planet that the trees will thrive and grow long enough and large enough to actually store a significant amount of carbon.26
Tree crop alternatives for food, feed, and fuel. In a prescient book in 1929, Joseph Russell Smith observed the ecological vulnerabilities of annual crops and called for "A Permanent Agriculture." This work highlighted the diversity of tree crops in the United States that could substitute for annual crops in producing starch, protein, edible and industrial oils, animal feed, and other goods as well as edible fruits and nuts—if only concerted efforts were made to develop genetic selection, management, and processing technologies. Worldwide, hundreds of indigenous species ofperennial trees, shrubs, and palms are already producing useful products for regional markets but have never been subject to systematic efforts of tree domestication and improvement or to market development. Since one third of the world's annual cereal production is used to feed livestock, finding perennial substitutes for livestock feed is especially promising.27
Exciting initiatives are under way with dozens of perennial species, mainly tapping intra-species diversity to identify higher-yielding, higher-quality products and developing rapid propagation and processing methods to use in value-added products. For example, more than 30 species of trees, shrubs, and liane in West Africa have been identified as promising for domestication and commercial development. Commercial-scale initiatives are under way to improve productivity of the Allanblackia and muiri (Prunus africanus) trees, which can be incorporated into multistrata agroforestry systems to "mimic" the natural rainforest habitat. Growing trees at high densities is not, however, recommended in dry areas not naturally forested, as this may cause water shortages, as has happened with euca lyptus in some dry areas of Ethiopia.28
Shifting biofuel production from annual crops (which often have a net negative impact on GHG emissions due to cultivation, fertilization, and fossil fuel use) to perennial alternatives like switchgrass offers a major new opportunity to use degraded or low-productivity areas for economically valuable crops with positive ecosystem impacts. But this will require a landscape approach to biofuels planning in order to use resources sustainably, enhance overall carbon intensity in the landscape, and complement other key land uses and ecosystem services.29
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