Dr Thomas J. Sauer, Research Leader at the National Laboratory for Agriculture and the Environment, U.S. Department of Agriculture details the importance of agroforestry for the environment
Agroforestry is a land-use practice that involves growing perennial woody vegetation (trees, shrubs, or hedges) integrated with forages, crops, fruits, berries and nuts, herbs, or medicinal plants on the same land. Many agroforestry systems mimic the multi-layered canopies of natural ecosystems that have tall trees above smaller trees or shrubs with a layer of non-woody plants beneath. A distinctive feature of natural ecosystems is the wide diversity of species that can be found even in an area of similar soils and climate. However, as agriculture and forestry evolved, the cultivation of crops and trees became specialised and separate leading to the modern production practices typified by monocultures of single crop or tree species grown on large parcels of land.
Why is there currently so much interest in a land-use system that represents such a stark contrast to modern agricultural practices? Climate change, food security, and dwindling land resources are the main drivers behind the surge of interest in agroforestry. Climate change effects impose 2 contrasting environmental stresses on agricultural production. Long-term, gradual shifts in temperature and precipitation (total amount and its annual distribution) may create “normal” conditions that are outside the optimal range for current crop species. The other climate-related environmental stress is extreme, episodic events. The severity of one isolated event (e.g. unusually high winds and heavy precipitation) may produce damage that results in near-complete crop failure and severe damage to soil and water resources. Only the most resilient of soils and agroecosystems will be able to adapt to shifts in climate or to fully recover from catastrophic events and return to pre-event levels of productivity.
Agroforestry systems are inherently more resilient to weather extremes than many traditional cropping systems. Agroforestry practices utilise perennial vegetation and multiple species that provide greater plant diversity with less vulnerability to climate stress than is provided by monocultures. The perennial woody vegetation itself also modifies the local microclimate. The trees and shrubs influence airflow and sunlight interception patterns, protecting the understory and adjacent plants from extremes in temperature and damaging winds. The deep rooting by the perennial vegetation also affords greater resilience to drought and increased exploitation of soil water and nutrients from soil layers not readily available to more shallow-rooted annual crops. Greater efficiency of agroforestry systems in water use and nutrient cycling is a key strength and further enhances their efficacy under the uncertainties of climate change.
Although agroforestry practices are inherently more resilient to environmental stresses, sharply increasing global demand for food, fuel, and fibre is creating intense pressure to produce more of all of these products. Meeting the food security challenge of feeding an estimated global population of 9 billion by 2050 is daunting enough without considering the competing demands for renewable bioenergy, timber, and fibre production. The diversity of species and design features of agroforestry practices offer greater opportunities to produce multiple commodities from the same parcel of land. The combination of species, their distribution/arrangement, and water and nutrient sources are also all manageable features. Agroforestry systems can thus be designed for anticipated changes in climate including greater fluctuations of or long-term trends in precipitation and temperature or to create systems that are more resilient to climate stresses by including species, design features, or management practices that are more adaptable.
The overriding objective of future agricultural land use will be on optimising productivity from the available land base. Agroforestry systems are easily adaptable to the concept of sustainable intensification. Each component can be managed to optimise production while the perennial species provide system resilience and ameliorate climate stresses. Soil degradation by erosion, salinity, and pollution and loss of prime quality farmland due to urbanisation and development continue to reduce the amount of land available for agriculture. Some new areas can be brought into production but often with lower productivity and at great cost due to the necessary investment in infrastructure (roads, irrigation, utilities, etc.).
By intensifying production on current lands using sustainable agroforestry practices, multiple products and ecosystem services (air and water quality, wildlife habitat, carbon sequestration, etc.) can be provided efficiently. In many cases, existing crop and forest systems can be modified to incorporate the production of multiple products from the same land. Some examples are thinning forests to allow grazing while the remaining timber matures (silvopasture) or planting rows of trees in a semiarid grain cropping system to improve water use efficiency and grow bioenergy feedstock (tree windbreaks).
One of the greatest climate-related threats to agriculture in the USA occurred during the 1930s when persistent drought, poor soil management, and severe wind erosion in the Great Plains region created a major environmental crisis. These “Dust Bowl” conditions were exacerbated by a deep economic depression and accompanying social disruption. To address this dire situation, the federal government developed a novel tree-planting program to bring physical and economic relief to 6 of the most severely-affected Dust Bowl states. The Prairie States Forestry Project (PSFP) was designed to alleviate drought conditions by creating multi-row tree windbreaks that would stabilise the soils and create a more favourable microclimate for crops. From 1935 to 1942 the PSFP program succeeded in planting over 217 million trees in almost 30,000 km of windbreaks. Many of these windbreaks planted over 80 years ago are still protecting crops in the region today. The principles behind the PSFP were largely borrowed from the Russian steppes, which have a long tradition of tree windbreak planting to protect crops from hot, dry summer winds that historically lead to repeated crop loss and famine.
Successfully addressing global climate change effects on agriculture will require a holistic, sustained approach incorporating a suite of strategies at multiple spatial scales and time horizons. In the USA of the 1930’s, bold and innovative leadership at high levels of government was needed to enact a unique program over an extensive area to successfully address severe drought conditions. Agroforestry practices offer excellent opportunities to adapt current agricultural production systems to future climates, build more resilient agricultural systems, and also provide climate change mitigation through carbon sequestration in biomass and the soil. Agroforestry also provides multiple additional ecosystem services including enhancing wildlife habitat, improving local microclimate and esthetics, and expanding renewable energy sources. The challenge now is to engage similar decision-making methods as demonstrated by the successful PSFP of the 1930s to craft effective policies and programs for adapting agriculture to global climate change effects and enhancing global food security.
Dr Thomas J. Sauer
U.S. Department of Agriculture – Agricultural Research Service, National Laboratory for Agriculture and the Environment