Overstory #255 - Climate Change Adaptation
Adaptation by Agroforestry
Agroforestry can add a high level of diversity within agricultural lands and, with it, an increased capacity for supporting numerous ecological and production services that impart resiliency to climate change (CC) impacts (see figure below) (Verchot et al. 2007). From a landowner’s perspective, the most valued services would be those that can dampen the negative effects of CC and weather extremes while augmenting the positive benefits provided by tree-based systems.
CC risk management is difficult in annual-only systems due to the increasing uncertainty and volatility of interannual variability in rainfall and temperatures. The mixing of woody plants into crop, forage, and livestock operations provides greater resiliency to this interannual variability through crop diversification produced seasonally, as well as through increased resource-use efficiency (Olson et al. 2000). Deep-rooted trees allow better access to nutrients and water during droughts and, when appropriately integrated into annual cropping or forage systems, may extract from a different pool of resources and/or from resources that would otherwise be lost from the system (van Noordwijk et al. 1996). Agroforestry increases soil porosity, reduces runoff, and increases soil cover, which can improve water infiltration and retention in the soil profile thereby reducing moisture stress in low rainfall years (Jose et al. 2009). During periods of excessive soil moisture, treebased systems can maintain aerated soil conditions by pumping out excess water more rapidly than other production systems, and when flooding eliminates an herbaceous crop for a season, the woody component can often survive and offer an economic return (Dimitriou et al. 2009).
Agroforestry practices are used to alter microclimates to produce more favorable conditions for crop, forage, and livestock production, and empirical results suggest these agroforestry-induced conditions could be critical in providing extra resiliency to shifting temperature and moisture regimes. Field studies have shown that air and soil temperatures too cold or too warm for forage growth can be favorably modified by trees in silvopasture systems to create an extended production period (Feldhake 2002; Moreno et al. 2007). Using a process-based model, Easterling et al. (1997) showed that windbreaks would increase dryland maize yields in Nebraska above corresponding unsheltered yields for most levels of predicted climate change.
Along with the impacts of weather extremes on production, increasing carbon dioxide (CO2) may reduce the ability of grazing lands to supply adequate livestock feed (USGCRP 2009). Morgan et al. (2004) found that higher levels of CO2 increased forage productivity but reduced forage quality because of the effects on plant N and protein content. The shading component in silvopasture systems, however, has been shown to improve forage quality by increasing protein content while reducing fiber (Kallenbach et al. 2006), which when combined with creating more favorable microclimate conditions could potentially result in higher forage quantity and quality than opengrazing lands during heat stress events.
With warmer temperatures, insect pests and plant diseases are expected to increase due to range expansion, higher winter survival, and increased number of generations per season (USGCRP 2009). Enhancing opportunities for biological pest control will become increasingly important and could be accomplished through agroforestry (Dix et al. 1995). For example, alfalfa intercropped with walnut supported twice as many predators and parasitic hymenoptera and half as many herbivores as did alfalfa alone (Stamps et al. 2002). Stamps and Linit (1997) reported that the greater niche diversity of agroforestry may support greater numbers and/or diversity of natural enemy populations than even polycultural systems of annual crops.
Increased heat, disease, and weather extremes are likely to reduce livestock productivity, and studies show that the negative effects of hotter summers will outweigh the positive effects of warmer winters (USGCRP 2009). By providing shade, silvopasture can reduce the energy expended for thermoregulation, leading to higher feed conversion and weight gain. Mitlöhner et al. (2001) found that cattle provided with shade reached their target body weight 20 days earlier than those without shade.
Climate change is expected to result in more erratic precipitation patterns that will ultimately lead to higher soil erosion rates. Where rainfall amounts increase, erosion and runoff will increase at an even greater rate: the ratio of erosion increase to annual rainfall increase is on the order of 1.7 (Nearing et al. 2004). Where annual rainfall decreases, system feedbacks related to decreased biomass production and soil drying could lead to greater susceptibility of the soil to erode. Modelling results indicated a 2ºC increase in annual temperature could increase wind erosion by 15% to 18% (Lee et al. 1996). Windbreaks, alleycropping systems, and riparian buffers are typically designed for reducing wind and water erosion (Garrett 2009).
Agroforestry systems can offer greater economic stability and reduced risk under CC by creating more diversified enterprises with greater income distribution over time. Like alley cropping, a silvopasture system mitigates risks associated with climate variability and fluctuating prices by providing short-term (forage and/or livestock) and long-term (timber) income sources (Cubbage et al. 2012).
Conserving biological diversity under shifting climates is a global priority (Korn et al. 2003). Agroforestry can play three major roles in supporting this priority: (1) providing habitat that offers a range of microclimate and resource refugia; (2) increasing landscape connectivity for species to migrate as climate changes; and (3) providing other ecosystem services, such as erosion control and water quality protection, that prevent the degradation and loss of surrounding habitat. Realizing these beneficial adaption services will require additional work to develop improved combinations and arrangements of species that better maximize facilitative interactions, while minimizing the competitive interactions between crop and trees (Jose et al. 2009).
Adaptation of Agroforestry
Despite the positive CC adaptation services that agroforestry can provide, these systems will likely be negatively impacted by the same forces. Plant stress, as well as shifts in woody plant disease, pest and natural enemy dynamics created by weather extremes, and the longer-term predicted shifts in climate, will play a dominant role in the persistence and performance of all agroforestry plants, herbaceous or woody (Fuhrer 2003; Allen et al. 2010). This is of particular concern with agroforestry as these practices require a long time to become established and fully functional. Losing one’s investment due to a stress and/or pest outbreak would be devastating. With limited research on CC-adapted plant materials and, more importantly, due to the longevity of the woody component, one is left with using diversity as a key principle in developing CC-adapted agroforestry plantings: in essence, selecting a variety of plants that will thrive under the many conditions created by shifting weather and climate change. Breeding programs can also expand selection options, such as the marker-assisted poplar breeding program in Canada that is utilizing the AgCanBaP collection (Soolanayakanahally 2010) to generate woody biomass feedstocks with high resource use efficiencies for present and future climates. Climate modeling results are indicating major shifts in tree species distribution (Iverson and Prasad 1998). Projecting out which agroforestry species that may be more suited to future conditions will need to become part of the CC-integrated planning and design process.
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This article was excerpted from the original with the kind permission of the publisher from:
Schoeneberger, M., G. Bentrup, H. de Gooijer, R. Soolanayakanahally, T. Sauer, J. Brandle, X. Zhou, and D. Current. 2012. Branching out: Agroforestry as a climate change mitigation and adaptation tool for agriculture. Journal of Soil and Water Conservation 67(5):128A-136A www.swcs.org
Michele Schoeneberger is research leader and soil scientist and Gary Bentrup is research landscape planner with the US Forest Service, USDA National Agroforestry Center, Lincoln, Nebraska. Henry de Gooijer is unit manager and Raju Soolanayakanahally is senior agroforestry researcher with the Agroforestry Development Centre, Science and Technology Branch, Agriculture and Agri-Food Canada, Indian Head, Saskatchewan, Canada. Tom Sauer is research soil scientist with the USDA Agricultural Research Service National Laboratory for Agriculture and the Environment, Ames, Iowa. James Brandle is professor of Shelterbelt Ecology, School of Natural Resources, University of Nebraska, Lincoln, Nebraska. Xinhua Zhou is statistician with the Department of Biostatistics, University of Kansas, Kansas City, Kansas. Dean Current is program director with the Center for Integrated Natural Resource and Agricultural Management, University of Minnesota, St. Paul, Minnesota.
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