Agroforestry

Preparers

Gary Bentrup, USDA National Agroforestry Center, US Forest Service, Lincoln, NE

Kate MacFarland, USDA National Agroforestry Center, US Forest Service, Burlington, VT

Introduction

Adapting to climate change requires a comprehensive approach that involves public and private lands. Agroforestry is the intentional mixing of trees and shrubs into crop and animal production systems to create environmental, economic, and social benefits. This private land management approach provides opportunities for shared stewardship on agricultural and forested lands, including those adjacent to public lands. Agroforestry allows land managers to integrate productivity and profitability with environmental stewardship that will contribute to healthy and sustainable landscapes.

The five widely recognized categories of agroforestry in the United States are 1) alley cropping, 2) windbreaks, 3) riparian forest buffers, 4) silvopasture, and 5) forest farming (also called multistory cropping) (Figure 1 and Table 1) (1). These agroforestry practices are used to enhance crop and livestock production; protect soil, air, and water quality; provide wildlife habitat; and diversify income sources.

Figure 1. Categories of common agroforestry practices used in the United States. Modified from (1).

Table 1. Description of agroforestry practices used in the United States. Modified from (1).

Based on a recent scientific assessment, agroforestry can contribute to climate change adaptation and mitigation by 1) reducing threats and enhancing agricultural landscape resiliency, 2) facilitating species movement to more favorable conditions, 3) sequestering carbon, and 4) reducing greenhouse gas emissions (1).

As an agricultural management option under climate change, agroforestry is unique in that it is a woody-plant based approach that adds functional diversity at various scales (2). Perennial components can create microclimates that benefit crops and livestock. The use of agroforestry to enhance resiliency is not a new idea. To mitigate one of the largest North American wind-erosion events in history—the 1930s Dust Bowl— the Prairie States Forestry Project planted over 18,600 miles of windbreaks in the Great Plains (3). Windbreaks continue to be used by farmers and livestock producers across the US to enhance crop yields and protect soil health, while also providing conservation benefits. Other agroforestry practices can play similar roles in different agricultural systems, delivering a range of functions to reduce threats and enhance landscape resiliency (Table 1).

Habitat fragmentation will likely impede the ability of many species to respond, move, and/or adapt to climate-related impacts (4). Agroforestry practices can function as forest corridors or stepping stones in agricultural landscapes that enhance habitat connectivity. For example, in the intensely farmed Tensas River Basin, Louisiana black bears (Ursus americanus luteolus) use riparian forest buffers to travel between forest patches to forage and breed (5).

One of the unique benefits of agroforestry is the opportunity to provide mitigation benefits along with adaptation in an integrated and synergistic manner (6). Agroforestry practices can reduce net greenhouse gas budgets by sequestering carbon in soil and biomass, decreasing fossil fuel usage by reduced equipment runs in fields, enhancing energy conservation around farm buildings, and enhancing efficiency of nitrogen fertilizer use (7). Agroforestry may offer valuable mitigation benefits in return for the amount of area in the agroforestry practice. For instance, woody riparian and hedgerow habitats on a Central Valley farm in California stored 18% of the farmscape's total carbon (C), despite occupying only 6% of the total area (8).

Likely Changes

Changes in climate patterns and weather variability pose substantial hazards for current U.S. agricultural systems (9, 10). Table 2 describes anticipated changes in different regions of the country with implications for crops and livestock raised in agroforestry systems (11). For example, significantly more hot spells may cause a rancher to add a few silvopasture paddocks to their grazing rotation. Similarly, fewer freeze days may cause a producer using apple trees in their agroforestry system to select different varieties.

Table 2. Observed and expected changes in weather by U.S. Region. Expected changes are the A2 scenario at mid-century (2041–2070 average). Modified from (11).

Options for Management

For farmers and ranchers, agroforestry practices provide perennial living structure that can offer adaptation strategies for different climate change risks depending on the specific practice (Table 3). While the mitigation services of agroforestry are valuable, they are rarely the driver of design and implementation by producers. Because most farmers and managers do not make management decisions primarily to mitigate climate change, hence these options for management are focused on adaptation.

Table 3. Climate change risks and associated adaptation services provided by agroforestry practices. Source (12).

Intense Precipitation and Flooding

Agroforestry can help landowners adapt to increasing intense precipitation events and lessen the negative impacts by expanding forest and tree cover that intercept rainfall, increase the amount of rain that infiltrates into the ground, and reduce the quantity, speed, and peak flows of runoff (2, 13). When compared with no till, cover crops, crop and livestock integration, and crop rotation, adding perennials increased water infiltration the most compared with annual crops (an increase of 59%) (14). In Missouri, fields planted with agroforestry contour buffers had a 28 to 30% reduction in annual soil loss (15).

Heat and Drought

Increasing humidity and higher day and nighttime temperatures pose a significant challenge for livestock and crop production systems (9, 10). Silvopasture and windbreaks can mitigate these challenges by reducing animal heat stress and maintaining quantity and quality of forage production under increasing temperatures (2). For instance, cattle provided with shade reached their target body weight 20 days earlier than those without shade (16). In cropping systems, agroforestry practices can reduce agricultural demand for water during dry periods. Windbreaks reduce crop water stress and conserve soil moisture by reducing windspeed across adjacent crop fields, thereby reducing evaporation losses from soil and transpiration by crops (17) and may be particularly valuable under water-stressed conditions (18).

Longer, hotter drought periods will reduce soil cover by crop and pasture vegetation and leave the soil increasingly vulnerable to accelerated erosion (8). Permanent vegetative cover in the form of drought-hardy agroforestry practices can maintain soil protection by complementing annual vegetation cover practices like cover crops which may face establishment challenges in times of drought (2, 19). Windbreaks, through a variety of physical processes, can reduce wind speed over the land surface and thereby reduce the mobilization and transport of dust and associated particulates (20, 21). The zone of reduced wind speed typically extends a distance equivalent to 10 to 20 tree heights downwind of a windbreak, so in large agricultural landscapes multiple barriers are often required to effectively control wind erosion (21).

In areas where rising stream temperatures may affect cold-water aquatic habitat, shade from riparian forest buffers can help maintain cooler water temperatures (22). Riparian forest buffers can maintain lower maximum summer stream temperatures by 3.3 °C compared with streams without buffers and lower summer mean stream temperatures by 0.6 °C based on a meta-analysis of 10 studies (23).

Winter Storms and Cold Temperatures

The consequences of extreme winter events can create challenges to producers, exemplified by the October 2013 storm that resulted in the deaths of more than 45,000 livestock in South Dakota (24). Reducing wind speed in the winter by using agroforestry practices, particularly windbreaks, can also lower livestock stress, improve feeding efficiency, and enhance survival during lambing and or calving season (2). Field windbreaks can help capture the moisture available in snow by slowing the wind and distributing the snow across the field. Snow capture and protection of crop plants from winter desiccation by windbreaks can increase wheat yields by 15% to 20% (25, 26).

Diversification

Agroforestry systems are multispecies mixes of perennials and annuals that are inherently more resilient to environmental stresses than annual-only cropping systems (27, 28). They have a higher degree of species diversity, larger root systems that hedge against climate extremes, and the ability to tolerate increased disturbance (Figure 2) (29). Where winter chilling requirements do not become limiting, food-producing perennial crops such as fruits, nuts, and berries can provide resilience to climate extremes. Species mixes spread biological and financial risks across crops and seasons. By providing continuous living cover, perennial crops may offer more resilient and sustainable options on marginal lands than annual crops that often require more soil disturbances (30).

Figure 2. The willows in this alley cropping system provided a harvestable product during an extremely wet year when the annual crop was lost due to flooding. Credit: Josh Gambel

Plant Adaptation

Successful agroforestry practices depend on the ability of the trees and shrubs to survive and grow well enough to provide the benefits sought. Local climate is a major factor in determining which woody species can be effective in an agroforestry practice, and a great deal of variability in this regard exists among different species. As climate changes, local climate can shift beyond tolerable thresholds for some species or varieties and move to within tolerable thresholds for other species or varieties. In a changing climate, agroforestry species must be able to survive and grow under both current and future climatic conditions. If species that are currently used cannot adapt to future climate, risk is high that the desired functional lifespan of the agroforestry practice will not be achieved (31).

When selecting plants for an agroforestry application, consider the multiple points of climate vulnerability: winter chilling requirements, springtime freeze risk, heat and water stress, pollination constraints, and disease and pest damage (32). Tools that model future local climate shifts can be useful when considering the suitability of species or varieties based on anticipated changes in temperature, precipitation and extreme weather. Reviewing plant material lists in nearby ecoregions that have a current climate similar to the predicated future climate of your planting area can be one way to identify species that may be potentially suitable for the changing conditions. Another effective adaptation strategy is simply increasing the plant diversity within a planting, which broadens the mix of genetic, phenological, and biophysical attributes (33).