Perennial polycultures for sustainable agriculture: Trees, shrubs, and herbs, oh my!

Reference: Kreitzman, M., Eyster, H., Mitchell, M., Czajewska, A., Keeley, K., Smukler, S., … & Chan, K. M. (2022). Woody perennial polycultures in the US Midwest enhance biodiversity and ecosystem functions. Ecosphere13(1), e03890.

Agricultural transitions: from annual monocrops to perennial polycrops

If you’ve ever driven through the rural midwestern United States in early spring, you might have noticed a lot of bare farms fields, with soil left exposed as the last of the winter snow melts. If you drive through the same area in mid-summer, you’ll likely see most of those fields planted with either corn or soybeans, creating a landscape with extremely low plant diversity. These two features – bare fields during colder months, and low plant diversity during the growing season – characterize the conventional agricultural system used on the vast majority of farmland worldwide. These agricultural ecosystems are designed to support only one function: crop yields. Unfortunately, this often comes at the expense of other important ecosystem functions, like maintaining healthy soils and providing habitat for beneficial insects, birds, and other organisms. The good news is that we can make agricultural ecosystems more sustainable and multifunctional by transitioning to systems that instead keep soils covered with living plants year-round, and have higher plant diversity.

Perennial polycultures are one way to achieve both of those goals. Compared to annual plants, which die at the end of the growing season each year (think corn, soy, and other common crops), perennial plants are long-lived and can survive for multiple, or many, years. Many natural ecosystems are composed of perennial plants, like forests and prairies. While the fields of corn and soy you often see in rural areas are composed of only one crop (monoculture), farmers can increase plant diversity by planting multiple species at the same time (polyculture). When perennial plants are planted in polycultures, they both keep the soil covered year-round, and increase plant diversity on farms. To help facilitate transitions to these types of systems, researchers from the University of British Columbia and the University of Wisconsin conducted a study to quantify environmental benefits from the systems, as well as determine if they pose any trade-offs, such as decreased yield.

Perennial polycultures combine multiple types of plant species that use space more efficiently than annual monocrop systems, and keep the ground covered year-round.

Cropping system comparisons

The research team, led by Dr. Kreitzman, visited 14 farms across the Midwest. On each farm, they measured ecosystem functions in a perennial polyculture field and a neighboring conventional field. In this way, they were able to compare the benefits of perennial polyculture systems to those of conventional systems. The perennial polycultures contained trees, shrubs, and an herbaceous ground layer (e.g., grasses or clovers), while the conventional fields were either corn, soy, or hay. The team measured multiple biodiversity and ecosystem function indicators, including soil health (e.g., compaction, fertility, carbon storage); erosion control; pest control; pollination; the diversity and abundance of soil microorganisms, plants, arthropods, and birds; and crop yields.

This map from the research article shows each of the fourteen farms that were included in the study.

Perennial polycultures take home the gold – well, almost

After comparing ecosystem functions in each type of system, the perennial polyculture systems were the undeniable winner in terms of the number and types of benefits they provided. Compared to the conventional systems, the perennial polycultures had more diverse plant, bird, invertebrate, and soil microbial communities, suggesting that they can help conserve and support biodiversity. They also had healthier soils, with less compaction and erosion, more nutrients, and greater carbon storage, making the soils more resilient to extreme weather and helping to reduce the amount of CO2 in the atmosphere. On top of all that, the perennial polycultures also had better natural pest control, reducing the need for harmful chemical pesticides.

The only drawback the researchers found was that the perennial polycultures had consistently lower crop yields than the conventional systems. However, this could be due to a number of reasons. The perennial polycultures included in this study were relatively young, meaning that most of the plant species hadn’t fully matured yet. It isn’t until those plants are fully mature that they begin producing higher yields of fruits and nuts, which means that as those systems get older, the yields will likely continue to increase. Perennial polyculture systems are also a fairly new practice in the U.S. Midwest compared to the conventional systems that have been optimized for maximum yields over many decades. If greater attention and research is put toward identifying strategies for increasing yields in perennial polyculture systems, that could help level out the playing field. On the bright side, the crops from the perennial polycultures were more nutritious than those from the conventional systems. While creating a more sustainable food system may seem a daunting task, this study demonstrates that perennial polycultures may be an important piece of the puzzle. 

Perennial polycultures are already common in some parts of the world, like in the tropics where this coffee farm is located. If perennial polycultures are used more in the U.S., they could produce a wide variety of nutritious crops, like fruits, vegetables, and nuts. Source: USDA

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