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Polyculture: The Benefits of Diversity for Climate Change

For further reading on the scientific underpinnings of this article, see the 2018 PNAS article “Plant diversity enhances productivity and soil carbon storage.” as well as this article, which mentions that polyculture systems can enhance carbon storage by 32-315% above monoculture systems.

Most of us grow our food using what is called monoculture. We plant one crop by itself, another crop somewhere else, and so on until the result are single stands of crops. Polyculture, on the other hand, achieves higher yields with less water and pesticide use in smaller spaces. For example, Native Americans planted maize, beans, and squash together because each of these “three sisters” had properties that enhanced the growth of the other two.

This is an image of a 1-acre polyculture forest, our first ever done. This plot of land in Durham, NC was entirely turf grass except for a single small tree in clay soil. Ticks were abundant on the property and wildlife was limited. Wildflowers were planted in the first year followed by an array of tree species year after year. Soil carbon was amended in some places by the addition of tree mulch. The image represents growth after 6 years, representing at least a 10x increase in plant and animal diversity. Ticks were replaced by spiders and a multitude of other insects, the soil was rich and dark, and several new species utilized the land including a resident groundhog.

Where there is physical structure in the landscape, there are opportunities to increase species diversity. The greater the diversity, the more likely a species will be able to provide a necessary ecosystem service. Some shade-loving plants, for example, can only exist in the understory of larger trees that screen out some of the sunlight. Shade also promoted the survival of more insects which were food for spiders and birds, and which outcompeted the previously abundant ticks. Shade would not have been possible without the growth of tall structures (trees).

With the benefits of polyculture known, why then do we insist on planting crops in monocultures? We are spending more money applying more fertilizer and pesticides per acre than we need. The only time we approach polyculture is in greenhouses, where the density of different plant species is extremely high. Out in gardens, we still plant individual plants in separate plantings, never together. Even indoor plants are kept in separate pots and never together.

The ecosystems of the world all exist as polycultures. Given the greater productivity of polyculltures for the least use of water, fertilizer, and pesticide, it stands to reason that greater productivity would translate into greater CO2 sequestration capability.

How We Determine Optimal Polycultures – Analysis and Experiment

We first must determine species composition and the total productivity of that combination of species. Next, we use an analysis method called ordination to determine which species are associated with higher productivity plots. Typically ordination is used to determine environmental factors responsible for a certain pattern of species distribution. We use it instead to determine species responsible for a certain pattern of productivity.

Variations on the polyculture are then tested to determine an optimal ratio of species. Going back to the three sisters example, perhaps it is best to have 3 squash, 2 beans and 2 maize in that ratio. When planting, that ratio will be sought, though for structural diversity those plants will be mixed together with random evenness instead of planted in isolated rows.

How Polyculture Can Help Us Take Action Against Climate Change

The increased productivity of polycultures represents an enhancement to the natural ability of ecosystems to sequester CO2. However, higher productivity does not translate into long-term CO2 storage unless it makes its way down into the roots into the soil as soil carbon. If the plants are eaten or used for fuel, this CO2 is re-emitted again. As long as it stays within the ground, we can count that as aiding the fight against climate change.

We have researched the use of fast-growing plants for carbon storage and are now looking at adding structure to a polyculture of plants, bacteria, and fungi to try sequestering more CO2 in the soil. Natural systems do an excellent job of taking up the majority of the world’s CO2. Enhancing the functioning of existing systems is much more cost-effective and quicker to store CO2 than building energy and emissions-intensive carbon sequestration projects from scratch.