The Power of Perennials

By managing photosynthesis, disturbance, and canopy structure, farmers can create regenerative ecosystems that capture more energy than traditional 2D cropland

As farmers and ranchers, we’re in the solar energy business.

The sun shines on the green leaves and stems of our crop plants and is the driver of photosynthesis. During photosynthesis, plants take in carbon dioxide from the air, and the energy from photosynthesis is used to connect carbon, hydrogen and oxygen and turn it into simple sugars. Simple sugars such as glucose are then reassembled to make more and more complex molecules. The product of these processes, whether in the form of a grain or a leaf or a vegetable, as well as the minerals contained in them, are the crop yields that we depend on for our livelihood. At the end of the day, a farmer or rancher’s yield and income are directly related to how much photosynthesis happens on our site.

Not all energy from photosynthesis turns into a marketable product, however. Some of the energy is used by the plant to make more complex molecules such as starches, oils, flavors, aromas, and colors, and for the general metabolism of the plant.

An eco-farm doesn’t run on synthetic fertilizer; it runs on the energy and nutrients from photosynthetically fixed carbon, cycling through life, death, and decay. The greater the photosynthesis, the more potential energy we have. The growth and metabolism of decomposer organisms—the lifeblood of our soil—is fueled by the energy stored in organic matter: the Net Photosynthetic Productivity (NPP).

What Is NPP?

Savannas capture a full 30 per cent more energy per acre than on cropland or pasture.

NPP represents the energy plants capture from sunlight through photosynthesis, minus what they use during respiration. This is stored in plant biomass first. Some is added to the soil in the form of root exudates, and some is consumed by graziers or decomposers and returned to the soil. NPP is equivalent to our energy “net margin.” Net margin, or net profit margin, measures the percentage of revenue remaining as profit after deducting all operating expenses.

The annual rate of biomass accumulation in live plants, live animals and soil organic matter in a particular land area is termed the Net Ecosystem Production (NEP). It’s the balance of how much carbon is stored versus how much is used in metabolism, lost through volatilization, or harvested.

When NEP is positive, carbon is building up. The ecosystem is storing more energy than it uses. But when it turns negative? That’s a flashing red light. A negative NEP means the system is breaking down faster than it’s growing — a telltale sign of degradation. This is the classic divide between agriculture that is regenerative versus that which is degenerative.

If you are adding more to your “ecosystem savings account,” you’re on the right path.

Disturbance: A Double-Edged Sword

Nature is no stranger to disturbances. Fires, storms, and grazing animals all play their role. Managed well, they create opportunities for regeneration. When mismanaged, though, they send NEP into the red. Clear-cuts, overgrazing, and monocultures strip ecosystems of their resilience. The amount of live biomass shrinks, organic matter decomposes faster than it’s replenished, and carbon escapes into the atmosphere.

When disturbance outpaces recovery, soil fertility declines. You’re no longer growing the carbon that fuels the decomposers. Without the decomposers, plant nutrients remain locked away in organic matter, unavailable to plants, and crop yields decline. Well-timed disturbance of the appropriate kind (mowing, tillage, grazing) is what we do to feed the decay cycle and strategically release nutrients for our crops and for building soil.

Since we’re in the business of capturing as much sunlight as possible and turning it into harvestable biomass, it is in our best interests to create a system that maximizes photosynthetic surface area and accumulates the most biomass. The “devil in the details” is that we have to figure out how to convert NPP into dollars!

The image accompanying this article is a list of various terrestrial biomes and the average NPP for those systems. Notice that cultivated land has an NPP of around 600 grams of photosynthate produced on every square meter of land. If you were to switch from growing annual grains to growing a perennial grassland harvested by graziers, you really wouldn’t be gaining any net photosynthesis. The NPP of grasslands is roughly the same as annual crops, but you should be better off financially because grazing doesn’t require equipment for tillage, cultivation and harvest. Custom-hire someone once to seed your pasture, then mow and fertilize with cattle.

Now take a look at the NPP for a savanna. Savannas include trees, shrubs, vines, and canes at various stages of growth within a matrix of grassland, with approximately 30-60 percent woody canopy closure. The grass grows well; animals have protection from the sun, wind, and rain; and the system captures approximately 900 grams of photosynthate per square meter. I.e., there’s a full 30 percent more energy capture per acre in a savanna system than on cropland. If slightly lower yields occur in our alley crop or pasture (research at the University of Missouri shows otherwise) we can afford it because we’re capturing 30 percent more energy.

As the trees and shrubs in a savanna grow, they develop a deeper, more convoluted canopy structure. This increases the photosynthetic surface area of the system, which increases until right before the canopy closes. Once the canopy closes, grasses will die out. Total NPP is greater (nearly twice that of an annual crop field, at 1,200 grams per square meter), but we’ll be in a forest, and the diversity of opportunities declines. You can do just about everything in a savanna that you could do in a forest, but you can’t do everything in a forest that you could in a savanna.

Remember: net canopy photosynthesis—and hence the amount of photosynthate available for growth, maintenance and reproduction—is a product of total canopy leaf area. Leaf area just prior to canopy closure sets the upper limit on canopy photosynthesis and therefore sets the limit on the amount of “gross photosynthetic income” that is available.

In an agroforestry system, whether we’re growing crops or grazing animals, our job becomes managing light and shade. We want to keep our system in the optimum range where there is enough light for productive crops and pasture, yet enough shade to get the benefits of cooling, wind protection, ample decay products and extra yields. Whenever the canopy is about to close and grass growth slows down, it’s time to harvest some trees (as saw logs, pulp, firewood, branches for animal feed, or mushrooms). Once the canopy is opened up, we can add more or different woody species to the system.

Once our system has reached this stage, it can be managed in perpetuity as a strategically disturbed ecosystem that harvests twice the sunlight as an annual crop field and that cycles nutrients in a guided way. We have become participants in the dance of life itself.