The remaining 85 to 90 percent are caused by humans, and slash-and-burn agriculture is one of the major reasons. Slash and burn is the practice of cutting down, burning off, and replacing existing forestland with crops. This method is also used as a forest fire mitigation technique. But does it work?

wildfire view from space

What is it?

Slash-and-burn agriculture, also called fire-fallow cultivation, starts by cutting down and burning plants in a forest or woodland to create a swidden. It's increasingly being used as a way to grow food. Wild or forested land is clear-cut and any remaining vegetation is burned away.

The resulting ash layer provides the newly cleared land with a layer of nutrients that helps fertilize the crops. However, the land is only fertile for a couple of years before those nutrients are depleted. Farmers must then abandon that land and move to a new plot, destroying more of the forest.

Slash and burn has been used in parts of the world for thousands of years. But as a burgeoning world population struggles to survive on the earth's dwindling natural resources, the impact of this type of farming is becoming unsustainable.

smoke from wildfires over the ocean

(Smoke from Pacific Coast wildfires, modified Copernicus Sentinel data 2020, CC BY-SA 3.0 IGO <https://creativecommons.org/licenses/by-sa/3.0/igo/deed.en>, via Wikimedia Commons)

Numerous problems with the method

Historically, problems with slash and burn agriculture have included deforestation, loss of habitat and species, increased air pollution, additional carbon in the atmosphere contributing to climate change, and an increase in exceptionally large unintentional fires.

The practice has also resulted in pronounced soil erosion accompanied by landslides, dust storms, and contaminated water. Without viable root systems to anchor soil, it washes away during heavy rains and blows away during droughts.

roof of a burning building

wildfire in Napa Valley

(2020 - top: Fairwinds Estate Winery, Napa Valley; bottom: Orange County, CA)

What's happening?

Every year another disastrous wildfire season seems to occur in the American West. In 2019, nearly 9 million acres were burned in the US alone. Uncontrolled fires, often accidentally ignited by people, decimate forests. While the amount of destruction last year was down significantly, many experts say that trend won't last.

Prominent political leaders such as California Governor Gavin Newsom and President Donald J. Trump have expressed very different opinions about the primary causes of these fires and how best to deal with them. However, there's no denying a problem exists.

Governor Newsom speaking with press

(California Governor Gavin Newsom talks to the media after touring the North Complex Fire zone on Friday, Sept. 11, 2020; photo by Paul Kitagaki Jr. via The Sacramento Bee)

Controlled burns

Most of us would agree that a forest fire is a disaster. However, some kinds of forest fires actually benefit the environment.

Partly because they can help stop an out-of-control wildfire, well-planned and well-managed controlled burns can be extremely beneficial for forest management. A technique called backburning involves setting a controlled fire in the path of an approaching wildfire. The flammable material is burned up and extinguished. With no fuel left to keep it going, the wildfire dies out.

firefighters starting backfires

Forest management

Throughout history, natural low-intensity wildfires occurred every few years. These fires burned up plant debris and dead trees, making way for young healthy trees and vegetation. The new growth, in turn, supported wildlife. Forest managers are now replicating this natural strategy when appropriate by starting manageable, slow-burning fires to make room for new growth that will help keep the forest healthy in the long run.

Good or bad?

This doesn’t mean all intentional wildfires are good. Many slash-and-burn fires used to clear land for agriculture are misguided at best and catastrophic at worst. Yet, these fires are being set every day to destroy large sections of forests. Obviously, they don’t only remove trees, they kill and displace wildlife, disrupt water cycles, destroy soil fertility, and endanger the lives and livelihoods of local communities.

Out of control

They can also rage out of control. In 1997, fires intentionally set to clear forests in Indonesia escalated into one of the largest wildfires in recorded history. Millions of acres burned, more than 100,000 people died, already threatened species like orangutans perished by the hundreds, and a haze of smoke and ash hung over Southeast Asia for months, reducing visibility and causing acute health problems.

All fire is risky. In order to minimize that risk as much as possible, controlled burns must be well-planned and ignited and maintained by trained professionals.

president trump and governor newsom inspecting burned areas

(President Donald Trump, second from right, reacts while taking in the damage from the Woolsey Fire alongside California Governor-elect Gavin Newsom, from left, Los Angeles County Fire Chief Daryl Osby, Cal Fire Deputy Chief Nick Schuler and California Governor Jerry Brown in Malibu, Calif., on Saturday, Nov. 17, 2018. Photo: Genaro Molina / Associated Press)

Best conditions for slash and burn

Slash-and-burn agriculture works best when a farmer has plenty of land he or she can afford to let lie fallow. It also works best when crops are rotated to help replenish soil nutrients. It's most successful in societies where people maintain a broad diversity of food sourcing, such as hunting, fishing, and gathering wild foods.

Environmental effects

Since the 1970s, swidden agriculture has been described as both a bad practice resulting in the progressive destruction of forests and an excellent practice to preserve and guard them.

A study of swidden agriculture conducted in Indonesia documented historical attitudes towards slash and burn and tested the resulting assumptions based on more than a century of slash-and-burn agriculture. They found that swidden agriculture can add to deforestation in regions if the maturity age of the trees removed is much longer than the fallow period. For example, if a swidden rotation is between 5-8 years and the trees have a 200-700 year cultivation cycle, slash and burn becomes one of several factors resulting in severe deforestation.

Slash and burn is a useful technique in some environments, but definitely not all.

people replanting burned forest

(mattmangum, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons)

Even the Arctic is burning

Forests around the world are burning due to slash-and-burn deforestation. Remote areas are allowed to burn indefinitely because they're difficult to access and pose very little threat. However, these fires are especially dangerous in the Arctic, a region that holds some of the biggest carbon stores anywhere on the planet. Burning those stores releases tremendous amounts of greenhouse gases and particles into the atmosphere, affecting the entire planet.

forest restoration in Iceland

(Above: Adalsteinn Sigurgeirsson (R), and Hreinn Oskarsson working on a reforestation project; Icelandic Forest Service photo)

Sustainable and renewable methods

Alternative sustainable methods do exist. These include alley-cropping, an agroforestry technique in which people plant food crops alongside trees. Planting a variety of food crops, creating buffer zones of native trees around existing forestland, and reclaiming degraded land through reforestation and other practices have all proven to work well.

(Alley cropping by Naresh Tevathasan, University of Guelph, Canada National Agroforestry Center, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons)