Slash and Burn Agriculture

There is nothing scarier for a rainforest lover than the sound of axes. Imagine you are exploring what you think is a trackless Amazonian wilderness. The forest seems like human hands have never touched it; the most incredible treasure trove of biodiversity on the planet and the Earth's lungs...superlatives abound.

And then you reach a clearing. Smoldering piles of vegetation are all about, the ground is covered in ash, and a lone tree is still standing, having been girdled, its bark removed, to kill it. Now that this 150-foot giant is dead, some men are hacking at it. Finally, it topples into the wound that has been opened in the forest. It's planting time!

Read on to find out that there is much more going on in this slash and burn example than meets the eye. You see, this was not the first time this "garden" (as the local people call it) was farmed.

Slash and Burn Agriculture Definition

Slash-and-burn agriculture is also known as swidden agriculture, forest-fallow agriculture, or simply forest fallow.

Slash-and-burn agriculture is one of the world's oldest agricultural techniques. Since humans learned to use fire over 100,000 years ago, people have burned vegetation for various purposes. Eventually, with the advent of plant domestication and before the invention of the plow, the most labor-efficient means of growing food in large areas was slash-and-burn.

Today, up to 500 million people practice this ancient form of agriculture, mostly for subsistence purposes and selling in local markets. Though the smoke and the forest destruction associated with slash-and-burn cause it to be much maligned, it is actually a highly complex and efficient form of food production.

Effects of Slash and Burn Agriculture

The effects of slash-and-burn depend directly on the factors below, so let's explore them.

Fallow Systems

Farmers have known for millennia that ash is nutrient-rich. Along a river like the Nile, the annual floods kept the soil fertile, but on rocky hillsides and even in lush tropical forests, wherever ash could be obtained from vegetation, it was discovered that crops grew well in it. After the harvest, the field was left fallow for a season or more.

"Or more": farmers recognized that, depending on the factors below, it was useful to let vegetation grow as long as possible until the land was needed again. More vegetation => more ash => more nutrients =>higher production => more food. This resulted in fallow plots of various ages across an agricultural landscape, ranging from this year's fields to fields growing up into forest "gardens" (which look like messy orchards), the result of planting various useful trees from seed or seedling the first year, along with grains, legumes, tubers, and other annuals. From the air, such a system looks like a patchwork quilt of fields, brush, orchards, and older forest. Every part of it is productive for local people.

Fig. 1 - A fallow area of brush has been slashed and is being prepared for burning in 1940s Indonesia

Short-fallow systems are those where a given area is slashed and burned every few years. Long-fallow systems, often called forest fallow, may go decades without being cut down again. As practiced in a landscape, the entire system is said to be in rotation and is a type of extensive agriculture.

Physical Geography

Whether or not a given area is slashed and burned and put into fallow rotation depends on certain geographical factors.

If the area is bottomland (flat and near a watercourse), the soil is probably fertile enough to be farmed intensively with a plow every year or two—no slash-and-burn necessary.

If the land is on a slope, particularly if it is rocky and can't be terraced or otherwise made accessible to plows or irrigation, the most effective way to produce food on it may be slash-and-burn.

Suppose the land is under a temperate forest, as in the eastern US prior to the 1800s. In that case, the first time it is farmed may be slash-and-burn, but after that, it may be necessary to farm it using intensive techniques with little to no fallow, plowing, and so forth.

If it is under tropical rainforest, most of the nutrients are in the vegetation, not the soil (tropical forest has no dormant period during the year, so nutrients are constantly cycling through the vegetation, not stored in the ground). In this case, unless a large labor pool is available for intensive methods, the only way to farm may be by slash-and-burn.

Demographic Factors

Long-fallow systems are ideal for extensive areas of forest or scrubland inhabited by small groups of semi-nomadic people who can move between fallow plots across their entire territory. A given plot farmed by an ethnic group comprising a few thousand people may not be touched more than once every 70 years. But the group's territory might need to be thousands of square miles in extent.

As populations increase, the length of time in fallow decreases. Forest can no longer grow tall or at all. Eventually, either intensification takes place (the shift to methods that produce more food in less space), or people have to leave the area because the fallow period is too short, meaning that there is too little ash to produce nutrients for crops.

Socioeconomic Factors

These days, rural poverty often is connected to slash-and-burn because there is no need for expensive machines or even draft animals, and it is highly labor efficient.

It is also associated with economic marginalization because the most productive lands in a region are often occupied by commercial ventures or the most prosperous local farmers. People with capital can afford labor, machines, fuel, and so forth, and so can increase their production to keep profits up. If slash-and-burn farmers inhabit such areas, they are pushed off the land into less desirable areas or leave for the cities.

Advantages of Slash and Burn Agriculture

Slash-and-burn has many advantages for farmers and the environment, depending on where it is practiced and how long the fallow period is. The typically small patches created by single families mimic the dynamics of forests, where treefalls happen naturally and open gaps in the forest.

As mentioned above, only rudimentary tools are necessary, and in new slash areas, even pests that afflict crops may not yet be a factor. In addition, burning is a cost-effective way of removing whatever pests may be present at the onset of the planting season.

In addition to producing bounteous crops of grains, tubers, and vegetables, the true advantage of a long-fallow system is that it allows a forest garden/orchard to be created, wherein natural species re-invade the space and mix with perennials planted by people. To the untrained eye, they may look like "jungle," but they are, in reality, complex forest-fallow cropping systems, the "gardens" of our introduction above.

Negative Effects of Slash and Burn Agriculture

The main scourges of slash-and-burn are habitat destruction, erosion, smoke, rapidly falling productivity, and increasing pests in short-fallow systems.

Habitat Destruction

This is permanently damaging if vegetation is removed quicker than it can recover (on a landscape scale). While cattle and plantations are probably more destructive in the long run, the simple fact of increasing human populations and diminishing length of fallow means that slash-and-burn is unsustainable.

Erosion

Much slash-and-burn happens on steep slopes just before the rainy season, when planting happens. Whatever soil exists is often washed away, and slope failure can also occur.

Smoke

Smoke from millions of fires obscures much of the tropics every year. Airports in major cities often have to close, and significant respiratory problems result. Though this is not from slash-and-burn alone, it is an important contributor to some of the worst air pollution on the planet.

Fig. 2 - Satellite image of smoke plumes from slash-and-burn plots created by Indigenous people still using long-fallow rotation along the Xingu River in the Amazon Basin, Brazil

Plummeting Soil Fertility and Increasing Pests

Plots that don't lay fallow long enough don't produce enough ash, and dropping soil fertility from ash necessitates using costly chemical fertilizers. Also, crop pests eventually show up to stay. Almost all slash-and-burn plots now in the world must be heavily fertilized and sprayed with agrochemicals, causing many human health and environmental problems from runoff and absorption through the skin, among other things.

Alternatives to Slash and Burn Agriculture

As the intensification of land uses occurs in an area, sustainability is necessary, and old slash-and-burn techniques are abandoned. The same land needs to be able to produce every year or two for the people who are farming it. This means crops must yield more, be pest resistant, and so forth.

Soil conservation is a must, particularly on steep slopes. There are many ways to do this, including terracing and living and dead vegetation barriers. The soil itself can be fertilized naturally using compost. Some trees need to be left to regrow. Natural pollinators can be brought in.

The alternative is often wholesale abandonment or conversion to another use, such as cattle ranching, coffee or tea plantations, fruit plantations, and so forth. One best-case scenario is the return of the land to forest and protection within a national park.

Slash and Burn Agriculture Example

The milpa is a classic slash-and-burn agricultural system found in Mexico and Central America. It refers to a single plot in a given year and to the fallow process whereby that plot turns into a forest garden, then is slashed, burned, and replanted at some point.

Fig. 3 - A milpa in Central America, with corn, bananas, and various trees

Today, not all milpas are in slash-and-burn rotation, but they are based on fallow systems that evolved over thousands of years. Their principal component is corn (maize), domesticated in Mexico over 9,000 years ago. This is usually accompanied by one or more types of beans and squashes. Beyond this, a typical milpa may contain fifty or more varieties of useful plants, both domesticated and wild, which are protected for food, medicine, dye, animal feed, and other uses. Every year, the composition of the milpa changes as new plants are added, and the forest grows up.

In Indigenous Maya cultures of Guatemala and Mexico, the milpa has many sacred components. People are seen as the "children" of maize, and most plants are understood to have souls and to be related to various mythic deities that influence human affairs, weather, and other aspects of the world. The result of this is that milpas are more than sustainable food production systems; they are also sacred landscapes that are critically important for maintaining the cultural identity of Indigenous people.