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In many areas of South America, Africa, India and Southeast Asia, "slash-and-burn" agriculture is employed whereby every year the stubble from the previous year is burned along with other live cuttings.

In North America, this does not seem to have ever happened. It is true that in New England (where I am from) the old-time farmers cut down all the trees and burned them to get pot ash, but this seems to have been a one time event. Once land was cleared of trees, it was not burned over and over again. Also, in many cases land was cleared without burning at all, the trees and stumps simply being hauled away.

Is there some difference in climate that results in tropical areas being subjected to repeated annual burnings whereas lands in North America are not? What causes the difference?

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  • $\begingroup$ In California's Sacramento Valley, it used to be (1970s) common practice to burn off the stubble from rice fields (and perhaps other crops) after harvest. Could be true of other areas as well. $\endgroup$ – jamesqf Nov 18 '16 at 18:27
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Dry season

It is easy to burn things in the dry season, not so much where it is wet. Eastern North America generally qualifies as wet, certainly in the winter, less so in the summer. This isn't to say that you CAN'T do slash and burn where it is wet, since it is done in rainy Papua New Guinea an the Amazon, but it makes more sense as a strategy when there is a clear rainy and dry season. A forest or scrubland area will be cut in the wet season, allowed to dry in the dry season, and then burned, returning valuable nutrients to the soil. The soil can then be farmed for several years without needing further nutrients. The burning during dry is key, because it allows the burning to be done between two growing seasons (the wet season). In Eastern North America, if you tried to burning during the dry season (summer), you would lose that growing season, and have to find another source of food that year.

The other side to the dry season is the wet season. Seasonal climates will almost all see much more rain in at least one month than anywhere in the eastern US, which has remarkably stable precipitation all year. Therefore, the washing of nutrients out of the soil happens more quickly in the tropics.

Soil quality

Many places where slash and burn agriculture are prevalent had poor soil quality. Tropical environments tend to have more leached soils due to heavy seasonal rains (or heavy year-round rains). That means that burning is an effective way to add soil nutrients. Areas brought under cultivation from forest-land in Northern Europe have remained productive under cultivation for at least 2000 years (in the case of northern France and England) due to the fertile soil layer. I suspect Eastern American soils have similar vitality.

The rainforest, on the other hand, retains almost no minerals and can be exhausted quickly. The only way to effectively farm is to cut down and burn the minerals in the living plants and trees to return them to the soil.

Leguminious plants

This is a little more theory than fact, but in the tropics, there are many leguminous trees, while in the northern latitudes there are many leguminous ground plants. African savannah-forests are dominated by acacia, mopane, and miombo. These are all leguminous trees that add nitrogen to the soil. No comparable array of dominant tree species are available in the northern latitudes. In America and Europe, soil fertility was traditionally restored with vetch, clover, alfalfa, or (nowadays) soybeans. Those are single season plants; in the case of clover and vetch the can be grown as a meadow in a year or two of fallow. The trees in the tropics, on the other hand, need a longer time to establish. Therefore, it makes relatively more sense to do a yearly fallow rotation in the northern latitudes, and a slash and burn many-year rotation in the tropics.

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Slash and burn has been used in North America and still is at times. I see regular field burning near me where fields are particularly rocky and tilling if impractical for the crops grown. It also occurs when there is intrusions of some noxious weeds that are controllable through fire as effectively and much cheaper than chemical controls.

In general, I would consider slash and burn in disfavor because of it consumptive nature. The practice does provide some otherwise missing nutrients, but it consumes large quantities of biomass while doing so. More favored in many regions it to use that biomass to supplement the soil by tilling it in and allowing it to aerate the soil while slowly breaking down and augmenting the soil. Between being fortunate enough to have good soil to begin with, or the ability to augment with either natural or chemical supplements, many North American farms do not need the quick fix burning can supply so opt for the slower, longer term solution of turning in the organic materials.

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The term "slash and burn" is generally not used to describe the field burning that is currently done in North America. Burning fields is illegal in some states. It is allowed in other states, but is usually regulated under a Smoke Management Plan. Smoke exposure is a serious health concern and particulate matter pollution is regulated under the Clean Air Act in the USA.

Many states only allow field burning at certaim times, based on atmospheric ventilation and current fire potential on the natural landscape.

  • If there is an atmospheric temperature inversion near the surface, or stagnant air characterized by no winds, then burning is not permitted.
  • If the natural landscape is covered with dry fuels, burning will not be permitted so as to reduce the risk of wildfire.

Another common practice is for government owned lands to have prescribed fire put to the ground, which simulates natural conditions and removes invasive species (e.g. on rangelands).

When field burning or broadcast prescribed burns are not possible, pile burns are conducted (e.g. piles from timber slash, orchard tear-outs, mixed brush, etc.). Pile burns ensure that the fire does not go wild and that there is enough heat flux to allow the smoke plume to break through a temperature inversion.

So, while North America does not typically employ "slash and burn" nomenclature, there is certainly a lot of slash being burned.

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