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Why did slash-and-burn farming limit population density in many forest regions?

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Map of South America and the Caribbean showing the distribution of Aboriginal cultural groups, with regions color-coded by cultural type including Andean, Central American, tropical forest, and various nomadic cultures such as aquatic nomads, forest hunters and gatherers, and shellfish gatherers. Major group names are labeled across the continent and surrounding islands. — South American forest Indian Distribution of aboriginal South American and circum-Caribbean cultural groups.

Slash-and-burn agriculture, also known as shifting agriculture, historically limited population density in tropical forest regions primarily due to the inherent limitations of the soil and the necessity of fallow periods.

In this system, indigenous farmers clear small patches of forest and burn the vegetation to enrich the soil with ash. However, tropical soils are typically infertile and lose their productivity after only a few years of cultivation. Once the soil is exhausted or overrun by weeds, the plot must be abandoned to allow the forest to regenerate, a process that can take about a decade before the site can be reused.123

Because this method requires large areas of land to remain fallow at any given time to maintain sustainability, it cannot support high population densities in a fixed location. Unlike intensive agricultural systems, traditional slash-and-burn agriculture relies on the community moving to new, isolated parts of the forest to allow previously used land to recover, which inherently restricts the number of people a specific territory can sustain.23