Anthonares

[1491, Amazon, Archaeology, Charles Mann, Rainforest, Soil, Sustainability, Terra Preta]

Rarely does one see the words “Amazon” and “sustainable” in the same sentence, let alone the same title. To the modern mind, the Amazon is synonymous with two things: astounding ecological diversity and rapacious environmental degradation. As I’m learning from Charles Mann’s masterwork 1491, that view is not now as it was long ago.

But it wasn’t until just a few decades ago that archeologists began to set aside their views of the Amazon region as one peopled by primitives capable only of limited, unproductive slash-and-burn (or swidden) agriculture. Why the Amazon held on so long to its secrets is a complicated story darkened by the colonialist biases of past centuries, and frustrated by the apparent lack of written language amongst the peoples of that region. Nevertheless, our picture of those people and their history has begun to change. What we’re finding is that the people who once occupied the Amazon region, survived only in name by modern descendants, may have lessons for us today.

There is so much that can be said about the accomplishments of a people who carved out not only a subsistence but a flourishing civilization in an environment that today we deem virtually incapable of sustaining modern human existence. But because what I know of the subject comes from one page-turning chapter of 1491, I’ll just say one thing: Terra Preta de Indio.

Soil, the Bedrock of Human Life

We are trained throughout our lives to regard verdant forests and rich savannas as the ecological basis for the astounding richness of species and numbers in the natural world. But with one very important exception, it is the soils beneath (or more appropriately, at the base of those biomes) that nourishes those biomes.

Soil is not just a bunch of loose material that is somewhere between a mountain and ocean-bottom sediment in the rock cycle. Soil is a thin layer of heavily chemically altered material that can be almost entirely organic matter, can often store nutrients with remarkable efficiency, and is home to a bewildering variety of microbial life itself vital to higher order life forms. You may have heard the analogy that if the Earth were the size of a large globe, then the thin shell of atmosphere that enables life would be no thicker than a few coats of varnish. Well, here’s another one, if the Earth were the size of said globe, the soil that all life depends on would be no thicker than a few atomic layers of gold plating (I’ll take a gold-plated globe over a varnish one any day).

Soil, the Bane of Modern Amazonia

We all now know the catch-22 of the tropical world: that mighty sea of green rests upon a soil so poor that within a decade all cleared farmlands become useless. Not only that, but under the heat of the tropical sun, the soils are essentially baked into enormous brick pavements. The social forces that drove farmers into the forests become every year more urgent, and so each relocation means a larger clearing, and more destruction. (image credit: Andrew Zimmerman, University of Florida)

The jarring dichotomy of the world’s most lushest landscapes perched atop its most barren soil is for many impossible to swallow. But its true. Those ecosystems have evolved over untold millenia to convert the waste or windfall from one species into the nutrition for another. Trees have root systems capable of extracting virtually all of the minerals from the soil, and unlike most other biomes of the world, atmospheric inputs of nitrogen and phosphorous can become important (and they’re small, really small).

Small perturbations in this ecosystem are absorbed by natural negative feedback processes (the tendency of a process to return a system towards its natural state). Large perturbations cannot be absorbed readily, and can fester into positive destructive feedback. Large open clearings are pounded by meters of annual rain and quickly washed of all organic carbon and remaining soil nutrients. This miniature wet desert becomes a cancerous wound in the dry season as the break in the canopy desiccates the soils around it. This is where people step in. We clear large plots of land, those plots are soon discarded and abandoned, and left fallow they only further the rapid degradation of the forests.

Terra Preta de Indio

Finally, you thought I’d never get to the point, right? The catch-22 I mentioned led archaeologist Betty Meggers to posit her “law of environmental limitation of culture”:

The level to which a culture can develop is dependent upon the agricultural potentiality of the environment it occupies

This seemingly explained the available evidence in the 1950s, but pointedly disagreed with the accounts by early Spanish “explorers.” To rectify this situation, those early accounts were disregarded as either misguided, exaggerated (understandable given the difficulties of Amazonian ethnogrophy), or altogether false.

But, as Mann begins his chapter entitled “Amazonia”, “The biggest difficult in reconstructing the pre-Columbian past is the absence of voices from that past.” We have no writing, few stone artifacts (thanks to the geology), and very little reliable oral tradition. Yet since the 1950s, more evidence has arisen to support an altogether different view of the Amazon. It was full of people.

In order to support those people, probably numbering in the millions, the Amazon had to produce more food than mere slash-and-burn agriculture could supply. Slash-and-burn agriculture produces only relatively small perturbations in the forest system that are quickly reclaimed, and thus it is deemed “sustainable.” Yet in burning the felled trees, assuming a society had the technology to fell them-a stretch with stone axes-,most of their carbon, nitrogen and phosphorous are released into the atmosphere. And a plot so treated can take over a century to recover. A slash-and-burn agriculture could not support millions, even in an area as vast as the Amazon.

So, to solve this dilemma, Amazonian societies did something no other culture has ever done in world history: they created good soil. These groups cleared plots in the forest, yes, but instead of burning them they charred them slowly. The charcoal was then mixed with the soil where it not only added carbon, but hosted microbes, and increased the soil’s capacity to hold nutrients even during a downpour. Instead of planting rows of maize in their new soil, they planted hundreds of varieties of domesticated trees. Among these trees they planted their staple crop, manioc root.

Not content merely with plentiful fertile soils, they used pottery to build these soils up out of the floodplain (similar to the Beni people mentioned discussed in the link at the beginning). This process required continued investment of resources over hundreds of years. Millions of pots were smashed in order to raise their fields and dwellings and reclaim the land.

Terra preta is a rich, dark soil sometimes several feet deep. The picture at right is of one such soil, (it features soil biogeochemist Wim Sombroek), credit Johannes Lehmann at Cornell. It’s an aberration in the Amazon uncorrelated to geography, climate, geology, or ecology. In other words, it’s man-made. We have, then, in this soil an example of humans living in large numbers within an environment and actually improving it. Not only are terra preta soils rich farmland, but they sustain the most productive areas of the rainforest once abandoned.

Wilderness Lost?

From this knowledge, researchers have begun to speculate as to what fraction of the Amazon basin is truly wilderness. Their results, though highly speculative, are startling. One widely-cited estimate is that 11.8 percent of the non-flooded Amazon forest is man-made. In other words, about an eighth of the dryland Amazon is orchard.

What a stunning shift this insight will cause when eventually it is accepted by the larger ecological community (it may seem that I am jumping onboard here a bit quickly, but Mann presents a very compelling case). Our conservation practices will change, our idea of development will too-hopefully for the better. But most of all, terra preta serves as an example of how we can alter our landscape in a way that not only keeps us alive for a few years, but improves it for our children.

The final chapter of the book is more about this subject, and I anticipate it being another very interesting read, so stay tuned!