Forests at Their Limit

One scientist’s ground-level view.

By George M. Woodwell

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Step with me from the cool moist forest of the Amazon basin into the edge of a thousand-acre tract of soybeans. Feel the weight of the tropical sun on that open field. Touch the bare earth between the soy plants and realize that one would not walk with bare feet more than a few steps on such a surface. Squinting in that brightness, retreat with me quickly into the comfort of the forest, where five layers of foliage shield us from a sun that burns directly overhead year-round. Wonder with me that those leaves, even those at the top of the towering canopy, are not burning under the relentless heat of the sun that we felt in the field. Every breath of air in the forest is moist. Evaporation is occurring everywhere in all those layers of leaves, cooling the entire forest.

We are traveling in the southeastern Amazon basin in the Brazilian states of Mato Grosso and Pará. Southward is the transition from rich moist tropical forest to the diminished and drier woody vegetation called cerrado. It is this great arc of the southern part of the basin that is under pressure from the massive expansion of industrial agriculture to enter world markets for soy products. The soils are deep and rich, and there is, for the moment at least, no need for irrigation. As we look out across the landscape, we can see that fully half the land has been turned from forest to soy. The roots that once tapped reserves of water and nutrients tens of feet into the soil are no longer there. The deep canopy of trees cooled by evaporation of tons of water per hour is no longer feeding water and energy into the atmosphere. The entire landscape has transformed from cool to hot, from moist to dry, from diverse and rich to uniform and simple ... and hot, hot, hot.

Hotter and drier here, we observe, means hotter and drier there, just downwind, for the local climate is affected as well when the clouds do not build up and storms do not occur. Drier there means that the moist forest downwind, ever vulnerable to disruption, is drier too. In a dry time, even the moist forest can burn. The first fire is small, a ground fire. The next fire, more severe, burns dead trees and girdles others that die. The forest is diminished and opened to the sun and becomes warmer and still drier. Soon it becomes savannah, an open forested shrubland more akin to cerrado than the tropical moist forest we admired. So the transformation marches ahead of the bulldozer and the chain saw, gradually moving the climate as well as the frontier of deforestation.

But the story is even larger, for the warming of the earth is being driven by increases in the gases in the atmosphere that absorb radiant heat, especially carbon dioxide, a major product of both combustion and decay. The systematic destruction of the forest and the organic matter in soils adds to the accumulation of heat-trapping gases in the atmosphere. Globally deforestation is contributing about 1.5 billion tons of carbon to the annual accumulation in the atmosphere of about five billion tons from all human activities. Those five billion tons are the amount of carbon that must be removed from current releases to stabilize the composition of the atmosphere immediately. Stopping deforestation would make a major contribution toward that objective. This is the first step in correcting the problem. We are on a slippery slope. The expansion of agriculture into primary forests, especially in the tropics, is already destabilizing climates globally. The changing climate will ultimately destabilize the new agriculture as well.

We have clearly discovered one of the major biophysical limits of our planet. It is time for us all to appreciate that the earth’s remaining primary forests globally, especially the tropical forests, must be preserved intact as essential elements in the continued functioning of the human habitat. The growing global problems now embrace every citizen’s local interests and welfare. All nations and all communities need to join in a common effort to resolve the global problems that have been made in ignorance, through collective efforts to abuse nature for short-term human purposes.

Exchanging forest for soy, or some other non-forest monocrop, may seem an attractive deal at the moment. But it will be a very short and expensive moment.

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George M. Woodwell, an ecologist, is founder and director emeritus of the Woods Hole Research Center in Woods Hole, Massachusetts.  
 
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