Forests at their limit: one scientist's ground-level view.

• Author: Woodwell, George M.
• Position: SPECIAL REPORT

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 Para. 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...