IN 1984, ON THE OUTSKIRTS OF PAR" agominas, a bustling and violent cattle town in the eastern Amazon, I walked through a pasture, brown and dusty from drought, into a 500-acre island of virgin forest. The moist greenness of the leaf canopy that reached more than 100 feet above my head and the squishy dampness of the humus, dead leaves, and branches on the ground were a world apart from the parched African grasses and gaunt white zebu cows I had just seen. Although I didn't realize it at the time, it is in understanding the differences between these two worlds--the rainforest and the cattle pastures that are replacing them--that we begin to realize how the lives of people around the world are tied to the fate of Amazon forests. The climate of the Amazon and the climate of the planet are both dependent upon the deeply rooting, drought-resistant trees that comprise Amazon forests, just as the survival of these trees depends upon climate. The risk is that the early symptoms of climate change will act synergistically with logging, fire, and drought to replace much of Amazon rainforests with fire-prone scrub vegetation, accelerating global warming in the process.
To penetrate the complex web of relationships between climate and rainforest, we must begin with a lesson in tree physiology. Hug a tree on a warm, sunny day, and your arms surround thousands of tiny, little tubes full of water that is racing silently skyward, like soda up a straw. Everyone has seen these tubes, which biologists call "vessels" and "tracheids." They help form the grain in wood. Seasonal variations in the diameters of these tubes make up the growth rings we see on the cut surfaces of tree trunks. Wood is the plant world's most successful invention for accomplishing two extraordinarily difficult tasks. Its remarkable strength allows trees to position their leaves to capture sunlight far above the ground. And its exquisite plumbing network of vessels and tracheids supplies these leaves with water absorbed from the soil.
For many years it was assumed that Amazon trees are not very good at absorbing water from the soil because of their very shallow root systems. Back in 1984, as I returned to that 500-acre forest island nearly every day of the five-month dry season, the mismatch between the assumption and what I was seeing slowly sank into my graduate-student mind. Some simple calculations led me to predict that the towering, green trees had to be absorbing moisture from at least 25 feet beneath the ground surface, well beyond the two- or three-foot rooting depth assumed by most. Otherwise these trees would have turned brown and gone dormant, just like the African forage grasses planted in the neighboring pasture. Twenty-five feet was the depth of soil needed to store the amount of water that the forest was releasing to the atmosphere through "transpiration"--the evaporation of water from leaves into the air--and that was not being supplied by the meager, dry-season rains that had fallen. I hired some well-diggers from Paragominas, a town south of Belem, to test my calculations and look for deep roots. One hearty digger dug down 68 feet, aided by an industrial fan that pumped fresh air into his damp, dark, grave-sized hole. The last tree roots disappeared 60 feet beneath the ground's surface. By 1992 my research team had dug dozens of deep holes across the Amazon with similar results. The assumption of shallow rooting in Amazon trees was put to rest in 1994 as we published our results in the journal Nature despite stiff resistance from some reviewers of our controversial findings.
Deep roots are far more than a botanical curiosity. For by allowing Amazon forests to remain green and lush during the severe seasonal droughts that affect about half of the Amazon region each year, these cryptic tree organs facilitate the release of enormous amounts of water to the atmosphere through transpiration. Thanks to deep roots, Amazon trees can supply the atmosphere with vapor year round, and this vapor is the most important ingredient of rain clouds in this region. …