A Carbon Sink, Sinking

What comes to mind when you think about deforestation in the Amazon? Probably clear-cutting, burning, ranching, maybe the increasing acreage for soybean cultivation. Those things are bad enough, but a new study published in Nature suggests we may have found yet another way to destroy the rainforest: choking the trees on carbon dioxide, the very thing they feed on. It’s the botanical equivalent of making foie gras.

For nearly 30 years, a team of scientists at the University of Leeds in the U.K. has counted and measured almost every tree on 321 plots in the Amazon—a total of around 189,000 trees. They were trying to determine how much carbon the rainforest was absorbing from the atmosphere and how much it released into the air from decaying vegetation. The plots showed no signs of human disturbance, so changes in vegetation could not be blamed on farmers or ranchers (at least not directly).

Throughout the 1980s and for most of the ’90s, the trees’ growth rate increased—likely the result of increasing carbon dioxide levels in the air. Scientists have long known that the carbon fertilization effect of our excess emissions would help plants grow faster. It also, however, seems to make them die faster.

“It’s a difficult phenomenon to prove, because trees have very long life spans,” says Roel Brienen, the study's lead author. “But if you pump more carbon dioxide into a mature forest, trees will produce seeds and flowers at a younger age, which suggests they will complete their life cycle faster.

Tree-ring research also indicates that trees that grow faster tend to die younger. And, as Brienen points out, cross-species comparisons within both the plant and animal kingdoms suggest that living fast really does lead to dying young. Mice mature in only two or three weeks and die before age three. Elephants, in contrast, take up to 13 years to mature but live well into their 40s, if not longer.

Back to those trees. During the 1980s and ’90s, the accelerating growth and death rates were roughly in balance, so the rainforest continued to absorb and store carbon at a high rate. Beginning around the turn of the millennium, however, death pulled far ahead of growth on the monitored plots. Over the past decade, the amount of carbon absorbed annually by the Amazon has declined by 30 percent. Brienen and his team still aren’t sure what caused the imbalance. A couple of droughts probably contributed, but the slowdown in growth happened before the first major one in 2005, and plots that weren’t affected by severe drought saw the same drop-off. It’s also possible that decreased nutrients or changes in temperature are to blame.

We don’t know what will happen next. The shrinking of the Amazon’s carbon-storing capacity has been fairly linear over the past decade. Will it level off, making the rainforest carbon-neutral over the long term? Or will the trend line cross the x-axis, as decaying trees and stalled growth transform the Amazon into a major source of atmospheric carbon?

It’s a terrifying thought. While there’s debate about just how much carbon is stored in the Amazon, it’s a massive amount by any measure. Cars, cows, and coal are polluting the atmosphere efficiently enough. We don’t need trees getting in on the act.

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