THE STORY OF ROOTS

Trees have become a source of continuous surprise. Only weeks after researchers demonstrated that old forest giants actually accumulate more carbon than younger, fast-growing trees, British scientists have discovered that the great arbiters of long-term global temperatures may not be the leaves of an oak, a pine or a eucalypt, but the roots.

The argument, put by a team from Oxford and Sheffield Universities in the journal Geophysical Research Letters, begins with temperature. Warmer climates mean more vigorous tree growth and more leaf litter, and more organic content in the soil. So the tree’s roots grow more vigorously, say Christopher Doughty of Oxford and colleagues.

They get into the bedrock, and break up it up into its constituent minerals. Once that happens, the rock starts to weather, combining with carbon dioxide. This weathering draws carbon dioxide out of the atmosphere, and in the process cools the planet down a little. So mountain ecosystems – mountain forests are usually wet, and on conspicuous layers of rock – are in effect part of the global thermostat, preventing catastrophic overheating.

The tree is more than just a sink for carbon, it is an agency for chemical weathering that removes carbon from the air and locks it up in carbonate rock.

That mountain weathering and forest growth are part of the climate system has never been in much doubt: the questions have always been about how big a forest’s role might be, and how to calculate its contribution.

Keeping climate stable

US scientists recently studied the rainy slopes of New Zealand’s Southern Alpsto begin to put a value on mountain ecosystem processes. Dr Doughty and his colleagues measured tree roots at varying altitudes in the tropical rain forests of Peru, from the Amazon lowlands to 3,000 metres of altitude in the higher Andes.

They measured the growth to 30 cms below the surface every three months and did so for a period of years. They recorded the thickness of the soil’s organic layer, and they matched their observations with local temperatures, and began to calculate the rate at which tree roots might turn Andean granite into soil.

Then they scaled up the process, and extended it through long periods of time. Their conclusion: that forests served to moderate temperatures in a much hotter world 65 million years ago.

“This is a simple process driven by tree root growth and the decomposition of organic material. Yet it may contribute to the Earth’s long-term climate stability. It seems to act like a thermostat, drawing more carbon dioxide out of the atmosphere when it is warm and less when it is cooler”, Dr Doughty said.

If forests cool the Earth, however, they might also warm it up.