Trees no savior for global warming
Researchers test theory that forests will be 'carbon sinks,' and find smaller benefits than computers forecast.
Stands of North Carolina pines are sending a sobering signal to governments preparing for a new round of global climate talks in Bonn in July.
The message: Don't count too heavily on forests to bail you out of reducing greenhouse-gases emissions. This verdict comes via experiments in which forest plots were subjected to sustained increases in carbon dioxide (CO2).
The news could alter computer models used to forecast how much heat-trapping carbon gas can be "locked up" in tree trunks, leaves, and soil. To date, the models have predicted that until 2050, forests - particularly those in the northern hemisphere - will soak up much of the CO2 that humans pump into the air.
But after nearly eight years of experiments, Duke University researchers are less optimistic. "Our results are clear. Forests will be a modest, if not disappointing, sink for carbon dioxide," says William Schlesinger, a Duke biogeochemist who led one of two teams reporting results yesterday in the journal Nature.
The results come as the Bush administration is trying to craft an alternative to the 1997 Kyoto protocols, which would commit industrial countries to cut greenhouse-gas emissions by 2012 to levels 5 percent lower than those in 1990. Conflicts over how much credit countries should receive for their forest "carbon sinks" scuttled efforts to agree on ground rules for the protocols in talks last November.
In March, the President effectively rejected the treaty. It has also drawn opposition in the US Senate, which would have to ratify it. Earlier this week, a senior administration official was quoted as saying he hopes to have an alternative ready for Mr. Bush to take to Europe in June. Leaders there have heaped scorn on the administration for backing away from the Kyoto accord.
Scientists began the ongoing North Carolina experiments in 1993, with a prototype plot to see how forests would react to CO2 levels projected to exist by 2050.
By 1996, the experiment included three more stands of loblolly pine, with an array of towers injecting C02 into the air overhead, plus three control stands that received no extra carbon.
The trees initially responded to higher CO2 levels by producing an 24 to 34 percent more wood than the control trees, but growth tapered off to marginal increases after about three years. The limiting factor appeared to be soil nutrients. When the researchers added nitrogen fertilizer to half the trees in one CO2-rich ring, their growth accelerated compared to the unfertilized trees. Trees not receiving extra CO2 also grew faster when fertilized, but not as fast at the trees getting more fertilizer and more CO2. Moreover, a drought in 1999 cut by 25 percent the amount of CO2 sequestered.
The notion that plant growth depends on moisture and soil nutrients, as well as what's in the atmosphere, is something of a "no-duh" proposition, says Ram Oren, leader of the Duke team measuring tree growth . "Even farmers in Mesopotamia should have known that."
But these factors have not been adequately captured by computer simulations.
A team led by Dr. Schlesinger looked at how effectively forest soils would lock up carbon dioxide. While they found that leaves and other forms of forest litter built up more quickly in the CO2-rich sites, the gas also returned quickly to the atmosphere - in about three years.
(c) Copyright 2001. The Christian Science Monitor