Carbon stockpiles question idea that forests will counteract global warming
Mature trees might not offset global warming by mopping up excess carbon dioxide, suggests a forest study in Switzerland. They may already have all the carbon dioxide that they need.
If the phenomenon is widespread, it could be a setback to those hoping that existing and newly planted forests will buffer mounting CO2 levels. Whether these carbon sinks will work is much debated because of its relevance to the Kyoto Protocol, in which countries compensate for their greenhouse-gas emissions by planting forests.
Günter Hoch of the University of Basel, Switzerland, and his colleagues tested the assumption that current CO2 levels limit plants' photosynthesis and growth. During spring's furious budding, for instance, deciduous trees were thought to rely heavily on carbon reservoirs of sugars, starches and fats because atmospheric CO2 is insufficient.
Trees in a 100-year-old forest near Basel use only an average of 33-45% of their carbon stores during the year, the group found1. The team measured the carbon content of leaves and wood from the 30-metre-tall trees over three years.
The finding suggests that, in this forest at least, trees are already amply supplied with CO2 and are unlikely to absorb much more if levels rise in future. "The trees are growing fat at the current CO2 concentrations," says Hoch.
"It's very nice work," says Richard Norby, who studies forests' response to climate change at Oak Ridge National Laboratory in Tennessee. But he believes that trees can still swallow extra CO2, even if they are not currently running down their supplies - just as we can eat excess food even if we are full or fat.
The prevailing scientific view is that existing woodland will compensate for rising CO2 levels to some degree, but by how much remains uncertain. One study, on trees grown in artificially high carbon dioxide concentrations, hinted that they may not soak up much excess gas - limited supplies of other nutrients, particularly nitrogen, seem to curb any growth spurt2.
Experiments are already under way in Basel to explore whether artificially boosting carbon dioxide will spur extra growth there - Hoch believes that space above or below the ground could restrict the trees' growth.
Newly planted forests are also expected to take up some extra CO2, but it is not clear how much or for how long. "It's naïve to think they'll solve the problem, but it won't hurt the situation," says Norby.
Hoch's study is one of the most comprehensive analyses of back-up carbon stores in trees. The team looked at leaves, branches and trunks in ten species, including deciduous oak, beech and maple plus evergreens spruce, fir and pine.
Deciduous species store sufficient carbon to replace all of their leaves at least four times over, the researchers calculate. Evergreens stockpile enough for only half a new set of needles. Deciduous trees may need a bigger reservoir in case they lose leaves to hungry insects, Hoch suggests.
Forest researcher Ram Oren of Duke University in Durham, North Carolina, is not yet convinced that trees use such excess carbon stores - some may be effectively inaccessible because they are difficult to shunt from trunk tissue to buds, he points out. Hoch is testing this possibility.
© Nature News Service / Macmillan Magazines Ltd 2003