Ability of trees to offset CO2 questioned

Tree planting is often seen as a means of offsetting human carbon emissions, but recent research has cast doubt on its effectiveness.

Results from a decade-long experiment by scientists at North Carolina’s Duke University suggest that trees’ ability to lock up excess CO2 is highly dependent on weather and soil conditions.

During the Free Air Carbon Enrichment (FACE) experiment, pine trees bathed in extra carbon dioxide grew more tissue, but did not necessarily store significant quantities of carbon.

Instead, the trees’ capacity to bank CO2 was influenced by the weather and large scale forest fertilisation efforts. This suggests that planting more trees may not be successful in slowing the pace of climate change.

Water availability

"If water availability decreases to plants at the same time that carbon dioxide increases, then we might not have a net gain in carbon sequestration," said Ram Oren, FACE project director and professor of ecology at Duke University.

"In order to actually have an effect on the atmospheric concentration of CO2, the results suggest a future need to fertilise vast areas," Oren added. "And the impact on water quality of fertilising large areas will be intolerable to society. Water is already a scarce resource. "

During the study, four forest plots at the FACE site were administered one and a half times today’s concentration of CO2. Ecologists then monitored the rate at which trees accumulated carbon, which was compared to plots receiving no extra gas.

"Elevated CO2 could increase the production of foliage but this would lead to only a very small increase in ecosystem carbon storage" Heather McCarthy

The research team found that trees in the pine-dominated forest plots that were treated produced about 20 per cent more biomass on average. But since the amounts of available water and nitrogen nutrients varied substantially from plot to plot, using averages could be misleading.

"In some areas, the growth is maybe five or ten per cent more, and in other areas it's 40 per cent more," Oren said. "So in sites that are poor in nutrients and water we see very little response. In sites that are rich in both we see a large response."

Furthermore, CO2 enrichment had no effect on the proportions of carbon atoms that found their way to different components of plant systems - wood, leaves, roots and underlying soil. Only a few of those components will store carbon over time, noted Oren and Heather McCarthy, a gradate student.

"Carbon that's in foliage is going to last a lot shorter time than carbon in the wood, because leaves quickly decay," McCarthy said. "So elevated CO2 could significantly increase the production of foliage but this would lead to only a very small increase in ecosystem carbon storage."

The researchers found that extra carbon dioxide equally had no influence on what foresters call "self thinning" - the tendency of less-successful trees to die off as the most-successful grow bigger.

"We didn't find that elevated CO2 caused any deviation from this standard relationship," said Mc Carthy.

You’ve read it. Now review it.

Back to top

Back to top