New research casts doubt on US’s best hopes for combating climate change

The two projects, published in the 24 May edition of the science magazine, Nature, have cast doubt on the use of carbon sinks, where carbon can be stored in vegetation or soils within forest ecosystems, the storage of carbon being determined by the fraction that is sequestered in persistent organic materials, such as humus. The plan is believed to form the main proposal of the Bush Administration’s forthcoming plans for combating climate change, issued in response to withdrawal from the Kyoto Protocol. The largest such project to date has just been announced by General Motors in Brazil (see story in this week’s ‘World Section’).

In one research project, forest plots of loblolly pine exposed to high CO2 concentrations, nearly half of the carbon uptake was allocated to short-lived tissues, largely foliage. These tissues fall to the ground and decompose, normally contributing only a small portion of their carbon content to refractory soil humic materials. Although the researchers reported a significant accumulation of carbon in the litter layer of experimental forest plots after three years of growth at increased CO2 concentrations, the fast turnover times of organic carbon in the litter layer appeared to constrain the potential size of this carbon sink. The researchers concluded that “significant, long-term net carbon sequestration in forest soils is unlikely”.

The second research project which concentrates on carbon sequestration in northern mid-latitude forests, commonly found in the United States. Researchers discovered that, contrary to expectations that large increases in carbon sequestration from carbon dioxide fertilisation are expected in these forests, they generally lie in sites of moderate to poor fertility, where tree growth is often limited by nutrient supply, in particular nitrogen.

In two forest experiments on maturing pines exposed to elevated atmospheric CO2, the CO2-induced biomass carbon increment without added nutrients was undetectable at a nutritionally poor site, and the stimulation at a nutritionally moderate site was transient, stabilising at a marginal gain after three years. However, a large synergistic gain from higher CO2 and nutrients was detected with nutrients added. This gain was even larger at the poor site, where it was threefold higher than the expected additive effect, than at the moderate site, where it was twice as high.

The conclusion reached was that fertility can restrain the response of wood carbon sequestration to increased atmospheric CO2 and that estimates of increases in carbon sequestration of forests are “unduly optimistic”.