Tree planting warning over global warming

Trees may not live up to expectations for storing carbon dioxide

By BBC News Online's environment correspondent Alex Kirby

Hopes of using forests to tackle global warming - by storing excess carbon - have received a setback.

Researchers in the US are shedding doubt on how effective trees are in absorbing carbon dioxide (CO2) and then releasing oxygen back into the atmosphere.

And they say they have identified factors that limit the ability of these natural "sinks" to soak up CO2.

Their findings could have huge implications for attempts to tackle climate change. The US and its supporters currently believe sinks can reduce CO2 levels significantly.

The researchers, whose work is reported in the journal Nature, looked at the growth rate of a plantation of loblolly pines on an experimental plot belonging to Duke University, North Carolina, US.

They found that trees growing in air enriched to contain about 0.06% CO2, considerably more than the current 0.036%, increased their growth rate for only three years, before resuming their normal rate.

Nitrogen's importance

What the researchers found limited the trees' capacity to respond to carbon fertilisation was a shortage of other nutrients, especially nitrogen. The availability of water was also important.

Forests need more than carbon to grow fast

When they made nitrogen available, the results were impressive.

They write: "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 (threefold higher than the expected additive effect) than at the moderate site (twofold higher)."

Foliage uptake

Another group of researchers examined the same forest plots to see how effective the leaf-litter layer and soil were at absorbing CO2.

They found that nearly half the carbon uptake went into short-lived parts of the trees, mainly foliage.

The total amount of litter did increase in a carbon-enriched atmosphere, but the rate at which it broke down also increased. And the carbon then went back into the atmosphere rather than into the soil.

They say: "We report a significant accumulation of carbon in the litter layer of experimental forest plots after three years of growth at increased CO2 concentrations.

"But fast turnover times of organic carbon in the litter layer (of about three years) appear to constrain the potential size of this carbon sink.

Reliance on sinks

"Given the observation that carbon accumulation in the deeper mineral soil layers was absent, we suggest that significant, long-term net carbon sequestration in forest soils is unlikely."

Forest floors retain little carbon

The November 2000 international climate talks in the Dutch capital, The Hague, were meant to finalise the workings of the global climate treaty, the Kyoto Protocol. But they ended in failure, with the role of carbon sinks one of the main sticking points.

The US and the other members of the so-called Umbrella Group (Japan, Switzerland, Canada, Australia, Norway, New Zealand and Russia) wanted to rely considerably on sinks in meeting their Kyoto targets for reducing emissions of greenhouse gases that may be warming the global climate.

The European Union and others opposed this, arguing that open-ended use of sinks to absorb CO2 could allow countries to avoid making any actual emission cuts at all.

In terms of international diplomacy that argument appears academic, because of President Bush's insistence that the US will not implement the protocol anyway.

But for the scientists and policymakers who are seeking practical ways of limiting what they see as the human contribution to climate change, it remains important. If sinks can help to absorb worthwhile amounts of carbon, many people will be very relieved. On this evidence, it is far from certain that they can.