Temperate forest soils may be poor carbon store in elevated CO2 world
19 February 2009
Mediterranean and North American temperate forests transfer more carbon into soils when atmospheric carbon dioxide levels are high. But most - or even all - of this carbon seeps back into the atmosphere within days.
These are the findings by British, Italian and American scientists published in the journal Forestry.
The fate of carbon in soils is an important issue for researchers and governments keen to control atmospheric CO2 concentrations. Forests account for more than three quarters of the carbon stored on land and most of this carbon is stored below ground. In future, if forest soils do not take up as much carbon as expected, more carbon will remain in the atmosphere.
Lead author Dr Martin Lukac from the NERC Centre for Population Biology says, 'We can more or less conclude that in high CO2 conditions trees grow faster, taking in more atmospheric CO2. But what we also observe, is a concurrent increase in soil CO2 efflux, which is the carbon coming out of the ecosystem'.
Poplar plots in Italy were given high doses of carbon dioxide.
A large part of carbon fixed by trees goes to the roots, the fine roots, and then symbiotic fungy that colonise the root tips of most tree species.
'Carbon is then transferred to the soil where it is metabolised by soil biota. Most of it comes back to the atmosphere very quickly,' explains Lukac. 'Sometimes it only takes three-to-four days for molecules of carbon dioxide taken up from the air by tree leaves to return to the atmosphere through the soil'.
This discovery was made using Free Air Carbon dioxide Enrichment, or FACE experiments at specially designed forest plantations.
During each of the experiments, the researchers continuously pumped CO2 upwind from circular plots in the forest, tens of metres in diameter, to maintain an artificially high CO2 environment for the trees. This method allowed researchers to monitor the forest's response to a high CO2 atmosphere while maintaining the natural ecosystem.
Carbon dioxide in these tanks is released to shroud the forest plots.
The researchers concentrated on three sites in the United States - Wisconsin, Tennessee and North Carolina - and one site located in Viterbo, in the Lazio region of central Italy. The Italian site, entitled EuroFACE, used fast growing species of poplar, normally grown in the Po River region in northern Italy. These needed constant irrigation to survive in the hot climate.
The local climatic conditions also affected the working hours of the EuroFACE scientists.
'Because we did the experiments mostly in the summer, we had to get used to working early in the morning and late in the evening, it was so hot in the day we couldn't really do anything,' recalls Lukac.
Each of the US sites also had its own distinctive character, using trees of different species and age and soils of different type and texture to those at the EuroFACE site. Despite these differences, the researchers always found the same result - enhanced carbon turnover in a forest growing in high CO2 atmosphere.
The question of how much extra carbon will be stored by temperate forest soils in a future high CO2 world remains a tricky one. The duration of these experiments is short relative to a lifecycle of around 100 years for a temperate forest, and the response of forest soils over such a long period remains best guess. These results are further complicated by other environmental factors such as changes in temperature and moisture in the soil.
Future work aims to extend the climatic range of the results by exploring the response of tropical or boreal forests to changes in atmospheric CO2.
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