Transient nature of CO₂ fertilization in Arctic tundra

THERE has been much debate about the effect of increased atmospheric CO₂ concentrations on plant net primary production^1,3 and on net ecosystem CO₂ flux^3-10. Apparently conflicting experimental findings could be the result of differences in genetic potential^11-15 and resource availability^16-20, different experimental conditions^21-24 and the fact that many studies have focused on individual components of the system^2,21,25-27 rather than the whole ecosystem. Here we present results of an in situ experiment on the response of an intact native ecosystem to elevated CO₂. An undisturbed patch of tussock tundra at Toolik Lake, Alaska, was enclosed in greenhouses in which the CO₂ level, moisture and temperature could be controlled²⁸, and was subjected to ambient (340 p.p.m.) and elevated (680 p.p.m.) levels of CO₂ and temperature (+4 °C). Air humidity, precipitation and soil water table were maintained at ambient control levels. For a doubled CO₂ level alone, complete homeostasis of the CO₂ flux was re-established within three years, whereas the regions exposed to a combination of higher temperatures and doubled CO₂ showed persistent fertilization effect on net ecosystem carbon sequestration over this time. This difference may be due to enhanced sink activity from the direct effects of higher temperatures on growth^16,29-33 and to indirect effects from enhanced nutrient supply caused by increased mineralization^{10,11,19,27,34}. These results indicate that the responses of native ecosystems to elevated CO₂ may not always be positive, and are unlikely to be straightforward. Clearly, CO₂ fertilization effects must always be considered in the context of genetic limitation, resource availability and other such factors.