TRANSIENT NATURE OF CO2 FERTILIZATION IN ARCTIC TUNDRA

THERE has been much debate about the effect of increased atmospheric C O2 concentrations on plant net primary production(1,3) and on net ecos ystem CO2 flux(3-10). Apparently conflicting experimental findings cou ld be the result of differences in genetic potential(11-15) and resour ce availability(16-20), different experimental conditions(21-24) and t he fact that many studies have focused on individual components of the system(2,21,25-27) rather than the whole ecosystem. Here we present r esults of an in situ experiment on the response of an intact native ec osystem to elevated CO2. An undisturbed patch of tussock tundra at Too lik Lake, Alaska, was enclosed in greenhouses in which the CO2 level, moisture and temperature could be controlled(28), and was subjected to ambient (340 p.p.m.) and elevated (680 p.p.m.) levels of CO2 and temp erature (+4 degrees C). Air humidity, precipitation and soil water tab le were maintained at ambient control levels. For a doubled CO2 level alone, complete homeostasis of the CO2 flux was re-established within three Sears, whereas the regions exposed to a combination of higher te mperatures and doubled CO2 showed persistent fertilization effect on n et ecosystem carbon sequestration over this time. This difference may be due to enhanced sink activity from the direct effects of higher tem peratures on growth(16,29-33) and to indirect effects from enhanced nu trient supply caused by increased mineralization(10,11,19,27,34). Thes e results indicate that the responses of native ecosystems to elevated CO2 may not always be positive, and are unlikely to be straightforwar d. Clearly, CO2 fertilization effects must always be considered in the context of genetic limitation, resource availability and other such f actors.