Response of Eriophorum Vaginatum to Elevated CO_2 and Temperature in the Alaskan Tussock Tundra

Small greenhouses were used in the arctic to maintain Erioporum vaginatum—dominated tussock tundra for 10 wk at ambient CO 2 (340 μL/L), elevated CO 2 (510 or 680 μL/L), or elevated CO 2 and 4°C above ambient temperature (680 μL/L, ambient + 4°). These treatments represent present levels of atmospheric CO 2 and temperature, and those predicted for the next century. Within 3 wk, plants maintained at elevated CO 2 exhibited a physiological adjustment of their photosynthetic rate so that plants grown at ambient and elevated CO 2 levels had similar photosynthetic rates at their respective growth CO 2 concentrations. The reduction in photosynthetic capacity for plants grown at elevated CO 2 levels did not appear to be due to stomatal closure or end—product inhibition. Other possible mechanisms were not explored. Transpiration rates and water use efficiency did not differ among treatments in the generally wet environment of tussock tundra. Relative leaf growth rate and the seasonal pattern of growth were also unaltered, suggesting that the growth of mature tillers is not, under normal ambient conditions, limited by temperature or carbohydrate. However, new tiller production was significantly increased at elevated CO 2 , suggesting that the long—term effect of CO 2 enhancement in this sedge may be the production of a greater number of new tillers rather than an increase in the size or productivity of existing tillers. Our results are consistent with the notion that growth of Eriophorum vaginatum in the field is more limited by nutrient supply than by photosynthesis. We further suggest that photosynthetic rates reflect the sink activity. It is therefore very difficult to assign cause and effect between growth rates and photosynthetic rates.