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Title: INCREASING CO2 FROM SUBAMBIENT TO SUPERAMBIENT CONCENTRATIONS ALTERS SPECIES COMPOSITION AND INCREASES ABOVE-GROUND BIOMASS IN A C3/C4 GRASSLAND

Author
item Polley, Herbert
item Johnson, Hyrum
item Derner, Justin

Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/20/2003
Publication Date: 7/20/2003
Citation: Polley, H.W., Johnson, H.B., Derner, J.D. 2003. Increasing CO2 from subambient to superambient concentrations alters species composition and increases above-ground biomass in a C3/C4 grassland. New Phytologist. 160:319-327.

Interpretive Summary: The concentration of carbon dioxide gas in the atmosphere has increased by 37% since the beginning of the Industrial Revolution and is expected to increase by another 50% within the century. Plants use carbon dioxide gas to grow. Consequently, the ongoing increase in carbon dioxide almost surely is stimulating plant growth, but field tests are lacking. We measured aboveground growth of a Texas grassland that was exposed for 4 years to carbon dioxide levels ranging from those representative of the past to concentrations expected within the century to determine the relative sensitivity of plant production to historical and predicted increases in carbon dioxide levels. Plant production increased as carbon dioxide concentration increased in each of the 4 years of the experiment. Contrary to expectation, though, the increase in plant growth was not more sensitive to increases in carbon dioxide over low concentrations representative of the past than over elevated levels predicted for the next century. The total amount of nitrogen present in plants did not change as carbon dioxide concentration increased, indicating that plants became more efficient in using nitrogen to grow at higher carbon dioxide levels. Our results indicate that the increase in atmospheric carbon dioxide since industrialization already has stimulated plant growth on grasslands and that grasslands will remain sensitive to rising carbon dioxide levels for several decades.

Technical Abstract: Plant growth may be more responsive to atmospheric CO2 over subambient than over elevated concentrations, but tests in natural ecosystems are lacking. We measured aboveground biomass production of a C3/C4 grassland exposed for 4 years to a continuous gradient in CO2 concentration from 200-560 µmol mol-1 to test the prediction that CO2 enrichment increases plant production more over subambient than over superambient concentrations. Because plant response to CO2 may be sensitive to feedbacks with nitrogen (N) availability and plant species composition, we also measured N in aboveground tissues and assessed relationships between community composition and CO2 treatment. Vegetation changed considerably during the 4 years of this study. Across CO2 concentrations, species evenness and diversity and the heterogeneity in species composition increased with time as contribution of C4 grasses to total production declined from 60% during the initial year to 24-25% during the final 2 years. Community composition clearly influenced the response of biomass production to CO2, but CO2 did not consistently affect species abundances. Heterogeneity in vegetation contributed to scatter in relationships between production and CO2, but biomass increased significantly with CO2 concentration in 1997-1999 and increased marginally (P = 0.07) at higher CO2 in 2000. Significant increases in production were larger than those typically measured in CO2 experiments on grasslands, and ranged between a mean of 153 and 209 g/m2 per 100 part per million increase in CO2. Contrary to prediction, though, biomass was no more sensitive to subambient than to superambient CO2. Responses of N concentration to CO2 varied among species and functional groups of species and among years, but in no year did total N in aboveground tissues change with CO2 treatment. In years in which CO2 stimulated production, therefore, the nitrogen use efficiency of vegetation (ratio of aboveground production to aboveground N) increased with CO2 concentration. The continuous response of aboveground production and of plant N concentration to CO2 in this C3/C4 grassland implies that CO2 enrichment already has stimulated production and that grasslands will remain sensitive to rising CO2 for several decades.