CO2-caused change in plant species composition rivals the shift in vegetation between mid-grass and tallgrass prairies

Authors: Polley, Herbert; Jin, Virginia; Fay, Philip

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/10/2011
Publication Date: 12/18/2011
Citation: Polley, H.W., Jin, V.L., Fay, P.A. 2012. CO2-caused change in plant species composition rivals the shift in vegetation between mid-grass and tallgrass prairies. Global Change Biology. 18:700-710.

Interpretive Summary: The concentration of carbon dioxide (CO2) gas in air has increased by about 40% since the 19th century and is expected to reach double the pre-Industrial concentration during this century. Several experiments have demonstrated that increasing CO2 changes the contributions of plant species to biomass production of grasslands, but the types of species favored and mechanisms by which changes are mediated differ among ecosystems. We measured changes in the contributions of perennial forbs and grasses to aboveground biomass production of tallgrass prairie vegetation grown along a pre-Industrial to elevated CO2 gradient in central Texas USA. Vegetation was grown on three soil types and irrigated each season with water equivalent to the growing season average of precipitation for the area. The grass fraction of biomass production increased dramatically as CO2 increased on all soils. Contribution of the tall-grass indian grass (Sorghastrum nutans) to production increased at elevated CO2 on the two most coarse-textured of the soils studied. The increase in indian grass was accompanied by an offsetting decline in production of a shorter mid-grass, sideoats-grama (Bouteloua curtipendula). Increased CO2 favored the tall grass over mid-grass by increasing soil water content and reducing the frequency with which soil water dipped below threshold levels of limitation. Increasing CO2 from the pre-Industrial level to the concentration anticipated in 2050 thus led to a shift in species relative growth that is similar in character and magnitude to differences observed between mid-grass and tallgrass prairies along a precipitation gradient in the central USA. Our results indicate that rising CO2 contributed to vegetation change during the last century and will continue to influence vegetation in the near-future.

Technical Abstract: Atmospheric CO2 enrichment usually changes the contributions of plant species to biomass production of grasslands, but the types of species favored and mechanisms by which change is mediated differ among ecosystems. We measured changes in the contributions of C3 perennial forbs and C4 grasses to aboveground biomass production of tallgrass prairie assemblages grown along a field CO2 gradient (250-500 µmol mol-1) in central Texas USA. Vegetation was grown on three soil types and irrigated each season with water equivalent to the growing season mean of precipitation for the area. We predicted that CO2 enrichment would increase the forb contribution to community production, and favor tall-grasses over mid-grasses by increasing soil water content and reducing the frequency with which soil water fell below a limitation threshold. CO2 enrichment favored forbs over grasses on only one of three soil types, a Mollisol. The grass fraction of production increased dramatically across the CO2 gradient on all soils. Contribution of the tall-grass Sorghastrum nutans to production increased at elevated CO2 on the two most coarse-textured of the soils studied, a clay Mollisol and sandy Alfisol. The CO2-caused increase in Sorghastrum was accompanied by an offsetting decline in production of the mid-grass Bouteloua curtipendula. Increased CO2 favored the tall grass over mid-grass by increasing soil water content and apparently intensifying competition for light or other resources (Mollisol) or by reducing the frequency with which soil water dipped below threshold levels (Alfisol). An increase in CO2 of 250 µmol mol-1 above the pre-Industrial level thus led to a shift in the relative production of established species that is similar in magnitude to differences observed between mid-grass and tallgrass prairies along a precipitation gradient in the central USA. Grassland vegetation likely will remain sensitive to effects of CO2 on soil water content.