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ARS Home » Plains Area » Temple, Texas » Grassland Soil and Water Research Laboratory » Research » Publications at this Location » Publication #191400
Title: C3-C4 COMPOSITION AND PRIOR CARBON DIOXIDE TREATMENT REGULATE THE RESPONSE OF GRASSLAND CARBON AND WATER FLUXES TO CARBON DIOXIDE

Author
item Polley, Herbert
item DUGAS, WILLIAM - TEXAS AG EXPT STN
item MIELNICK, PATRICIA - TEXAS AG EXPT STN
item Johnson, Hyrum

Submitted to: Functional Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/11/2006
Publication Date: 5/25/2007
Citation: Polley, H.W., Dugas, W.A., Mielnick, P.C., Johnson, H.B. 2007. C3-C4 composition and prior carbon dioxide treatment regulate the response of grassland carbon and water fluxes to carbon dioxide. Functional Ecology. 21:11-18.

Interpretive Summary: The concentration of carbon dioxide (CO2) gas in air is increasing as the result of land use change and fossil fuel combustion. When exposed to a higher CO2 concentration, plants usually respond by increasing the rate at which leaves convert CO2 to plant carbon mass (CO2 uptake) and by reducing the rate at which leaves expend water (water loss). These initial responses to CO2 enrichment may not be sustained, however, if plants adjust their physiology or acclimate to higher CO2 levels. We sought to determine whether prior CO2 exposure affected the response of grassland CO2 uptake and water loss to CO2 change. To do so, we measured effects of temporarily changing CO2 concentration by 150 to 200 parts per million (about one-half of the present atmospheric CO2 concentration) on daytime net CO2 uptake and water loss of grassland plots in central Texas that had been exposed for more than 3 growing seasons to different CO2 levels ranging from low concentrations of the pre-historical period to elevated levels expected during the century. Following CO2 change, CO2 uptake was 11% greater and water loss was 12% smaller, on average, at high than low CO2. Proportional and absolute differences between CO2 uptake and water loss at the two CO2 concentrations did not depend on prior CO2 exposure. But, the absolute and proportional increase in both CO2 uptake and the ratio of CO2 uptake to water loss (water use efficiency) following short-term change in CO2 rose as the fraction of broadleaf herbaceous plants (forbs) in the plant canopy (forb plus grass) increased. Our results imply that sensitivity of this grassland to the continuing rise in atmospheric CO2 concentration will depend more on how management and other factors influence plant composition than on the duration of exposure to elevated CO2 levels alone.

Technical Abstract: Plants usually respond to carbon dioxide (CO2) enrichment by increasing photosynthesis and reducing transpiration. These initial responses to CO2 may not be sustained, however, partly because of constraints on the rate at which carbon can accumulate in plants and soil. We measured effects of temporarily increasing or decreasing CO2 concentration by 150 to 200 'mol mol-1 on daytime net ecosystem CO2 exchange (NEE) and water flux (evapotranspiration, ET) of C3/C4 grassland in central Texas, USA that had been exposed for more than 3 growing seasons to a CO2 gradient from 200 to 560 'mol mol-1. Grassland maintained at subambient CO2 (< 365 'mol mol-1) was exposed for two days to an elevated CO2 gradient (> 365 'mol mol-1). Grassland maintained at elevated CO2 was exposed for 2 days to a subambient gradient. When CO2 concentration was temporarily changed, NEE was greater (11%) and ET was smaller (12%), on average, at elevated than subambient CO2. Proportional and absolute differences between fluxes at the two CO2 concentrations did not depend on prior CO2 exposure, but the response of NEE and water use efficiency (NEE/ET) to short-term change in CO2 varied with the ratio of C3 to total (C3 plus C4) plant cover. The ratio of NEE at elevated CO2 to NEE at subambient CO2 increased from 1.0 when C3 plants comprised 35% of plant cover to about 1.2 when C3 plants comprised 100% of cover. The response of ET to elevated CO2 did not depend on C3/total cover, but the relative response of WUE to elevated CO2 increased from 1.09 to 1.34 as C3 cover increased from 35% to 100%. Carbon and water fluxes in the mesic grassland we studied remained responsive to CO2 enrichment after more than 3 growing seasons at different CO2 levels. For both NEE and WUE, however, CO2 response was determined mainly by the C3-C4 composition of vegetation, indicating that sensitivity of this grassland to CO2 enrichment depends more on vegetation dynamics than on physiological adjustments related to the duration of CO2 exposure.