Fluxes of CO2 and water vapor from a prairie ecosystem exposed to ambient and elevated atmospheric CO2

Increasing concentrations of atmospheric CO2 may alter the carbon and water relations of prairie ecosystems. A C4-dominated tallgrass prairie near Manhattan, KS, was exposed to 2 × ambient CO2 concentrations using 4.5 m-diameter open-top chambers. Whole-chamber net CO2 exchange (NCE) and evapotranspiration (ET) were continuously monitored in CO2-enriched and ambient (no enrichment) plots over a 34-d period encompassing the time of peak biomass in July and August, 1993. Soil-surface CO2 fluxes were measured with a portable surface chamber, and sap flow (water transport in xylem) in individual grass culms was monitored with heat balance techniques. Environmental measurements were used to determine the effect of CO2 on the surface energy balance and canopy resistances to vapor flux. In 1993, frequent rainfall kept soil water near field capacity and minimized plant water stress. Over the 34-d measurement period, average daily NCE (canopy photosynthesis — soil and canopy respiration) was 9.3 g CO2 m−2 in the ambient treatment adn 11.4 g CO2 m−2 under CO2 enrichment. However, differences in NCE were caused mainly by delayed senescence in the CO2-enriched plots at the end of the growing season. At earlier stages of growth, elevated CO2 had no effect on NCE. Soil-surface CO2 fluxes typically ranged from 0.4 to 0.66 mg CO2 m−2 s−1, but were slightly greater in the CO2_enriched chambers. CO2 enrichment reduced daily ET by 22%, reduced sap flow by 18%, and increased canopy resistance to vapor flux by 24 s m−1. Greater NCE and lower ET resulted in higher daytime water use efficiency (WUE) under CO2 enrichment vs. ambient (9.84 vs. 7.26 g CO2 kg−1 H2O). However, record high precipitation during the 1993 season moderated the effect of WUE on plant growth, and elevated CO2 had no effect on peak aboveground biomass. CO2-induced stomatal closure also affected the energy balance of the surface by reducing latent heat flux (LE), thereby causing a consequent change in sensible heat flux (H). The daytime Bowen ratio (H/LE) for the study period was near zero for the ambient treatment and 0.21 under CO2 enrichment.

[1]  Robert Clement,et al.  Momentum, water vapor, and carbon dioxide exchange at a centrally located prairie site during FIFE , 1992 .

[2]  D. Baldocchi.  A comparative study of mass and energy exchange rates over a closed C3 (wheat) and an open C4 (corn) crop: II. CO2 exchange and water use efficiency , 1994 .

[3]  Paul W. Leadley,et al.  Canopy photosynthesis of crops and native plant communities exposed to long‐term elevated CO2 , 1991 .

[4]  J. Pereira,et al.  Water Stress, CO2 and Climate Change , 1992 .

[5]  D. Paslier,et al.  Net Exchange of CO2 in a Mid-Latitude Forest , 1993, Science.

[6]  J. Barnes.  Impact of carbon dioxide, trace gases, and climate change on global agriculture , 1992 .

[7]  John D. Alexander,et al.  Plant water relations and the effects of elevated CO2: a review and suggestions for future research , 1993 .

[8]  F. S. Nakayama,et al.  Soil Carbon Dioxide Distribution and Flux within the Open-top Chamber , 1988 .

[9]  Jay M. Ham,et al.  Aerodynamic and surface resistances affecting energy transport in a sparse crop , 1991 .

[10]  P. Vitousek.  10 – Global Dynamics and Ecosystem Processes: Scaling Up or Scaling Down? , 1993 .

[11]  J. Ham,et al.  Heat balance sap flow gauge for small diameter stems , 1993 .

[12]  Response of plants to CO2 under water limited conditions , 1993 .

[13]  H. Mooney,et al.  PREDICTING ECOSYSTEM RESPONSES TO ELEVATED CO2 CONCENTRATIONS , 1991 .

[14]  R. Desjardins.  Carbon dioxide budget of maize , 1985 .

[15]  J. M. Norman,et al.  Soil surface CO2 fluxes and the carbon budget of a grassland , 1992 .

[16]  H. Rogers,et al.  Crop responses to CO2 enrichment , 1993 .

[17]  James W. Jones,et al.  Responses of Soybean Canopy Photosynthesis and Transpiration to Whole‐Day Temperature Changes in Different CO2 Environments1 , 1985 .

[18]  C. V. Bavel,et al.  Relative role of stomatal and aerodynamic resistances in transpiration of a tomato crop in a CO2-enriched greenhouse , 1987 .

[19]  Effects of carbon dioxide on photosynthesis, plant growth, and other processes , 1990 .

[20]  Alan K. Knapp,et al.  Biomass Production in a Tallgrass Prairie Ecosystem Exposed to Ambient and Elevated CO"2. , 1993, Ecological applications : a publication of the Ecological Society of America.

[21]  M. B. Kirkham,et al.  Leaf Photosynthesis Water Use of Big Bluestem under Elevated Carbon Dioxide , 1991 .

[22]  Alan K. Knapp,et al.  Photosynthetic and Water Relations Responses to Elevated CO2 in the C4 Grass Andropogon gerardii , 1993, International Journal of Plant Sciences.

[23]  B. Kimball.  Impact of carbon dioxide, trace gases, and climatic change on global agriculture. , 1990 .