C₃-C₄ Composition and Prior Carbon Dioxide Treatment Regulate the Response of Rrassland Carbon and Water Fluxes to Carbon Dioxide

• Published in: Functional ecology 2007-02, Vol.21 (1), p.11-18
• Main Authors: Polley, H. W., Dugas, W. A., P. C. Mielnick, Johnson, H. B.
• Format: Article
• Language: English
• Subjects: Acclimatization; Carbon dioxide; Ecosystem Ecology; Grasses; Growing seasons; Human ecology; Leaf area; Net ecosystem exchange; Photosynthesis; Plants; Terrestrial ecosystems
• ISSN: 0269-8463; 1365-2435

1. Plants usually respond to carbon dioxide (CO₂) enrichment by increasing photosynthesis and reducing transpiration, but these initial responses to CO₂ may not be sustained. 2. During May, July and October 2000, we measured the effects of temporarily increasing or decreas CO₂ concentration by 150-200 µmol 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 three growing seasons to a CO₂ gradient from 200 to 560 µmol mol⁻¹. Grassland grown at subambient CO₂ (< 365 µmol mol⁻¹) was exposed for 2 days to an elevated CO₂ gradient (> 365 µmol mol⁻¹). Grassland grown at elevated CO₂ was exposed for 2 days to a subambient gradient. Our objective was to determine whether growth CO₂ affected the amount by which grassland NEE and ET responded to CO₂ switching (sensitivity to CO₂). 3. The NEE per unit of leaf area was greater (16-20%) and ET was smaller (9-20%), on average, at the higher CO₂ concentration during CO₂ switching in May and July. The amount by which NEE increased at the higher CO₂ level was smaller at elevated than subambient growth concentrations on both dates, but relationships between NEE response and growth CO₂ were weak. Conversely, the effect of temporary CO₂ change on ET did not depend on growth CO₂. 4. The ratio of NEE at high CO₂ to NEE at low CO₂ during CO₂ change in July increased from 1·0 to 1·26 as the contribution of C₃ cover to total cover increased from 26% to 96%. Conversely, in May, temporary CO₂ enrichment reduced ET more in C₄- than C₃-dominated grassland. 5. For this mesic grassland, sensitivity of NEE and ET to brief change in CO₂ depended as much on the C₃-C₄ composition of vegetation as on physiological adjustments related to prior CO₂ exposure.