C3–C4 composition and prior carbon dioxide treatment regulate the response of grassland carbon and water fluxes to carbon dioxide
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- 1Plants usually respond to carbon dioxide (CO2) enrichment by increasing photosynthesis and reducing transpiration, but these initial responses to CO2 may not be sustained.
- 2During May, July and October 2000, we measured the effects of temporarily increasing or decreasing 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).
- 3The 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.
- 4The 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.
- 5For 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.