C 3 – C 4 composition and prior carbon dioxide treatment regulate the response of grassland carbon and water fluxes to carbon dioxide

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