In situ stomatal responses to long-term CO2 enrichment in calcareous grassland plants

A calcareous grassland community growing under full season CO? enrichment at low altitude in the Swiss Jura mountains was investigated for diurnal and seasonal variations of leaf diffusive conductance. A new CO2 enrichment method (Screen aided CO2 control, SACC) permitted irt situ leaf porometry under natural climatic conditions without disturbance of plants. At 600 ppm CO2, leaf conductance in the dominant species, Bromus erectus (a species so far not showing a growth response to elevated CO2) was reduced to half the values measured in controls. Tn contrast, leaf conductance in Carex flacca, a species of low cover (the only species so far exhibiting a dramatic growth stimulation by CO2 fertilization) remained almost unaffected by elevated CO2. Sanguisorba minor; Plantago media, and Cirsium acaule showed intermediate responses. Trifolium montanum, studied only on a single day, showed a reduction like Bromus. Differences between treatments were largest under humid conditions and disappeared during dry periods. In none of the species studied did stomatal density or stomatal index differ between treatments. A parallel investigation of whole ecosystem evapotranspiration indicated only small (< 10%) and non significant CO2 responses, suggesting that both aerodynamic effects at the canopy level and a great interspecific variation of leaf level responses overshadow the clear CO2 response of Bromus stomata. The different stomatal responses to CO2 enrichment are likely to alter species specific water consumption, and may thus affect community structure in the long run.