Carbon dioxide assimilation by a wetland sedge canopy exposed to ambient and elevated CO2: measurements and model analysis

Abstract

Summary The wetland sedge Scirpus olneyi Gray displays fast rates of CO2 assimilation and responds positively to increased atmospheric CO2 concentration. The present study was aimed at identifying the ecophysiological traits specific to S. olneyi that drive these CO2‐assimilation patterns under ambient and elevated CO2 conditions. The net ecosystem exchange (NEE) of CO2 between S. olneyi communities and the atmosphere was measured in open‐top chambers. We developed a new mechanistic model for S. olneyi communities based on published ecophysiological data and additional measurements of photosynthetic parameters. Our NEE measurements confirmed that S. olneyi communities have a high rate of summertime CO2 assimilation, with noontime peaks reaching 40 µmol CO2 m−2 ground s−1 on productive summer days, and that elevated CO2 increased S. olneyi CO2 assimilation by c. 35–40%. Using S. olneyi‐specific ecophysiological parameters, comparison with measured NEE showed that the model accurately simulated these high rates of CO2 uptake under ambient or elevated CO2. The model pointed to the Rubisco capacity of Scirpus leaves associated with their high total nitrogen content as the primary explanation for the high rates of CO2 assimilation, and indicated that the vertical‐leaf canopy structure of S. olneyi had comparatively little influence on CO2 assimilation.