CO2 environment, microclimate and photosynthetic characteristics of the moss Hylocomium splendens in a subarctic habitat.

In order to document the natural CO2 environment of the moss Hylocomium splendens, and ascertain whether or not the moss was adapted to this, and its interactions with other microenvironmental factors, two studies were carried out. Firstly, the seasonal variations of CO2 concentration, photosynthetically active radiation (PAR), tissue water content and temperature were measured in the natural microenvironment of H. splendens in a subarctic forest during the summer period (July-September). Secondly, the photosynthetic responses of the species to controlled CO2 concentrations, PAR, temperature, and hydration were measured in the laboratory. CO2 concentrations around the upper parts of the plant, when PAR was above the compensation point (30 μmol m-2 s-1), were mostly between 400 and 450 ppm. They occasionally increased up to 1143 ppm for short periods. PAR flux densities below saturating light levels for photosynthesis (100 μmol m-2 s-1), occurred during 65% (July), 76% (August) and 96% (September) of the hours of the summer period. The temperature optimum of photosynthesis was 20° C: this temperature coincided with PAR above the compensation point during 5%, 6% and 0% of the time in July, August and September, respectively. Optimal hydration of tissues was infrequent. Hence PAR, temperature and water limit CO2 uptake for most of the growing season. Our data suggest that the higher than normal ambient CO2 concentration in the immediate environment of the plant counteracts some of the limitations in PAR supply that it experiences in its habitat. This species already experiences concentrations of atmospheric CO2 predicted to occur over the next 50 years.