Forest floor vegetation plays an important role in photosynthetic production of boreal forests

We estimated gross photosynthetic production (GPP) of the forest floor vegetation in a 40-year-old Scots pine stand in southern Finland with three different methods: measurements of CO₂ exchange of single leaves of field and ground layer species, measurement campaigns of forest floor net CO₂ efflux at different irradiances with a manually operated soil chamber, and continuous measurements of forest floor net CO₂ efflux with an automatic transparent chamber system. We upscaled the measured light response curves from the manual soil chambers using the biomass distribution of the forest floor species, a modelled seasonal pattern of photosynthetic capacity and a model of light extinction down the canopy. Leaf gas exchange measurements as well as measurements of net CO₂ efflux with the manual chamber indicated saturation of photosynthesis at relatively low (50–400 μmol m^?2 s^?1) light levels. Leaf and patch level measurements gave similar rates of photosynthetic CO₂ fixation per unit leaf biomass suggesting that reduction in photosynthetic production due to within-patch shading was small. Upscaling of photosynthetic production to the stand level and continuous measurements with the automatic soil chambers indicated that momentary photosynthetic production by the forest floor vegetation in the summer was typically about 2 μmol m^?2 (ground) s^?1. Cumulative upscaled GPP over the period of no snow (from 20 April to 20 November) in year 2003 was 131 g C m^?2. Continuous measurements with the automatic soil chamber system were in line with the upscaling, the cumulative GPP being 83 g C m^?2 and the seasonal pattern of photosynthetic rate similar to that of the upscaled photosynthesis.