Effect of variations of PAR on CO2 exchange estimation for Scots pine

A set-up with 161 photosynthetically active radiation (PAR) sensors was used to investigate spatio-temporal variations of irradiance for five horizontal arrays within a Scots pine canopy. The measured PAR was converted to CO₂ exchange using of a shoot-scale photosynthetic response curve and the vertical distribution of the needle area. The net ecosystem exchange was simultaneously measured by the eddy covariance technique. The effect of spatial and temporal averaging of the PAR values and the number of sensors were analysed under different conditions as regards cloudiness and the shading by the foliage. In 1/2 h CO₂ exchange values for the entire canopy, a maximum overestimation of 30% resulted from a spatial averaging over horizontal arrays of 2–5 m and occurred under clear-sky conditions and significant foliage shading. Under partly cloudy conditions, the largest overestimation occurred for a case of little shading and the inaccuracy resulting from 1/2 h temporal averaging exceeded that of spatial averaging.