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

A set-up with 161 photosynthetically active radiation (PAR) sensors was use d to investigate spatio-temporal variations of irradiance for five horizont al arrays within a Scots pine canopy. The measured PAR was converted to CO2 exchange using of a shoot-scale photosynthetic response curve and the vert ical distribution of the needle area. The net ecosystem exchange was simult aneously measured by the eddy covariance technique. The effect of spatial a nd temporal averaging of the PAR values and the number of sensors were anal ysed under different conditions as regards cloudiness and the shading by th e foliage. In 1/2h CO2 exchange values for the entire canopy, a maximum ove restimation of 30% resulted from a spatial averaging over horizontal arrays of 2-5 m and occurred under clear-sky conditions and significant foliage s hading. Under partly cloudy conditions, the largest overestimation occurred for a case of little shading and the inaccuracy resulting from 1/2 h tempo ral averaging exceeded that of spatial averaging. (C) 2000 Elsevier Science B.V. All rights reserved.