Quantifying spatial variability of photosynthetically active radiation within canopies using a multipoint measuring system

A multipoint measuring system of photosynthetically active radiation (PAR) was used to investigate the effect of the number and position of measuring points in determining the dependence of the photosynthetic rate on PAR with in a Scots pine canopy. A regular grid of 400 measuring points was incorpor ated into a cuvette to observe the distribution of PAR falling on the needl es of a twig along with simultaneous measurements of the CO2 exchange. Meas urements were made in the middle layer of the crown, at a level characteris ed by a high spatial and temporal variability in PAR. The results show that measuring points covering the entire needle region in an even manner are n eeded for accurately quantifying PAR and reliable estimation of the photosy nthetic rate. The positioning of the measuring points is of primary importa nce: even a large number of measuring points can lead to high errors if the positioning is not representative in relation to the twig. (C) 1998 Elsevi er Science B.V. All rights reserved.