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

Abstract

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 within a Scots pine canopy. A regular grid of 400 measuring points was incorporated into a cuvette to observe the distribution of PAR falling on the needles of a twig along with simultaneous measurements of the CO 2 exchange. Measurements were made in the middle layer of the crown, at a level characterised 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 needed for accurately quantifying PAR and reliable estimation of the photosynthetic rate. The positioning of the measuring points is of primary importance: even a large number of measuring points can lead to high errors if the positioning is not representative in relation to the twig.