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

T Vesala*, T Markkanen, L Palva, E Siivola, S Palmroth, Pertti Hari

*Corresponding author for this work

Research output: Contribution to journalArticleScientificpeer-review

32 Citations (Scopus)


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 1/2h 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. (C) 2000 Elsevier Science B.V. All rights reserved.

Original languageEnglish
Pages (from-to)337-347
Number of pages11
JournalAgricultural and Forest Meteorology
Issue number4
Publication statusPublished - 25 Feb 2000
MoE publication typeA1 Journal article-refereed


  • irradiance distribution
  • PAR
  • photosynthesis
  • Scots pine
  • net ecosystem exchange
  • eddy covariance


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