Carbon dioxide exchange at leaf and canopy scale for agricultural crops inthe boreal environment

Within the framework of the Northern Hemisphere Climate processes Land-surf ace Experiment (NOPEX) a field experiment was conducted in May-June 1994 fo cusing on carbon dioxide exchange over agricultural land within the boreal zone. The carbon dioxide flux was monitored continuously above spring barle y and spring wheat crops. These measurements were supplemented by analysis of carbon dioxide fluxes at the leaf level. At the same time, canopy develo pment, the capture of photosynthetically active radiation (PAR) and atmosph eric fluxes of heat and water vapour, were also monitored. The carbon dioxi de fluxes on both leaf and canopy levels are presented in this paper. The r elationship between the canopy carbon dioxide flux and absorbed photosynthe tically active radiation (APAR) was found to be approximately linear over t he full range of APAR, giving a quantum yield of 0.025 and 0.027 mol CO2 pe r mol APAR for wheat and barley, respectively. In scaling the fluxes from l eaf to canopy level two approaches are examined. Both are based on the "big leaf" concept, using the light response of photosynthesis and the absorbed PAR. By comparing to the measured canopy fluxes it is found that the best agreement is obtained by scaling the light response of photosynthesis of th e leaves to the canopy using absorbed PAR directly. (C) 1998 Elsevier Scien ce B.V. All rights reserved.