Extrapolating gross primary productivity from leaf to canopy scale in a winter wheat crop

The objectives of this paper are to determine winter wheat gross primary productivity (GPP) by extrapolating to the canopy scale measurements of photosynthetic assimilation made at the leaf scale, to identify the uncertainties inherent in this method and to quantify their impact on GPP predictions. Crop development monitoring and photosynthesis measurements were conducted between I May and 19 July 2004 at the Carboeurope site of Lonzee, Belgium, with a portable porometer Li-Cor 6400. The model divided the canopy into 10 layers in which assimilation was computed on the basis of incident radiation and of assimilation to light response curves calibrated in the field. The model also took account of photosynthesis of stems and ears, senescent organ distribution and response of assimilation to leaf to air vapour pressure difference. Model estimates were compared with eddy covariance measurements performed at the site during the same period. The best agreement (regression slope = 1.13, R-2 = 0.94) between the two estimates was obtained by postulating a concentration of the senescent organs in the canopy bottom and a stem assimilation rate equal to 63% of the leaf assimilation. This ratio was found compatible with further leaf scale measurements. This led to a GPP of 1570 g C m(-2) during the crop development and maturation periods. The sensitivity analysis revealed that the main sources of uncertainties were linked to the photosynthetic capacity of the stems (an increase of 40% in the initial GPP) and ears (an additional increase of 15%) and to the senescent organ spatial distribution (impact of 79%). An overestimation of GPP during spring (270 g C m(-2)) was also observed, due to assimilation reduction at low temperature not be accounted for. Apart from this, the impact of the A-Q curve parameter uncertainties was found to be limited (impact on GPP always lower than 4%). (c) 2007 Elsevier B.V. All rights reserved.