Mitteilungen der Gesellschaft für Pflanzenbauwissenschaften Band 23

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In contrast to leaf measurements, canopy measurements of carbon assimilation
and transpiration were highly variable across the fields (Fig. 1). Initial analyses
suggest that the observed spatial and seasonal variability was mainly caused by the
heterogeneity in leaf area indexes and less by gas exchange rates per unit leaf area.
However, both crops differed in their reaction to drought stress: while wheat
responded mainly through irreversible reduction in green leaf area, the canopy
assimilation rates of sugar beets decreased only temporarily with no observed effects
of LAI. The rational is temporal stomatal closure due to high temperatures and high
vapour pressure deficit in beets. In wheat, a combination of heat and drought stress
as well as resulting decrease in nitrogen uptake lead to a smaller green leaf area and
an earlier senescence.
Fig. 1: Daily canopy CO2-assimilation during the growing season of three plots in
winter wheat (A – C) and sugar beet (D – F) fields.
The results of flux measurements revealed that canopy resistance is highly
variable within a field and during the growing season. These dynamics have to be
considered in modelling soil-vegetation-atmosphere processes in agricultural
landscapes.
Acknowledgements
This work has been funded by the DFG under the SFB TR32 (Patterns in Soil-
Vegetation-Atmosphere-Systems) (TRR 32/1 2010, TRR 32/2).