Sensitivity of field-scale winter wheat production in Denmark to climate variability and climate change

A simulation model of the direct effects of climate on winter wheat product ion and grain yield is presented. The model was calibrated using data from field experiments in Denmark. The model was validated using data from near optimally managed experimental plots with winter wheat from The Netherlands and Denmark. The model was further evaluated using data from 1971 to 1997 for 7 sites in Denmark. The model explained from 0 to 20% of the variation in detrended observed yields, depending on soil type. A regression analysis of observed yields against monthly climate data showed a positive effect o f temperature in October, November and January on grain yield, a positive e ffect of radiation in April and a strongly negative effect of precipitation in July. Only the positive effect of radiation in April was predicted by t he simulation model, probably because the indirect effects of climate are n ot taken into account by the model (e.g. effects of rainfall on lodging or Septoria disease). The sensitivity of simulated grain yield to changes in m ean temperature, temperature variability, precipitation, length of dry spel ls and CO2 concentration was analysed for 4 soil types using generated clim ate data from 1 site in Denmark. Yield decreased with increasing temperatur e. This decrease was strongly non-linear with temperature: change when usin g a fixed sowing date, but almost linear for the optimal sowing date. There was only a very small response to changes in temperature variability. Incr easing precipitation increased yields with the largest response on the sand y soils. Large changes in grain yield were also seen on sandy soils with ch anges in the length of dry spells. A comparison of the simulated responses to the direct effects of temperature and rainfall with those to the indirec t effects of these variables as estimated from the regression analysis show ed that the indirect and the direct effects had opposite effects and that t hey may almost cancel each other out. The simulated increase in grain yield due to increasing CO2 concentration in most cases exceeded the simulated r esponses to changes in climate variables.