Temperature variability and the yield of annual crops

Global production of annual crops will be affected by the increases in mean temperatures of 2-4 degreesC expected towards the end of the 21st century. Within temperate regions, current cultivars of determinate annual crops wi ll mature earlier, and hence yields will decline in response to warmer temp eratures. Nevertheless, this negative effect of warmer temperatures should be countered by the increased rate of crop growth at elevated atmospheric C O2 concentrations, at least when there is sufficient water. Of more importa nce for the yield of annual seed crops may be changes in the frequency of h ot (or cold) temperatures which are associated with warmer mean climates. T he objectives of this paper are to review evidence for the importance of va riability in temperature for annual crop yields, and to consider how the im pacts of these events may be predicted. Evidence is presented for the impor tance of variability in temperature, independent of any substantial changes in mean seasonal temperature, for the yield of annual crops. Seed yields a re particularly sensitive to brief episodes of hot temperatures if these co incide with critical stages of crop development. Hot temperatures at the ti me of flowering can reduce the potential number of seeds or grains that sub sequently contribute to the crop yield. Three research needs are identified in order to provide a framework for predicting the impact of episodes of h ot temperatures on the yields of annual crops: reliable seasonal weather fo recasts, robust predictions of crop development, and crop simulation models which are able to quantify the effects of brief episodes of hot temperatur es on seed yield. (C) 2000 Elsevier Science B.V. All rights reserved.