A.W. McKeown, J. Warland, M.R. McDonald, C.M. Hutchinson
There has been much discussion about the potential effects of climate warming on food production. Climate models can be used to predict the effect of global warming, but is there current evidence in yield data to suggest an effect of climate change? Cool season vegetables could be good indicators of changes, since they can be more adversely affected by temperature extremes than some warm season crops such as corn. Thus, effects from climate warming may develop first in these crops. The average yield per hectare for several cool season vegetables in Ontario, including broccoli (Brassica oleracea var italica L.), cabbage (Brassica oleracea var capitata L.), cauliflower (Brassica oleracea var botrytis L.), carrots (Daucus carota L.), onions (Allium cepa L.), and potatoes (Solanum tuberosum L.) was examined over the past 60 years. Overall, yields increased from the 1940's until the mid 1980's. Since then, yields per hectare have decreased and appear more variable in spite of modern cultivars and production practices. Yield of field tomatoes (Lycopersicon esculentum L) have increased. Coincidently, this has been the period of noticeably warmer climate. Are these changes in provincial yield data related to climate warming? Records in southern Ontario indicate a general increase in average air temperatures. Yield of cole crops, rutabaga and potatoes were found to decrease with warmer average temperature, number of days > 30 ° C, and with fewer days with precipitation. Higher temperatures probably will increase heat related quality disorders and possibly reduce vitamin content. Part of the decrease in yield might be a result of other factors, such as drought, soil compaction, or changes in insect and disease pressure. However, these would be expected to also change and interact with climate warming. Implications of these observations will be discussed.
McKeown, A.W., Warland, J., McDonald, M.R. and Hutchinson, C.M. (2004). COOL SEASON CROP PRODUCTION TRENDS: A POSSIBLE SIGNAL FOR GLOBAL WARMING. Acta Hortic. 638, 241-248
DOI: 10.17660/ActaHortic.2004.638.31
cool season vegetables, climate, yield, quality

Acta Horticulturae