241 Cool Season Crop Production Trends: a Possible Signal for Global Warming A.W. McKeown Department of Plant Agriculture, University of Guelph, Simcoe, Ontario Canada J. Warland Department of Land Resource Science, University of Guelph, Guelph, Ontario Canada M.R. McDonald Department of Plant Agriculture, University of Guelph, Kettleby, Ontario Canada Keywords: cool season vegetables, climate, yield, quality Abstract 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 o 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. INTRODUCTION Ontario’s average yearly temperature is 0.5 to 0.6 o C warmer now than in 1900 (Smith et al. 1998). Few studies exist on long term yields, climate or potential effects of climate warming on vegetables. Historical yield data might be used to estimate past climate effects and gain some insights on potential effects of Greenhouse warming. Nieuwhof, (1969) cited work from the early 1900's showing that cabbage(Brassica oleracea var capitata L.) yield was related to amounts of precipitation in the growing season. Field crops were studied using historical data back to 1895, and precipitation was a limiting factor (Andresen et al., 2001). El- Nino Southern oscillation (ENSO) events were shown to affect corn (Zea mays L.) yields primarily due to changes in precipitation (Phillips et al. 1999). Florida and Texas vegetable production was also shown to be effected by ENSO events (Park et al., 2002). Additionally, vegetables are generally regarded as shallow rooted, and more prone to heat/drought stress than deeper rooted field crops (Stone and Rowse 1982). Soil compaction is known to reduce yields (Wolfe et al. 1995) and interact with climate especially under wet conditions (Morse 1993). There are few growth models available for studies on vegetables similar to those for cereals (Andresen et al., 2001; Phillips et al. 1999). Montieth, (1981) found that cool season cereal yields were higher in cool areas than warmer ones in the UK and we can surmise that this effect would be similar for warming climates. Higher yields from climate warming and Proc. XXVI IHC – Sustainability of Horticultural Systems Eds. L. Bertschinger and J.D. Anderson Acta Hort. 638, ISHS 2004 Publication supported by Can. Int. Dev. Agency (CIDA)