Corn (Zea mays) is a crop central to the region's economy. Crop production in the NCR is an important resource as a national supply of food and by-products as well as a component of the nation's export marketing strategy. The role of weather as a cause of the variability of crop production at local, regional, and national scales is a subject of considerable concern. Variability in the annual yield of crops such as corn is a useful indicator of regional climate patterns because plant growth and biomass accumulation are primarily dependent on weather (temperature, precipitation, and solar radiation). Other causes of variability include soils (texture, water-holding
capacity) and technology (genetic manipulation, chemical subsidies, irrigation). In agricultural ecosystems, crop productivity or yield refers to the utilizable part of the plant (Tivy 1990). Four major ecological factors determine crop yield in agro-ecosystems: weather, water availability, "negative" biological factors (pests and diseases), and nutrients (Olson 1982; Tivy 1990).
Yield (t/ha) over time (1972-1997) for corn and the spatial distribution of average corn yield is shown in figure 4.1. The interannual variation is largely driven by meteorological factors, whereas the general increase in yield can be attributed to changes in technology (figure 4.1a). The average spatial distribution of corn yield (figure 4.1b) defines the boundaries of the Corn Belt and reflects a combination of the geographic distribution of prime soils and optimal climate for the growth and development of corn.
At regional scales (1,000,000 km2), climate is the main driving variable of the ecological system (Burke et al. 1991; Bailey 1996). Water is the single most impor tant limiting factors for crop yields worldwide (Tivy 1990). Water shortages cause varying levels of crop stress, contingent on the developmental stage of the plant (Doorenbos and Kassam 1979).
In 1988, an agricultural drought occurred in the North Central Region (Kunkel and Angel 1989; Kunkel 1992; Petersen et al. 1995; Zangvil et al. 2001). This one-year drought was a primary causal agent of a general crop failure in the Corn Belt. Corn yields were unusually low, resulting in a significant reduction in U.S. corn production. Regional mean corn yield in 1988 was 4.2 t/ha, approximately 2 t/ha below the previous and following years (see figure 4.1a). The drought was triggered by meteorological anomalies, including above-average temperatures associated with below-average rainfall that occurred unusually early in the growing season, compared with other years in the climate record.
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