Long Term Effects of Drought on Ecosystems

Several studies characterize the ecological effect of the drought of 1988. Tilman and Downing (1994) provided documentation of drought effects on plant communities other than those in agriculture. They characterized the influence of the 1988 drought on plants at the Cedar Creek LTER in Minnesota, and they measured the effects of drought and the dynamics of recovery from drought against a known baseline. Indeed, it was not until 1993, the fifth year after the 1988 drought and the twelfth year of the Cedar Creek LTER, that the effects of the drought on the species richness in successional grasslands were no longer discernible. However, the effects of the 1988 drought were still clearly evident in the oak savanna complex in 1993. About 30% of mature pin oaks died during the drought, compared to only 10% of bur oaks. Most of these dead trees are still standing. Tilman and Downing (1994) concluded that this major shift in oak species composition and reduction in oak canopy cover that will likely have an impact on these savanna ecosystems for decades to come.

In wooded vegetation a short-term (one-year) drought, depending on timing and severity, may cause plant mortality or weaken the plant system, predisposing the crop to insect herbivory or disease. In row-crop ecosystems, a short-term (one-year) drought can have a significant effect on yield, thus reducing productivity in the year of the anomaly. The long-term effects of a short-term drought on annual rotational agronomic systems are generally minimal. A drought may be local or regional. A drought that occurs over a large geographic region for a relatively short time period (several months compared with several years), such as the 1988 drought (May and June), can be economically devastating but not ecologically catastrophic. However, as Tilman and Downing (1994) illustrate, even a short-term drought can have cascading ecological consequences. Other unanticipated ecological consequences may occur. For instance, a drought may stimulate the need for irrigation as a means to override the effects of the drought. The addition of irrigation can affect ecosystem function by adding water to soils in dry ecosystems, thus stimulating changes in ecosystem flows and functions; irrigation can also affect the water table by mining groundwater and river systems to provide water to crops, particularly in ecosystems unaccustomed to large amounts of water. Crops such as corn that require high amounts of water. Corn grown in the western portion of the region is under stress in most years (see figure 4.7b), and this was especially the case in 1988 (figure 4.7a). Thus it is necessary to override the climate (via irrigation) each year to sustain "profitable" corn yields.

When a short-term drought transitions into a prolonged agricultural drought, this can have a cascade of effects by modifying ecosystem function. Poor agricultural practices and dry soil, coupled with wind, can erode soils, resulting in their redistribution. This agricultural management practice in the 1920s and 1930s caused soil erosion at regional scales, resulting in widespread ecological and economic consequences. Herbivore population fluctuations are associated with moisture and heat cycles. For instance, grasshopper egg survivorship and development of the eggs is intimately associated with soil moisture regimes and temperature (Mukerji and Gage 1978), and grasshopper populations respond to combinations of warmer than average spring temperatures (early hatch) and warmer than average temperatures in September, especially in northern regions. These longer periods of warm weather provide maximum potential for increased numbers of eggs to be laid in the soil. Because eggs can develop after oviposition in fall, they hatch earlier in spring, resulting in increased crop loss potential due to herbivory of crops during their early growth stages. Thus climate is a major contributing factor to pest outbreaks (Gage and Mukerji 1977).

Climate Variability in the North Central Region 71 Climate Change Implications

The drought that occurred in 1988 was clearly different from other droughts during the 20-year period analyzed (1972-1991). The 1988 drought occurred in May and June, whereas in other years, significant periods of stress occurred later in the crop-growing season (July-August). The severity of the stress and subsequent loss in productivity was due to the inability of young seedlings to tap soil moisture reserves prior to a stress period. Long periods of intense heat without precipitation during early phases of plant growth resulted in crop mortality. In the western Corn Belt (see figure 4.9b), rain-fed corn did not survive. However, the existence of the Ogallala Aquifer and the irrigation infrastructure developed to support row-crop agriculture in this short-grass prairie ecosystem enabled irrigation to override the stress induced by the 1988 drought in some parts of the region. In these areas, yields in 1988 were comparable to years when moisture was adequate for good corn production (see figure 4.9b, bottom left). However, the ecological costs and subsequent economic costs of depleting aquifer resources have not been fully evaluated. The 1988 drought was a one-year drought compared to the multiyear "dust bowl" drought that occurred in the 1930s. During the past 60 years, a significant multiyear drought has not occurred in the NCR, thus the probability of such a mul-tiyear drought is high.

There has been considerable debate regarding the effect that a changing climate will have on agricultural productivity in the United States. In an assessment of the adaptation of agriculture to climate change, Rosenberg (1992) argues that agriculture may be both negatively and positively impacted by a changing climate and that additional information is needed at regional scales to provide a more complete assessment. More recent assessments of the impacts of climate change (NAST 2000) address the potential effects on agriculture in the Midwest and the Great Plains. The NAST (2000) report suggests that crop productivity may increase as a result of enhanced CO2 in the northern reaches of the Midwest (eight of the NCR states) but may decline in southern portions of the Midwest. Four of the NCR's 12 states are also in the Great Plains. The NAST (2000) report predicts that higher evapotranspiration will result in decreased water availability, a problem for both the Midwest and the Great Plains.

The analysis presented in this chapter does not address whether climate change will have an impact on agriculture in the NCR. Instead, it shows that if significant changes in temperature and precipitation regimes occur, then organisms such as plants and insects, which depend on these variables for growth and survivorship, will respond based on ecological principles. An increase in temperature associated with a decrease in precipitation will result in a larger HPR and thus will cause an increase in stress to most biological communities, inducing them to adapt. More complete and higher quality data are needed to improve our ability to make more accurate ecological assessments. The LTER climate network, data archives, and associated ecological observations will satisfy part of that need.

Acknowledgments I express my appreciation to Manuel Colunga and Brian Napoletano for earlier reviews of the manuscript. Gene Safir made many helpful suggestions and contributions. Work associated with this chapter was accomplished with support from the NSF-LTER program (DEB 98-10220), the Michigan Agricultural Experiment Station, and the members of the USDA Regional Research Committee NC94. I also greatly appreciate the review, provision of additional literature, and suggested revisions to the chapter made by Peter Lamb.


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