Timing of Short Term Climatic Events

The chapters in this section support the hypothesis that the timing of short-term events is important in partially determining the kind and degree of ecosystem re sponse that might occur. At the Arctic LTER site, early snow melt is an issue. Hob-bie et al. report that every few years soil warming, caused by early snow melt or by warm summers, results in both a synchronous flowering of arctic cotton and an increase in leaf area that occurs across the entire area of northern Alaska. The corn study particularly illuminates the importance of timing of soil moisture deficit conditions in determining the degree of impact of drought on agriculture. During May and June, seedling and early root growth are vulnerable to periods without moisture, especially if temperatures are high. The severity of the stress in the early growing season and subsequent loss in productivity was due to the inability of young seedlings to tap soil moisture reserves prior to a stress period. In another instance, Gage reports that depending on timing and severity, drought 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. Timing is important in the forest ecosystem as well. Kloeppel and his colleagues claim Southern Pine Beetle (SPB) populations, as with most insect species, are cyclic, but the coincidence of SPB outbreak or epidemic populations and stress brought on by severe and sustained drought can have monumental impacts on pine populations.

In subsequent chapters, we will see several other examples of the importance of timing of a climatic event at other time scales. This issue relates, in part, to the framework question, What preexisting conditions will affect the impact of the climatic event or episode? In many cases the exact state or stage of seasonal development of an ecosystem will create a preexisting condition that will control the intensity of the effect of the climatic event or episode.

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