Patterns of Drought During the Growing Season May August

To examine the annual patterns of HPR during the 20-year period of record, the average HPR was computed for May, June, July, and August, based on all 1055 locations in the climate database. These months were selected because they encompass months when plant stress will have a significant impact on plant productivity, particularly in agricultural crops. The monthly patterns of HPR, computed for May-August in the North Central Region, are provided in figure 4.5. In May, relatively high HPR values occurred in 1977, 1980, and 1988, indicating high potential

Figure 4.5 Calendar month time series of HPR for May-August 1972-1991.

plant stress early in the growing season. In June, the value of HPR was greater than the mean only in 1988. In July, values of HPR were above average in 1974, 1975, 1980, and 1983, whereas patterns of HPR in August were above average in 1973, 1976, 1983, and 1984. In 1988, both May and June exhibited high values of HPR. These early season periods are important for the seedlings, which require moisture for root growth and thus are more vulnerable to mortality because they have a lower tolerance for stress.

Further examination of the HPR, calculated at different spatial scales (county, section, division, and domain) reveals important patterns (figure 4.6). In figure 4.6a-d, the average HPR for the time interval (1972-1991) within each of the ecoregion classifications is contrasted with the HPR values in 1988. The cooccurrence of high values of HPR in May and June within each of the ecological classifications demonstrated a general phenomenon (May-June peak) across all levels of the spatial hierarchy. The HPR values were less extreme at the County level (Kalamazoo County) with a value of ~12 and greatest in magnitude (~ 22) at the ecological section level (South Central Great Lakes). Had this analysis not been restricted to ecoregions that are associated with the LTER, plant stress potential would have been even more extreme, particularly in western portions of the NCR where the ecoregion division is classified as short-grass prairie (Temperate Steppe).

Figure 4.6 Patterns of HPR at ecoregion scales (county, section, division, and domain) contrasting the average HPR for the time interval (1972-1991) with the HPR values in 1988.
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