Ecological Scaling of Climate Variables

To scale the climate data to ecological regions, each of the ecological categories defined by Bailey (1996) (section, province, division, and domain) was associated with each of the 1055 counties in the NCR. Table 4.2 shows the data points, located at the county centers, in the database of daily weather and annual corn yields in the Bailey ecoregion classification scheme over the 20-year period. This analysis focuses on those ecoregions associated with the LTER in southwest Michigan (Kala-mazoo County). The lighter areas in the maps shown in figure 4.2 represent these ecoregions. This hierarchy provides a comparative method to evaluate weather and corn yield patterns associated with the measurements made at the KBS LTER. Although the KBS-LTER was not initiated until 1989, this scaling provides a template for evaluation of the performance of corn yields under a variety of ecological management regimes.

Figure 4.4 shows the monthly patterns of mean temperature (C), precipitation (mm), Heat Units (>10 C), and HPR for the period 1972-1991. Each variable is presented for each of the five levels of regional hierarchy corresponding to the ecological scales plus the county scale (see table 4.2 and figure 4.2). Mean temperature and degree-days (>10°C), as expected, follow a regular pattern with peak occurrence in July (month 7), whereas the variability in precipitation, and hence HPR, is more stochastic.

Table 4.2 Hierarchical organization of Bailey ecoregions and associated weather station locations in the North Central Region

Bailey Ecoregion

Ecoregion Code

Ecoregion Name

Counties (Weather)

County

26077

Kalamazoo

1

Section

200010

South Central Great Lakes

42

Province

2000

Eastern Broadleaf Forest

361

Division

200

Hot Continental

399

Domain

2

Humid Temperate

842

MONTH MONTH

Figure 4.4 Monthly patterns of mean temperature (C), precipitation (mm), Heat Units (>10C), and HPR for the period 1972-1991.

The HPR exhibited the most irregular patterns where the single county observations (Kalamazoo County) had the highest value of HPR, followed by the Section (see figure 4.4d). Because of the focus on the evaluation of the1988 drought, we anticipated that the HPR values would be most indicative of potential stress to plant communities. The heat/precipitation ratio can be a useful indicator of potential stress to biotic communities.

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