Historically, maps have been drawn based on primary use areas, showing broad geographic areas, e.g. Compendium of Common Forages maps within Forages: An Introduction to Grassland Agriculture, 7th ed. (2018), John Wiley & Sons, Inc.
More highly detailed maps, based on quantitative climatic and soil factor tolerances and using GIS spatial grids, provide information on where species are suitable for a variety of intended uses.
The following collection of maps were developed by a group of Oregon State University scientists, using the PRISM-generated collection of climate factor grids and the NRCS soil characteristics database.
Suitability curves were developed for each clover species for three climate variables (average annual precipitation, average July maximum temperature, and average annual extreme low temperature) and three soil variables (drainage class, pH, and salinity). For each variable and each species, the curves were fit using estimated yield data across the full range of values for the given variable.
The coefficients for the model equations were applied to spatial data layers representing each climate and soil variable, resulting in spatial outputs of percent yield for each of the clover species and each climate and soil variable. The percent yield layers were then classified into four suitability classes, as follows:
100%-75% - Suitable
75%-50% - Moderately suitable
50%-25% - Marginally suitable
25-0% - Not suitable
Finally, three "hybrid" suitability layers were produced for each clover species based on combinations of 1) the three climate variables, 2) the three soil variables, and 3) all six climate and soil variables together. These combined suitability layers were created by selecting for each location the lowest suitability value of the included variables, with the idea that the overall suitability for a species will be limited by the most restrictive factor.
QUANTITATIVE TOLERANCES GIS-BASED MAPS
The contiguous USA
Climate Factors
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Soil Factors
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Combined Factors
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Minimum Temperature

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pH

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All climate and soil factors (most limiting)
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Maximum Temperature

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Drainage

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All soil factors (most limiting)
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Precipitation

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Salinity

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All climate factors (Most limiting)
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