Drought and Heat Delimiters

The suitability map for the United States (Fig. 3-8a) differs significantly from the traditional map (Fig. 3-1) with respect to increased detail and expansion of areas shown to be suitable.  The major "fescue belt" is retained, however, in the east-central and southeastern United States. This is consistent with the predominant role of tall fescue as a cool-season forage in the humid transition zone (West and Waller, 2007). Smaller ranges of moderate to high suitability occur in the northern Rocky Mountains, Pacific Northwest, and Southwest.

The map was refined through numerous iterations to be consistent with experts' observations. The suitability of tall fescue in most of the western states is limited by low precipitation and high potential evapotranspiration, but substantial additional land areas would be suitable if irrigation were economically feasible. Among the cool-season perennial grasses commonly cultivated as forages, tall fescue exhibits the greatest ability to grow, or at least survive, during periods of high evaporative demand, due in part to its deep and robust root system (Sleper and West, 1996).

PRISM-Internet Mapserver Species Suitability maps rely on accurate and meaningful climatic and soil delimiters, and proper station-weighting. Gaps in our knowledge and incomplete consideration of all factors driving growth and survival can cause anomalies in comparison with known zones of adaptation and suitability. For example, the U.S. map shows moderate to high suitability in the Gulf Coast from southeast Texas to the Florida Panhandle, whereas in practice tall fescue would be marginally suited at best in that area because of sustained high summer temperatures and humidity, and a preponderance of highly competitive, perennial, subtropical grasses. Additional functions in future iterations of the Mapserver application, to allow for inputs of high temperature duration and for disease susceptibility, would help address this inconsistency.

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Fig. 3-8a. Tall fescue suitability map for US using all climatic and edaphic factors.


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