Ponderosa Pine in the Northern 

 Rocky Mountain-Intermountain Region 



Climate and Soil of the Habitat 



Ponderosa pine grows under a wide range of conditions typified by varying rainfall, 

 high temperatures and low humidities during the growing season, mountainous topography, 

 and great diversity of soil types and depth (Curtis and Lynch 1965). 



Climate. --In typical ponderosa pine habitats, average annual temperatures vary 

 from 40° F to 48° F and average annual precipitation from 12.7 to 26.8 inches (table 1) . 

 The average amount of precipitation from June through September depends on the locality. 

 Idaho City, Idaho, for example, receives one-third more precipitation per year than 

 Darby, Montana, but barely half as much during the June-September period. In southern 

 Utah, the June rainfall is sparse but increases during the following 3 months, almost 

 reversing the pattern of many other places. 



In certain places, such as central Idaho, the June-through-August climate is 

 severe. Forest temperature may be 110° F in the shade, relative humidity 3 percent, 

 and total July and August precipitation as little as 0.02 inch. 



Most sites are subject in the early or late growing season to soil moisture - 

 stress and high soil surface temperatures. Tree growth primarily depends on water 

 stored in the soil from melted snow and from spring rains. Summer precipitation, 

 whether light showers or high-intensity rainstorms, is probably of little use to tree 

 seedlings because of interception, surface evaporation, and runoff. Soil moistures 

 below the wilting point have been measured at a depth of 60 inches in the granitic 

 soils of central Idaho. Soils on south aspects dried out and reached the wilting 

 point sooner than on north aspects. 1 



Soil. --From northern Montana to southern Utah there is wide diversity of soil 

 origins and hence soil types. Parent material includes quartzite, argillite, schist, 

 basalt, andesite, granite, limestone, and sandstone. Probably the soils most subject 

 to drought are the coarse-textured, highly erosible granitic types of central Idaho 

 and southwestern Montana. The soils of western Montana on which ponderosa pine is 

 found are the Gray Wooded Great Soil Group (Cox 1957). They are characteristically 

 about 50 inches deep, with a pH of 6 and a "well-developed platy A 2 horizon, a blocky 

 B 2 horizon with a distinct accumulation of clay, and overall gray appearance." The 

 soils developed from argillites and quartzites tend to be shallow and coarse with low 

 water-holding capacity. The medium-textured soils that develop on morainic materials 

 are deeper and their moisture-holding capacity is higher. 



Unpublished data, Div. Watershed Manage. Res., Intermt. For. § Range Exp. Stn. 



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