CHAPTER X 



WESTERN YELLOW PINE TYPE 



General Conditions. — The timber type in which western yel- 

 low pine is the dominant species is found all the way from the 

 Canadian line to the Mexican border. Patches of greater or less 

 extent occur in all the states west of the Great Plains. Large 

 solid bodies do not, however, occxu: in Nevada. 



With such a range the climate of_ the type must necessarily 

 vary within wide limits. For example, the annual average 

 temperature varies from 45 degrees to 60 degrees while the num- 

 ber of months during which there is no growth on account of cold 

 range from three in southern New Mexico and Arizona to seven 

 in northern Montana. In fact the only climate factor which is 

 fairly uniform is the annual precipitation. This never exceeds 

 25 inches and has a normal range between 15 and 20 inches. Its 

 distribution is not, however, nearly so regular. In the southwest 

 44 per cent of the annual precipitation comes in the summer 

 months of June, July and August while in the northwest only 

 25 per cent falls during the same period. Such a wide range of 

 climatic conditions has naturally restilted in considerable varia- 

 tion in the form of the species. In the warmer and drier parts of 

 its range where the scant rainfall is barely sufficient the cones are 

 small and the seeds encased in thick scales. Where there is more 

 abundant moisture the cones are larger and do not retain the 

 seeds so tenaciously. 



The yellow pine type is confined to the foothill region of the 

 Rocky Mountains and hence occurs on the lower slopes of the 

 higher mountains. These are usually alluvial in origin or at 

 least covered with loam, sand or gravel to a good depth. The 

 exact effect of the soil upon western yellow pine has not been 

 definitely determined because it grows thriftily on a variety of 

 soils provided they are well drained. Swampy or alkaline soils 



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