140 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE. 



Hence spruce seeds have ample time ior germinating and are in no 

 haste to extend their roots deeply, often only penetrating the humus 

 layer and establishing contact with the mineral soil by the end of 

 the first summer. The rapid drying of a well-insolatea soil, on the 

 other hand, demands prompt germination and rooting of the seedling. 

 Yellow-pine seeds do not germinate more promptly than those of 

 the other Rocky Mountain species, but the seeds are large, the seed- 

 lings large and strong, and the root is often extended 3 or 4 inches 

 into the soil before the cotyledons appear above ground. Quite often 

 the roots of yellow-pine seedlings must extend to a depth of a foot 

 and below most of the grass and weed roots before a stable supply 

 of moisture is assured. 



In all probability moisture becomes an important factor in limit- 

 ing the downward extension of the yellow-pine forest, though here 

 we have no proof that excessive temperatures may not as often be 

 the immediate cause of the death of seedlings. Moisture is probably 

 controlling where other vegetation is already established; heat is 

 probably controlling on denuded sites. 



CONCLUSIONS. 



Each site in the various forest types of a mountain region such as 

 has been studied and reported in this bulletin by means of the several 

 stations presents a unique complex of all the conditions which com- 

 prise the atmospheric and soil habitat. Two sites may not vary in 

 a single one of the many conditions which are enumerated as wind, 

 humidity, temperature, soil moisture, etc., without having some 

 effect on the vegetation. In fact when the reproduction of a small 

 forest area is studied in detail most surprising and often unaccount- 

 able differences in the number, size, and species of the seedlings 

 growing thereon may be noted in different parts of the area, even 

 though it be only a few yards in extent. It seems, therefore, almost 

 hopeless to attempt to define every factor which is accountable for 

 vegetation of a given type. The best that can be hoped for is that 

 the maxima or minima, or, in other words, the extreme conditions 

 of any nature which a given species will tolerate, may be depicted 

 by a study of the present kind, or by a series of such studies properly 

 correlated. To a limited extent this has been accomplished in the 

 discussion of each factor 'and in the recapitulation. Since it is a 

 common human desire to have complex matters made to appear 

 simple a single generalization may be indulged in, as a kind of sum- 

 mary, after which the exceptions may be noted and the specific 

 problems of each species may be defined as they are seen at this 

 stage. 



A review of the facts that have been presented leaves little doubt 

 that the several tree species of the central Rocky Mountains are con- 

 trolled in their distribution almost wholly by the degree of insolation 

 of the site, the resultant temperatures, and the closely related surface 

 moisture conditions. 



In a general way the forest zones correspond with air-temperature 

 zones, and the considerable differences between the air temperatures 

 of north and south exposures at the same elevation might explain the 

 corresponding differences in the forest types. However, even when 

 air temperatures are measured close to trie ground, the critical differ- 

 ences between sites are not brought out. Air moves so freely from 



