10 FLORA OF THE YELLOWSTONE NATIONAL PARK. 



vegetation does not seem to be affected in the least by a tem- 

 perature of 10"^ below freezing, but a little later in the season 

 its effects are apparent, when the plants have lost much of their 

 vitalit3\ The change from the luxuriance of August to the 

 deca}^ of September is abrupt. Generally by Sept. loth snow 

 has fallen to lie upon the ground for a day or two. 



FORESTS. 



The Yellowstone Park lies in the Rocky Mountain belt of 

 coniferous forests, geographically termed the Interior Pacific, 

 and which trending northwestward, unites in northern Washing- 

 ton Territory with that of the Pacific coast, forming a broad 

 belt w^hich still farther north in British America merges into the 

 northwest extension of the Atlantic forest. The common and 

 most wide-spread tree of the Park is the Black Pine, Pinus 

 3Iurrayana, Balf. (P. contorta, Dough, var. llurr-ayana, Engelm.). 

 It is the only tree forming extensive forests to the exclusion of 

 other species. It reaches its greatest development on the dryer 

 plateaus between 7000 and 8000 feet, here forming 90 per cent, 

 of the forest. It is not generally over two feet in diameter, 

 with a height of 60-100 feet, and is found from the lowest alti- 

 tudes up to 9500 feet. Over the lower and dryer portions it is 

 found with the Red Fir (Pseudotsuga Bouglasii, Carr), and in 

 hio-her and moist situations with more or less Picea and Abies. 



It is the onl}^ tree here that seems to be able to reproduce 

 itself to any extent after having been destroyed by fire, doubt- 

 less owing to the protection afforded the seeds by the hard and 

 indestructible cones. In many burnt districts an almost impen- 

 etrable growth of 3'Oung trees have sprung up. Probably 65 

 per cent, of the whole forest area is composed of the Black 

 Pine. 



Pinus fiexilis, James, generally occupies the dry gravelly 

 rido-es from 7500 feet upward, especially above 8000 feet. At 

 low elevations it grows with the Black Pine and higher with 

 Picea and Abies. It is found as low as 6000 feet at the Mam- 

 moth Hot Springs with Juniperus Virginiana, these two species 

 forming exclusively the timbered portion of the formation. 



Pinas albicaulis, Englm. {P.Jiexilis,X3ii'. albicaulis, Englm.), 



