46 BULLETIN 460, U. S. DEPARTMENT OF AGRICULTURE. 



Moreover, the habit this tree has of long retaining its cones in a 

 closed state enables the species to perpetuate itself even after severe 

 fires have killed the mother trees, because, if not too badly burned, 

 many of the charred cones then open and liberate their seed. Vast 

 areas in the central and eastern parts of its range once heavily 

 stocked with Pinus strobus and P. resinosa are now re-covered with 

 jack pine, which is often the only growth on these barrens. 



LONGEVITY. 



Jack pine is comparatively short-lived, the largest trees probably 

 not exceeding from 125 to 150 years of age. As a rule, however, the 

 age of such trees can not be determined accurately, because they are 

 decayed at the heart. The oldest sound trees ordinarily cut rarely 

 exceed 110 years in age. Trees from 6 to 15 inches in diameter grown 

 in an open stand are from 25 to 85 years old, while 6-inch trees grown 

 in a dense stand are from 70 to 75 years old. 



KEY TO SPECIES. 



Leaves borne singly (or some of them 2 in a bundle) Pinus monophyUa 



Leaves borne in bundles of 2 : 



Cones armed with delicate prickles Pinus contorta 



Cones without prickles — 



Cone scales widely open after the cones are ripe Pinus edulis 



Cone scales mostly closed, or only part of them open, after the 



cones are ripe Pinus banksiana 



Leaves borne in bundles chiefly of 3 (sometimes in bundles of 2, 4, or 5) : 

 "With a prominent basal sheath — ■ 



Cone scales armed with stout prickles — ■ 



Leaves mostly from about 4 to 7 inches long Pinus ponderosa 



Leaves mostly from about 8 to 15 inches long Pinus apacheca 



Without a basal sheath : 



Cones borne on long, slender stems Pinus chihuahuana 



Cones borne on very short stems Pinus cembroides 



Leaves borne in bundles of 5 and cones with prickles : 



Cone scales armed with long, delicate prickles Pinus aristata 



Cone scales armed with short stout prickles Pinus arizonica 



Leaves borne in bundles of 5 and cones without prickles : 

 Cones opening at maturity and shedding their seeds — 



Cone scales strongly reflexed at the tips Pinus strobiformis 



Cone scales mostly not reflexed at their tips — 



Seeds with long, well-defined wings Pinus monticola 



Seeds with very short rudimentary wings Pinus flexilis 



Cones remaining closed at maturity Pinus albicaulis 



