20 BULLETIN 820, U. S. DEPARTMENT OF AGRICULTURE« 



stagnation is not very serious.^ Under the shade of mature stands 

 few jack pine seedlings appear, for their establishment and growth 

 depend on their obtaining abundant light. 



The rate of growth of seedlings, especially seedlings of intolerant 

 species, has much to do with the success or failure of the species, 

 since it determines the amount of light they will receive in compe- 

 tition with other vegetation. Compared with other northern pines, 

 jack pine grows rapidly, sometimes 6 inches or more the first year, 18 

 inches the first two years, and 3 feet the first three years. ^ On the 

 the average, 1-year seedlings are 2 inches, 2-year-old seedlings 6 

 inches, and 4-year seedlings from 1 to 3 feet high. These last are 

 not always high enough, however, to compete successfully with 

 hardwood seedlings, brush, and weed growths of many kinds which 

 are abundant on good soils. 



SUSCEPTIBILITY TO INJURY. 



FIRE. 



Up to the age of 60 or 70 years fire is the greatest source of dam- 

 age to jack-pine stands. These stands, occurring as they do on 

 dry, sandy soils, are especially liable to fire damage after leaf litter 

 and dead twigs have accumulated for several years. Fires in dense 

 young stands often become ci'own fires, which burn with great fierce-, 

 ness and destroy the entire stand. In older and more open stands 

 ground fires occur, which kill a few of the trees and lessen the vitality 

 and resistance of those left. 



Jack pine resists fire better than Norway and white pines because 

 it grows more rapidly in diameter and height during its early years, 

 and the young trees of a given age have thicker bark. 



WIND, FROST, AND SNOW. 



Jack pine trees do not often suffer from wind-throw where soil con- 

 ditions have helped to develop a deep-going root system. On shal- 

 low and wet soils, hovv^ever, the root system is flat and the tree is rery 

 susceptible to wind-throw. Where the trunks are tall and slender 

 or where the trees have been weakened by insects or disease, severe 

 storms cause considerable breakage. 



Jack pine is not especially susceptible to damage from frost and 

 snow, but heavy snowfalls and ice storms cause some injury to young 

 slender saplings growing in dense stands. In the plantations of 

 Nebraska and Kansas jack pine sometimes suffers from those snow 

 and hail storms which come late in May and break off the leaders 

 or new terminal shoots. 



1 Supervisor H. C. Hilton, of the Michigan National Forest, reports a case of complete stagnation of jack 

 pine, a 20-year old stand containing from 50,000 to 100,000 trees per acre of an inch or less in diameter. Such 

 cases, however, are very exceptional. 



2 Jack pine grovtrs 2 to 3 whorls of branches each year (see fig. 4), while white and Norway pines only grow 

 one. 



