JACK PINE. 7 



Most of the soil on which jack pine occurs has little or no accumula- 

 tion of humus. Accumulation of humus would mean, in some cases, 

 improved soil conditions and the ousting of jack pine l)y more per- 

 sistent, longer-lived species. 



Jack pine requires at least 10 per cent of water in the soil for its best 

 growth. On very dry soils young stands of jack pine may do well at 

 the outset, but the growth is not sustained, the trees do not reach 

 large size, and the stand becomes more open because of competition 

 for soil moisture. To a limited extent jack pine grows in swamps 

 also, but in a stunted form. 



Jack pine is intolerant of shade, somewhat more so than Norway, 

 and considerably more so than white pine. For this reason it is 

 unable to reproduce itself under the shade of mature trees. Jack pine 

 stands which are dense up to the age of from 30 to 50 years thin out 

 rapidly thereafter even on good sites. On lands suited to white or 

 Norway pine, but which become seeded up to dense stands of jack 

 pine following lumbering and fire, these species, through their 

 superior tolerance and persistence in growth, gradually seed in and 

 push themselves up through the jack pine and kill it out entirely, not, 

 however, before the jack pine has reached merchantable size. 



FORM AND DEVELOPMENT. 



In dense stands jack pine is a tall, slender tree, with a short crown' 

 and a long bole. In open stands it tends to develop a dome-shaped, 

 wide-spreadmg crown and a short bole. The dead branches are very 

 persistent, normally remaining on the trunk from the live crown 

 almost to the ground durmg most of the life of the tree. 



The size of a mature jack pme tree varies gi-eatly m different 

 portions of its range. In northern and western Minnesota, where 

 jack pine forests attain their best development m the United States, 

 dominant forest-grown trees which reach maturity when from 80 to 

 100 years old are usually from 60 to SO feet tall and from 10 to 15 

 inches in diameter at breast height. Older trees are frequently found 

 which are 85 feet high and 15 or 16 inches in diameter. Trees 90 

 feet in height and over 20 inches in diameter are sometimes found, 

 but these are the exception, and indicate better soil conditions than 

 usually obtain where jack pine grows. After reaching 80 years of 

 age, and sometimes sooner, jack pine deteriorates rapidly. 



On the jack pine plains in the northern half of the Southern Penin- 

 sula "of Michigan there are many miles of pure young jack pine forest 

 in which there are few, if any, trees over 75 feet in height or more than 

 14 inches m diameter. Frequent fires have thinned many of these 

 stands and as a result large numbers of the trees are scrubby and 

 branchy. On the Michigan National Forest in the Lower Peninsula 

 jack pine averages from 4 to 6 inches in diameter and from 30 to 40 



