JACK PINE. qi 
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 by 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 along bole. In open stands it tends to develop a dome-shaped, 
wide-spreading crown and a short bole. The dead branches are very 
persistent, normally remaining on the trunk from the live crown 
almost to the ground during most of the life of the tree. 
‘The size of a mature jack pine tree varies greatly in different 
portions of its range. In northern and western Minnesota, where 
jack pme forests attam their best development in the United States, 
dominant forest-grown trees which reach maturity when from 80 to 
100 years old are usually from 60 to 80 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 in 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 
