218 



WOOD AND FOREST. 



balsam fir have the remarkable power of growing slowly under heavy 

 shade for many years, and then of growing vigorously when the light 

 is let in by the fall of their overshadowing neighbors. This can 

 plainly be seen in the cross-section of balsam fir, Fig. 63, where the 

 narrow annual rings of the early growth, are followed by the wider 

 ones of later growth. A common sight in the dense woods is the 



maple sending up a 

 long, spindly stem thru 

 the trees about it and 

 having at its top a lit- 

 tle tuft of leaves, Fig. 

 64. By so doing it- 

 survives. The fact that 

 a tree can grow with- 

 out shade often deter- 

 mines its possession of 

 a burnt-over tract. The 

 order in the Xorth 

 Woods after a fire is 

 commonly , first, a 

 growth of fire weed, 

 then -raspberries or 

 blackberries, then as- 

 pen, a very intolerant 

 tree whose light shade 

 in turn permits under 



it the growth of the spruce, to which it is a "nurse," Fig. 65. In 

 general it may be said that all seedling conifers require some shade 

 the first two years, while hardwoods in temperate climates, as a rule, 

 do not. .^: 



This matter of tolerance has also much to do with the branching 

 of trees. The leaves on the lower branches of an intolerant tree will 

 not thrive, with the result that those branches die and later drop off. 

 This is called "cleaning," or natural pruning. Intolerant trees, like 

 aspen and tulip, Fig. 66, clean themselves well and- hence grow with 

 long, straight boles, while tolerant trees, like spruce and fir, retain 

 their branches longer. 



The distribution of a species may also be determined by geograph- 

 ical barriers, like mountain ranges and oceans. This is why the 



Fig-. 63. Cross-section of Balsam Fir, 

 Showing- Fast Growth After Years of 

 Suppression. Notice the width of the 

 annual ring's in later age compared 

 with early. U. S. Forest Service. 



