218 



WOOD AND FOHICST. 



balsam fir have the lemarkable power of growing slowly under heavy 

 shade for many yeais, 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 

 liaving at its top a lit- 

 tle tuft of leaves. Fig. 

 (U. By so doing it 

 survives. The fact that 

 a tree can grow with- 

 out shade often deter- 

 mines its possessioji of 

 a Ijurnt-over tract. The 

 Oilier in the Xorth 

 A\'oods 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 s[iruce, to which it is a '•nurse," Fig. 65. In 

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

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

 do not. 



'i'liis matter of tolm-ance has also much to do "with tbc liranching 

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

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

 This is called "cleaning," or natural ]iruning. 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 bv geograph- 

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



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

 Showing- Fast Growth After Years of 

 Suppression. Notice the width of the 

 annual rincrs in later age compared 

 with early. U. S, Forest Service. 



