THE NORTHERN HARDWOOD FOREST. 13 



For the species in Table 4 such a scale is approximately as follows: 



Most extensive: 



1. Aspens. 



2. Gray birch. 



3. Paper birch. 



4. Fire cherry. 



5. Black cherry. 



6. White pine. 



7. Yellow birch. 



8. Black birch. 



9. White ash. 



Most extensive — Continued, 



10. White elm. 



11. Red spruce. 



12. Basswood. 



13. Sugar maple. 



14. Red maple. 



15. Iron wood. 



16. Hemlock. 

 Most intensive: 



17. Beech. 



Red spruce and yellow birch are examples of species which though 

 in most respects intensive, are also extensive, under favorable con- 

 ditions. Both often reproduce in even- aged second-growth stands 

 on clearings, while the spruce, and to a less extent the birch, are 

 able to start seedlings within the forest. 



The extensive species are obvioulsy well adpated for quickly 

 reclaiming burned or otherwise cleared land, and not for competition 

 with intensive species. (PI. VI.) Aspen and paper birch are rapidly 

 displaced by maple, beech, or hemlock, or, in fact, any others of the 

 " characteristic " species of the northern forest whose reproduction 

 may happen to start beneath them. 



The intensive reproducers hold their ground when once they have 

 gained it; but they differ among themselves in aggressiveness and 

 persistance. Sugar maple is the most generally aggressive repro- 

 ducer throughout the characteristic beech-birch-maple type. 

 This is undoubtedly due to its combined tolerance and seeding 

 qualities. Beech, which is probably more tolerant, does not bear 

 large seed crops annually, and much of the seed produced is destroyed 

 by animals. Yellow birch, which does bear each year, is less tolerant 

 than maple. Its light-winged seed are so widely dispersed, however, 

 that many fall where the crown shade is light enough to permit the 

 development of seedlings. These are adaptable to a great variety 

 of seed-bed conditions, from sandy soils burned free of humus to 

 duff-covered clay loams, and even moss-covered bowlders, decayed 

 stumps, and logs. Yellow birch thus accomplishes through its 

 reproductive aggressiveness often more than beech can accomplish 

 through its extreme shade endurance. White elm and basswood 

 both require much light for growth and especially for seed production. 

 The elm seeds, with their surrounding wings, are light, thin disks, 

 fitted to be distributed quite widely by the wind; the tree bears 

 annually and abundantly. Basswood seeds are produced in less 

 abundance, and at first glance seem poorly adapted for wind dis- 

 persal. They are suspended in clusters of as many as six large 

 spherical fruits beneath a single bract, apparently insufficient in 

 size for a long flight; but when the seed clusters fall the bract becomes 



