THE NORTHERN" HARDWOOD FOREST. 11 



shade tolerance, beginning with the least tolerant, and gives the 

 characteristics of each which influence its reproduction inside and 

 outside of the forest. 



These characteristics are subject to variation. Tolerance, for 

 example, is greater in seedlings than in old trees, in the south than 

 in the north, and in fertile than in dry situations. The frequency 

 of seed years and the fertility of the seed produced depend to a 

 large extent on climatic factors, and the amount is influenced by 

 these and by the light supply; even the annual seed bearers do not 

 produce the same amount each year. The extent of seed distribu- 

 tion depends on the height and exposure of the crown, the buoyancy 

 of the seed, and the strength and steadiness of the winds at seed 

 time. Growth of both seedlings and sprouts is influenced by the 

 length of the growing season, the fertility of the soil, and the humidity 

 of the air. All of these variations, by affecting the aggressiveness 

 of particular species in competition with others, modify the compo- 

 sition of the stand. The variations caused by physical factors (soil, 

 precipitation, temperature, etc.), though they do have an influence 

 during the youth of the stand, are active especially in determining 

 the character of the old-growth forest, and are chiefly responsible 

 for differences in its composition at different latitudes, longitudes, 

 altitudes, and exposures. Those caused by physiological factors 

 are especially active in the establishment and subsequent history of 

 temporary stands. 



The temporary stands formed by species aggressive outside the 

 forest give way, after they have developed, to species which are 

 aggressive inside the forest. A convenient classification might be 

 based upon this difference in aggressiveness, the trees being called, 

 respectively, extensive or intensive reproducers, according to whether 

 they are more aggressive outside or inside the forest. The separation 

 of the species into these classes would then be made on the basis 

 of the last two columns of Table 4. Extensive reproducers are 

 intolerant of shade, and are generally small, rapid-growing, short- 

 lived, and light-foliaged, and have a tendency to form even-aged 

 stands (PL VI). Intensive reproducers are tolerant in tendency, 

 of slow growth and long life, and form uneven-aged stands with 

 dense crown cover (PI. V). To be sure, these characteristics exist 

 among the different species in all degrees between the extremes, 

 so that a hard and fast line can not be drawn between the extensive 

 and intensive reproducers. Some species even are extensive under 

 certain conditions and intensive under others. Nevertheless the 

 divergent tendencies are perfectly evident and a scale can be drawn 

 the extremes of which are almost exclusively extensive and intensive. 



