SYLVIC CHARACTERISTICS. 11 



should be well established. On account of its intolerance of shade the 

 stem clears itself of branches thoroughly when the trees are crowded. 



Reproduction. Yellow poplar reproduces chiefly by means of seed. It 

 reproduces to a limited extent by means of sprouts but only from small 

 trees until they reach the size of poles ; the sprouting of old trees is un- 

 reliable. Sprouting is best in full sunlight. The sprouts from trees 

 which are shaded are weak. The seeds, which are borne in small cones, 

 are about one inch long, including the slender wing, and are so light that 

 they may be blown several hundred yards from the parent tree. The 

 seeds from the top of the cone and from the exterior will not germinate. 

 These are the last to fall from the tree, often remaining until the follow- 

 ing spring, and in collecting seed for planting they should be discarded. 

 Trees begin to bear fertile seed when about 40 years old. Seeds are pro- 

 duced on older trees in great abundance and almost annually, but a large 

 proportion, as a rule about 90 per cent., are infertile. They ripen in Sep- 

 tember and October and are in great part distributed at once by the wind. 

 They largely germinate during the following spring and the early part 

 of the summer, but some seem to lie over on the ground until the second 

 spring. Germination takes place wherever the soil is moist, and the seed- 

 lings easily establish themselves, but they fail to survive the dry periods 

 of the first summer unless on a moist or shady site. Stocking, conse- 

 quently, is better on north slopes, in coves and hollows or in lower slopes, 

 than on the drier upper slopes and on southern exposures. It is also best 

 where the mineral soil has been exposed, as in old fields or where the soil 

 has been upturned by logging or by hogs. A thick layer of dry leaves is 

 unfavorable. 



Diseases and defects. While old trees of yellow poplar have many de- 

 fects, such as wind-shake, frost crack, and stained wood, there are, so far 

 as known, no destructive fungous diseases or insects which attack and 

 kill trees. 



Sensitiveness to fire. Seedlings and small trees of yellow poplar are 

 extremely sensitive to fire, being more easily injured even than chestnut. 

 This is due primarily to the thin and smooth bark of the young tree of 

 yellow poplar. An additional reason for their great injury is that the sap 

 and inner bark of yellow poplar become active earlier than in the asso- 

 ciated trees. This greatly exposes this species to early spring fires, which, 

 when they take place during dry and windy periods before the forest fol- 

 iage is out, are very hot and destructive. Fortunately yellow poplar is 

 somewhat protected by being largely confined to the damp hollows and 

 coves. Seedlings of yellow poplar which are killed down by fire will usu- 

 ally sprout from the root, unless they are shaded. This power, however, 



