TIMBER: FORESTRY 399 



369. Composition of the forest. 1 Forests, whether of conif- 

 erous or of dicotyledonous trees, may be either pure or mixed. 

 A pure forest is one which consists almost entirely of a single 

 kind of tree; a mixed forest, one which contains two or more kinds. 

 Nearly pure forests of white pine (Fig. 321) and of long-leaf 

 pine (Fig. 260) are not uncommon. Few kinds of North 

 American hard woods grow unmixed with other species of 

 trees, but some of our birches, oaks, and maples occasionally 

 do so. 



Mixed forests, however, are the rule, and these may consist of 

 only two or three kinds of trees, as beeches and maples ; oaks 

 and hickories ; oaks, elms, and ashes. Often many species of 

 several genera grow side by side, as sycamores, oaks, ashes, 

 black walnuts, elms, and hackberries, which occur in some rich 

 bottom lands of the Middle West. 



Beneath the crowns of the trees many kinds of shrubs and 

 undershrubs often flourish, and under these grow herbaceous 

 plants in greater or less abundance. Their numbers depend 

 on several factors, such as the light supply, the moisture 

 supply, and the fertility of the soil. In the eyes of the for- 

 ester many of these plants which grow beneath the trees are 

 weeds, which more or less effectually hinder the growth of 

 seedling trees. 



370. Tolerant and intolerant trees. A tree which can endure 

 a good deal of shade is said to be tolerant. Examples are, 

 among conifers, the hemlock and the red spruce ; among hard 

 woods, beech and maple. Trees which require much light are 

 said to be intolerant. Examples are, among conifers, the white 

 pine and the larch; among hard woods, oaks, hickories, and 

 chestnuts. As a rule, seedlings require far less light to begin 

 life than is needed to enable the mature tree to reach its maxi- 

 mum size. So it often happens that seedling trees may struggle 

 on for years on the forest floor, making but little growth until 

 the decay and fall of overshadowing trees, their destruction 



1 This section applies especially to forests of temperate zones, those of 

 tropical climates being often very complex in their make-up. 



