20 



vania are decidedly mixed in their composition. The conifers are 

 found oftener in pure stands than the broad-leaved species. Since 

 the forests of this State are composed largely of hardwood species it 

 is rather unusual to find pure stands. Occasionally one may find 

 small pure stands of such species as Pitch Pine, White Pine, or Red 

 Cedar and rathei* extensive ones of Chestnut. 



About 125 species of trees are native to the State of Pennsylvania 

 but not more than 25 species are of sufficient importance to deserve 

 to be developed in our future forests. A large proportion of our 

 native trees is found as undergrowth. They form dense and some- 

 times almost impenetrable thickets. This dense and complex struc- 

 ture of our upderwood aids considerably in increasing the number 

 of participants in our forests. Dense and tall undergrowth tends to 

 protect the soil from erosion, to con-serve the fertility of the soil, 

 and to afford shelter to birds which prey upon the insect enemies 

 of the forest; but it may also impede the utilization of forest pro- 

 ducts and make the tending of the forest more difficult. 



The tendency of forestry is to eliminate the undesirable species. 

 We should eliminate cautiously since the despised species of to-day 

 may be prized to-morrow. If tliis process of elimination is devel- 

 oped on an intensive scale, it means a reduction in the number of 

 species and, in extreme cases, leaves only one species, i. e. a pure 

 stand. Tliis is especially true where a forester aims to establish a 

 stand by artificial seeding or planting. He is apt to choose a species, 

 which he thinks will give the highest returns. If he selects the 

 proper species and it is not injured during its development he may 

 possibly obtain satisfactory results. Before establishing stands one 

 should consider the subjoined advantages of pure and mixed stands. 

 The principal advantages of pure stands are: — 



1. Pure stands are easier and cheaper to establish. 



2. Pure stands are easier to tend and manage. 

 The principal advantages of mixed stands are: — 



1. Mixed stands utilize the available plant food in the soil and 

 air more fully. Close utilization of the factors of the habitat 

 and keen competition by every forest tree upon its neighbor 

 are requisites for optimum quantity and quality production. 



2. The forester can meet the demands of the market better with 



a few species than with one species. 



3. A larger number of trees per unit of area is usually found in 

 a mixed stand than in a pure stand. 



i. j\Iany species are less subject to damage by wind, frost, fire, 

 fungi, and insects, in mixture than when grown pure. 



5. Trees usually develop a better form if mixed properly than if 

 grown pure. 



6. Mixed stands are more attractive than pure stands. 



