NATIVE WOODY PLANTS OF THE UNITED STATES 11 
SELECTION OF SPECIES FOR PLANTING 
PRIMARY SUCCESSION 
The factors governing survival must be recognized when a plant, 
is placed among others to compete with them for a place in the sun. 
These factors or survival values are measured in terms of species 
requirements, although we often state them by speaking of the toler- 
ance of a plant for shade, sun, acid or alkaline soil, drought, and 
moisture. Because no experiments have been conducted, it has been 
assumed by some technicians that there is no scientific basis on which 
to judge the ability of one species to compete on the same ground and 
under the same conditions with another species. This is by no 
means true. 
For at least 20,000 years in the northern United States, and for a 
much longer time in the southern part, the species comprising the 
flora of the country have been. competing for position, settling them- 
selves little by little into their respective ecological niches and asso- 
ciations. On any given area of ground there has grown a succession 
of associations of plants which, as they have contributed to the 
gradual modification of the soil (or rock) on which they grew, have 
eventually been replaced by plants of a different association. After 
sufficient time, an association of plants has appeared which, under 
the existing environmental conditions appears to be the best adapted 
to the area. This group is often known as a climax, and although it is 
recognized that a completely stabilized environment never exists, the 
climax regions for the country have been more or less accurately 
mapped. Associations of grasses are considered to be the climax for 
the Plains region; certain climax associations of deciduous trees 
appear in the eastern part of the country; evergreen trees constitute 
a climax for the western coastal region; and there is a xerophilous 
association of succulents and leathery species that forms the climax 
in the deserts of the Southwest. It should be kept clearly in mind 
that every plant association, whether climax or preclimax, is the out- 
come of long ages of natural selection. 
Enough work has been done to enable us to predict the climax for 
most areas with some confidence, but it may be many years before 
the date of the climax can be predicted, if that ever becomes possible. 
In a general way we know what species tend to occur together, and on 
what sites they are to be expected, in any given succession in any part 
of the country. Careful observation of a species in its native, un- 
disturbed (or disturbed) habitat will usually demonstrate where 
how it fits into its environment. According to Shantz (504, p. 
3OV): 
A thorough understanding of the natural vegetation climax and of the second- 
ary stages leading to its re-establishment when it is once destroyed is the best 
basis for a revegetation and erosion-control program. 
For many years plant ecologists have been engaged in determining 
the fundamental principles concerned with the initiation, develop- 
ment, and maturity of plant associations on given sites. The slow 
encroachment of lichens and mosses on bare rock, followed by gradual 
appearance of higher plants, the development of soil, and the inva- 
sion and succession of later associations tending toward a stable inter- 
