PLANT ASSOCIATIONS 



439 



agencies. In all such cases of denuded land areas the first inva- 

 ders have a free field, without competition on the part of other 

 plants, which is a very important factor in the success of invaders 

 into old plant communities. The main restrictions on the occu- 

 pation of these naked surfaces are those imposed by the nature 

 of the invading plants themselves and by the environmental con- 

 ditions obtaining in the invaded area. Thus, strictly water-loving 



FIG. 285. Invasion of Pinus ponderosa into plains grassland, 

 Black Forest, Colorado 



After Clements 



plants will not thrive on an upland tract, and plants accustomed 

 to medium, or mesophytic, conditions will not grow or thrive in a 

 desert soil. In other words, the invading species must be more or 

 less closely adapted to the soil and climate of the invaded region 

 in order to survive and become a permanent member of a new 

 plant community or association. The first invaders are usually 

 annuals or biennials furnished with mobile seeds, fruits, or 

 vegetative parts. These are in turn succeeded by hardy peren- 

 nials like the grasses, which may establish a permanent plant 



