steady-state climax. The actual directional change is gradual 

 and continuous instead of in a series of discrete steps, although 

 subjective vegetation stages can be recognized. The course of the 

 directional change can usually be predicted within a reasonable 

 degree of reliability (Figure 5-4), although the change is not 

 straight or specific, and is subject to a great deal of variation. In 

 some cases successional stages may be skipped or added, or the 

 course of change may be compressed or extended. Use of the 

 term "succession" need not imply the necessity for a large num- 

 ber of stages as in a hydrosere or a xerosere. 



An interruption in the sequence of communities may occur. A 

 part or all of the community and habitat may be so changed that 

 an earlier stage of the sequence or a less complex stage occupies 

 the area; this abrupt change may be caused by fire (Figure 5-5), 

 clearing by man, mining, grazing by livestock, wind throw, flood- 

 ing deposition, landslip, or snowslides. Then a directional sequence 

 starts to run its course again from that stage; this is called second- 

 ary succession. The course and rate of directional change within a 

 sere may be rather rapid between some stages (Figures 5-6 and 

 5-7). In the late summer and fall following cultivation of fields 

 on the Piedmont, North Carolina, crabgrass {Digitaria sanguinalis) 

 is usually dominant. During the first year of abandonment, where 

 the fields are not severely eroded, horseweed {Leptilon canadense), 

 conspicuous because it is 4 to 6 ft tall, and crabgrass are usually 

 dominant, and ragweed {Ambrosia elatior) may share this domi- 

 nance. During the second year, aster {Aster pilosus), approximately 

 6 ft tall, is usually dominant, and crabgrass and horseweed are 

 still present. During the third year, broomsedge {Andropogon vir- 

 ginicus) assumes dominance and maintains the dominance until 

 it is replaced by pines. In due time the climax oaks and hickories 

 occupy the area.^^^ 



The term "retrogression" is applied by some to a directional 

 change which is essentially the reverse of succession, others in- 

 clude such a change under succession. Retrogression may also be 

 considered partial or gradual destruction of a community, fol- 

 lowed by invasion of species characteristic of earlier stages. The 

 causative agent may be climatic, grazing by domestic animals, 



Habitat Patterns, Changes, and Climax • 153 



