a community may become so severe that a species may survive 

 only in an area outside of its optimum range, where competition 

 is reduced because it grows in a different season from most of the 

 plants in that community.^^^ 



It may be concluded that the various expressions of periodicity 

 and phenology of plants are means of adaptation to seasonal 

 changes in the physical environment. Usually, in order to live in 

 a given location, the plant must make full use of the favorable 

 environmental periods and endure or tolerate, by one means or 

 another, the unfavorable periods. 



Vitality (Vigor) 



Vitality relates to the condition of a plant and its capacity to 

 complete the life cycle, while vigor refers more specifically to the 

 state of health or development within a certain stage; a seedling 

 or mature plant may be vigorous, or it may be feeble and poorly 

 developed (Figures 1-2, 1-19, 1-26). In order to appraise the de- 

 gree of vigor one needs to know the appearance of normal plants, 

 preferably under optimum conditions, in the various stages of 

 growth. The vigor of species that are represented by only a few 

 individuals in a stand often merits special attention because they 

 may indicate the end of a previous stage of succession, the begin- 

 ning of a new stage, a change in conditions such as an increase 

 or decrease in soil moisture, attack by parasites, or some kind of 

 harmful grazing practice. One of the first signs in the improve- 

 ment of a depleted grassland in the annual weed stage is the ap- 

 pearance of scattered, newly established, vigorous perennial forbs 

 and grasses. 



A number of criteria may be used in determining the vigor of 

 plants such as the rate and total amount of growth, especially in 

 height; rapidity of growth renewal in the spring or following 

 mowing or grazing; quantity or area of foliage; color and tur- 

 gidity of leaves and stems; degree of damage by disease or in- 

 sects; time of appearance and number and height of flower stalks; 

 rate of growth and extent of the root system; appearance and de- 

 velopment of new stems and leaves; and the extent of dead por- 

 tions, especially in bunch- or mat-formers. 



90 • Tke Conmznunity 



