pogon stands in eastern Maryland. They should always be con- 

 sidered as part of the stand as a whole, instead of as independent 

 units, because numerous interactions take place between many 

 organisms in the different layers. All the synusiae and layers are 

 closely integrated with the entire stand. 



Plants in the lower strata may increase in size and number 

 when those in the upper stratum are removed by grazing or mow- 

 ing. For example, in ungrazed prairie in Missouri the short 

 Antennaria neglecta and Viola sagittata made up 1 to 4, and 0.4, per 

 cent of the stand, respectively, while in the grazed and mowed 

 prairie they made up 9.3 and 3.1 per cent respectively. Other 

 components of the lowest stratum, Trifolium pratense, T. repens, 

 and Oxalis stricta, were also abundant in the latter. ^^ Big sage- 

 brush {Artemisia tridentata) suppresses all plants in the stand when 

 its crowns cover about 40 per cent of the ground,'" but when it 

 has been destroyed by burning, uprooting, or spraying with an 

 herbicide, the plants in the field layer usually flourish and pro- 

 vide much forage. 



Layering of root systems may be caused by various factors, such 

 as differences in moisture content of the soil and the concentra- 

 tion of salts at various depths. This is common in many parts of 

 the western United States. For example, in the vicinity of Mil- 

 ford, Utah, three layers were evident: (1) the uppermost layer, 

 to a depth of 30 to 45 cm, with a salt concentration of less than 

 1000 ppm (bridge method) in which the roots of Bouteloua gracilis, 

 Hilariajamesii, and Sporobolus cryptandrus were restricted; (2) an in- 

 termediate layer, at between 40 and 65 cm, where the salt con- 

 tent was over 1000 ppm and in which were found some of the 

 larger roots of Atriplex confertifolia, Chrysothamnus stenophyllus, and 

 Eurotia lanata; and (3) the lowest layer, at 80 to 90 cm, where the 

 soil was more porous and contained less salt, and where roots of 

 Artemisia tridentata and Grayia spinosa were found. '^^ 



Periodicity (Phenology, Aspection) 



Periodicity refers to the regular seasonal occurrence of various 

 processes such as photosynthesis, growth, pollination, flower- 

 ing, and ripening of fruits and seeds; and the manifestations of 



Azkalytic: CHaracteristics of tHe CornnrnvaBmity • 85 



