[Chap. V LOCAL PLANT COMMUNITIES 41 



the growth of clover and weeds. If the bluegrass is carefully removed, 

 the weeds and clover rapidly increase in abundance. Similarly the pres- 

 ence of weeds interferes with the growth of bluegrass. When one re- 

 moves the leaves of a single large dandelion plant from a lawn for the 

 first time he is usually amazed by the area of soil from which bluegrass 

 was excluded by their presence. Trees with low branches may interfere 

 with the growth and dominance of bluegrass in a lawn unless the trees 

 are very small. Many people plant grass seed beneath such trees every 

 autumn. The grass grows well throughout autumn and during the follow- 



er WIDTH of CROWN ^ 



Fig. 23. Bluegrass may grow beneath a tree if all the branches of the tree are 

 several feet above the ground. The diameter of the root system of a tree in a lawn 

 may be as much as 5 times the diameter of the crown of the tree. 



ing spring, but by late June or early Jub' it becomes yellow and dies. 

 If all the branches of a tree are high above the ground, bluegrass may 

 continue to grow indefinitely beneath it (Fig. 23), 



Succession. Since each plant in a lawn is influenced in some way by 

 the presence of other plants, the relative abundance of bluegrass or of 

 any other species of plant in the lawn may not remain the same from 

 year to year. During one growing season some species may increase 

 while others decrease in abundance. That is, the composition of a mixed 

 population of plants is not constant; it changes in time as a result of the 

 mutual eftects of the plants upon each other and of increase and decrease 

 in light, water, soil fertility, and other external factors. It is this in- 

 stability of a mixed population of a community that concerns the owner 

 of a beautiful lawn. Gradual changes in the relative abundance of the 

 constituent species may in time result in a community that is very 



