COMMUNITIES AS INDICATORS. 75 



has been overtopped by the later layers. This naturally does not hold for the 

 primary layer of trees or shrubs and for the highest layer of herbs which 

 develops last. The practical value of shoot-layers as indicators is in con- 

 nection with the natural reproduction of forests and the selection exerted by 

 light upon the tree seedlings of a mixed forest, especially of conifers. 



Aspects. The character of a conmiunity changes with the season. This 

 is best shown in prairie where the characteristic subdominants reach their 

 maximum at different times, producing three or even four aspects, viz, pre- 

 vemal, vernal, estival, and serotinal (Pound and Clements, 1900 : 140). 

 Similar aspects occur in the herbaceous layers of forests. The mmiber 

 decreases with the altitude and latitude, so that arctic and alpine regions 

 usually show but two, spring and summer. The indicator significance of 

 aspects is partly a matter of the societies which characterize them,^ but they 

 have a seasonal value as well. This lies in recording the advance of the season 

 and in permitting the determination of departures from the normal rate. 

 The correlation of this with the behavior of crop-plants and with all processes 

 which deal with the renewal or rate of growth each year should have con- 

 siderable practical value. Phenological lists suggest these values, but are too 

 general and unrelated as a rule to be of much service (Lamb, 1915). 



