84 INVASION 



The investigation of invasion furnishes the basis for all 

 developmental and structural study of vegetation. Obser- 

 vational methods, though of great value in indicating the 

 causes, direction, rapidity, amount and results of invasion, 

 are in themselves insufBcient, and must at present be sup- 

 plemented and in time replaced by experimental methods. 

 For this purpose, the writer has extended the application of 

 the quadrat, the use of which was first suggested by Pound 

 and Clements ( J898:I9) for the accurate determination of abund- 

 ance. In this connection, the quadrat is now used experi- 

 mentally to follow in an exact and minute manner the 

 changes produced by invasion. The permanent quadrat 

 makes it possible to record from year to year the incidental 

 invasions on any one spot, or the changes of a succession as 

 the latter passes from one stage to another. If a quadrat 

 list showing the annual increase or decrease in abundance 

 of each species is all that is desired, the list quadrat, which 

 is five meters square, is sufficient. A permanent record of 

 the exact changes in a quadrat area is of much greater 

 value, however ; such a record is obtained by the use of the 

 chart quadrat. The size of the latter depends upon the 

 character of the vegetation: in open formations, and in 

 forests and thickets without dense undergrowth, the chart 

 quadrat should be two meters square, but in closed vege- 

 tation the accurate plotting of a quadrat is an arduous task, 

 and the area best adapted to this work is a meter square. 

 The permanent record of a chart quadrat is made upon centi- 

 meter plotting paper, the scale being one centimeter to each 

 decimeter, and a close-focus photograph is also made of each 

 quadrat charted. In practice, chart quadrats have been 

 made permanent by driving a properly labelled stake at the 

 north east corner of the quadrat. 



An invaluable aid to the study of invasion and succession 

 is the experimental or denuded quadrat. A chart quadrat 

 is staked out and plotted in the usual way, and a photograph 

 is made of it. The area is then completely denuded, especial 

 care being taken to remove all perennial parts. Such de- 

 nuded quadrats are then charted and photographed each 



