l82 THE FORMATION 



THE MIGRATION CIRCLE 



226. Purpose. The migration circle is designed to record the invasion 

 of species, since it operates outward from an individual or a group of plants 

 as a center. As migration takes place to a certain degree in all directions, 

 a circle is better adapted to the purpose than the quadrat. From the very 

 nature of invasion, migration circles should always be permanent in order 

 that the yearly advance may be accurately noted. Circles of this character 

 are important aids in the study of any vegetation, except, perhaps, one that 

 has practically become stabilized. Their great value, however, is found 

 in succession, where it is necessary to trace the movement of new individuals 

 away from the original invaders as centers of colonization. 



227. Location and method. The size of the migration circle is largely 

 controlled by the density of the vegetation, and in some degree by the height 

 of the species also, since this determines the trajectory of the disseminule. 

 In close formations, a circle of i-, rarely of 5-meter radius can best be used, 

 but in the more open initial stages of succession a radius of 5, 10, or, in ex- 

 ceptional cases such as open woodland, even 25 meters, affords the best 

 results. The location should always be made with a plant or group of 

 plants of the species to be studied as a center. This migration circle differs 

 from the quadrat in that it is used to show the movement of one, rarely 

 two or three species, and not the position of all the plants within it. The 

 center is permanently fixed by driving a labeled stake with the number of 

 tlie circle and the data. Two tapes the length of the radius are used for 

 recording. These are provided with the usual eyelets, 5 decimeters apart, 

 and are fastened on a peg in the top of the central stake so that they move 

 readily. At the outer ends they are staked 5 decimeters apart by a tape of 

 this length when the radius is i meter, and i meter when the radius is 5 or 

 ID meters. The record forms must be especially prepared on blank sheets 

 about 9 inches square. The scale is 10:1 for circles of i meter, and 100:1 

 for those of 5- and lo-meter radius. In the former, concentric circles are 

 drawn about the center at intervals of 5 decimeters, and radii are drawn 

 to the circumference at the same interval. In the larger circles, the inter- 

 vals are 1 meter. Each segment of the circle is read by means of the two 

 tapes, and the position indicated with reference to the concentric lines and 

 radii. When but one species is read, a tiny circle is used to denote the posi- 

 tion of each plant. If more than one is used, the symbols are those already 

 indicated for the quadrat. One tape is left in place and the other with the 

 segment tape is shifted to a new position, and the resulting segment is read 

 as before. The exact position of the base radius is fixed by a label stake, 



