METHODS OF MAPPING. 435 



A migration circle or axe may be located with reference to an individual, 

 a community or a distinct ecotone, such as the edge of a forest. In the case 

 of an individual, or a small family or colony, a circle, or better, a series of con- 

 centric circles, is marked out. This may be done by carrying a radius around 

 the object, or, better still, where the study is to take some time, by means 

 of concentric circles made by using a tennis-court marker. When the 

 migration is in a single general direction, as at a forest edge, concentric arcs 

 or parallel lines are used, as the size of the community demands. The deter- 

 mination of migration alone demands the most minute study, and hence it is 

 diflScult to carry it throughout a season. In the case of wind-borne germules, 

 it can best be determined after times of high wind, or, still better, at times of 

 varying velocities which are measured. The detection of seeds and fruits 

 in vegetation is so time-consuming, even when it is possible, that the study of 

 actual movement can best be made upon snow surfaces or upon the bare 

 ground. In the case of individual plants, especially trees, a denuded area in 

 the direction of usual movement is the most satisfactory. When the study of 

 migration receives the detailed attention which its importance warrants, it is 

 probable that quadrats artificially prepared to catch and retain the migrules 

 brought to them will be placed at definite intervals in the direction of migra- 

 tion. At present, the measurement of eflFective migration in terms of ecesis 

 furnishes the most convenient method. This has been used with signal success 

 in investigating the invasion of "natural parks" by Picea and Abies in the 

 Uncompahgre Plateau of Colorado (fig. 51) and recently by Hofmann (1916) 

 in studying secondary succession in the Pacific coniferous chmax. 



METHODS OF MAPPING. 



Methods. — Schroter (1910 : 127) has discussed at length the methods of 

 ecological cartography. Such macrographic methods are. not considered 

 here, as they have little or no bearing on the intensive study of succession. 

 This is due to the fact that succession has usually not been taken into account 

 at all, though it is obvious that the areas mapped may be readily distinguished 

 as climax and developmental. The methods indicated below are micrographic 

 in the sense of NUsson in that they deal with individual plants or communities. 

 They may be distinguished as (1) extensions of the quadrat method, (2) 

 ecotone and community mapping, and (3) combined quadrat and map method. 

 The first is illustrated by the method of squares and the gridiron method of 

 Oliver and Tansley (1904:228, Tansley 1904:200). The gridiron method 

 appears to be very similar to the use of perquadrats of 25 feet, but differs in 

 that the contours and the outlines of communities are indicated rather than 

 the individuals. The method of squares employs areas 100 feet square, 

 which are used to cover continuously the entire area to be mapped. Physical 

 features, contours, and the boundaries of communities are alone shown, though 

 the gridiron method may be used to furnish greater detail in various areas. 



Community charts and ecotone maps. — Commvmity charts (Thornber, 

 1901 : 126) resemble closely the gridiron maps of OHver and Tansley. They 

 are made by means of pacing or by tapes. They may be employed to map in 

 some detail the dense vegetation of an associes or association, but their greatest 

 value lies in indicating the position and growth of families and colonies in 

 bare areas. Community charts combine quadrat and map methods, in that 



