INSTRUMENTAL METHODS. 437 



refinement. For the present the methods of cartography already in use for 

 macrographic maps will sufl&ce, but it seems clear that these must be largely 

 worked over when maps come to be used to show primary developmental rela- 

 tions. 



INSTRUMENTAL METHODS. 



General considerations. — ^While there has been a notable advance in the 

 use of instruments since the appearance of "Research Methods in Ecology" 

 (1905), the instrumental study of vegetation is still far from the rule. This is 

 strikingly true of succession, for the additional reason that developmental 

 studies themselves are still exceptional. As indicated in the discussion of 

 reactions, the use of instruments in studying successional processes was begun 

 in America more than a decade ago, but it is only during the last two or three 

 years that instnunentation has become a general procedure in this coimtry 

 and in England. Elsewhere, even in Scandinavia, where developmental 

 studies have long been the rule, the instrumental study of successional pro- 

 cesses is still infrequent. There are evidences, however, that this condition 

 is disappearing, and we can look forward confidently to a time when succession 

 will become the basic method of vegetation research, and when it will use instru- 

 ment and quadrat as its most indispensable tools. 



The chief use of instruments so far has been in attempting a complete 

 or partial analysis of the habitat. All careful work of this sort furnishes data 

 for succession, but much of it is difficult of application or interpretation. 

 As a consequence, the use of instruments in developmental study must be 

 directed to the critical processes in succession. These are reaction, ecesis, and 

 competition. The first of these is clearly the most important, because of its 

 control of the movement of successive populations, but its effect in plant 

 terms is measured by ecesis and competition also. The critical effect of reac- 

 tion is felt at the time of germination, and when competition between the 

 mixed populations of a mictimn is passing into the dominance of the next 

 stage. Hence the measiu-ement of reaction has its greatest value when it is 

 directed to these two points. It must also be recognized that reaction is itseK 

 a complex process, in which all of the factors of the habitat may be concerned. 

 Here, again, it is essential to keep in the main path, and to concentrate upon 

 the primary reactions which direct the actual sequence of stages. As has been 

 shown in Chapter V, the primary reactions are upon water and light, and upon 

 the stability of the soil, though the latter can perhaps best be measured in 

 terms of humus and water-content. In some cases, reaction upon nutrient 

 content plays a primary r61e, as perhaps also that upon water by which it 

 becomes acid. It is clear that these two reactions may also be intimately 

 bound up with each other. In initial and medial stages the edaphic reactions 

 are controlling, but in the final stages of scrub and forest formations the light 

 reaction is decisive. At the same time the water reaction can not be ignored, 

 as Fricke has demonstrated (p. 93). The local climatic reactions of forests 

 may ultimately prove of much importance, but they would seem to play only 

 a subordinate part in the development of a particular sere. 



The instrumental study of succession must be made chiefly in the reaction- 

 level. This is the level which is bisected by the surface and is characterized 

 by the maximum effect of reaction. It is the level also in which the critical 



