Fig. 82. A mixed formation of aspens and spruces (Populus-Picea-mictium), preceding the final spruce forest of a burn succession.
Puzzling cases of mixture resulting from position occur toward the limits of facies which occupy extensive areas. Bouteloua oligostachya, and Andropogon scoparius extend from the prairies through the sand-hills and plains, and into the foot-hills of the Rocky mountains. Their abundance at once raises a question as to the validity of the prairie, sand-hill, plain, and foot-hill formations. If these two grasses were controlling, and equally characteristic throughout, then the entire stretch would have to be regarded as a single formation. Since they are often absent, or mixed with other facies of greater importance, they can not be considered the sole tests of the formation. This view is reinforced by the fact that prairie, sand-hill, plains, and foot-hill all have their characteristic principal and secondary species, in addition to facies that are more or less typical. In certain formations, doubtless, Bouteloua and Andropogon are relicts, in others invaders, while in the formations actually constituted by them they are dominant. The final solution of such problems is quite impossible, however, until the comparative study of large areas can be based upon the accurate detailed investigation of the component formations.
Experimental Vegetation
356. Scope and methods. The experimental study of the formation as a complex organism rests upon methods essentially similar to those discussed under experimental evolution. The scope of the two fields is practically the same, moreover, in that both deal with the experimental development of an organism and the structures that result. The actual problems are naturally very different, since the formation is a complex of individual plants, but the fundamental basis of habitat, function, and structure is common to both. However, the functions now to be considered are aggregation, invasion, competition, etc., and the structures, zones, consocies, societies, communities, and families. The latter may properly be regarded as adaptations called forth by the adjustment, i. e., aggregation, migration, ecesis, etc., of the formation to the physical factors of the habitat. As consequences of measured factors, formational adjustment and adaptation must themselves be carefully measured and recorded. For these purposes, the methods of quadrat and transect, of chart, photograph, and formation herbarium are used. Invaluable as they are for any scientific inquiry into vegetation, such methods form the very foundation of experimental study in which accuracy is the first desideratum.
It has already been shown that nature’s own experiments in the production of new forms furnish the best material for experimental evolution. This statement is equally true of experimental vegetation. The formation of new habitats by weathering and transport, and the denuding of old ones, yield experimental plots of the greatest value. This is likewise the case in the great majority of formations, where invasion or competition is active. These are the phenomena that must be considered in any careful study of vegetation, but in taking them up from the experimental standpoint, greater attention must be paid to detail, and the changes must be followed closely for a longer time. The method that makes use of existing changes in vegetation is designated the method of natural habitats. In contrast with this is the method of artificial habitats, in which the habitat itself is definitely modified, or a group of species actually transferred to a different habitat. Many problems of vegetation can be attacked with greater success under control than in the field. This is particularly true of competition, in which results can be obtained most readily by means of the method of control habitats, as carried on in the plant house.
METHOD OF NATURAL HABITATS
357. Natural experiments. Every family as well as every community constitutes an experiment in competition; the same statement necessarily holds for the larger groups, society, consocies, and formation, which are composed of families and communities. The last also make it possible to study competition in two typical instances, viz., in the family, where the individuals are of one kind, and in the community, where they belong to two or more different species. The community, moreover, is a product of invasion, and it furnishes material for the study of this function, as well as for that of aggregation and competition. Practically every formation shows some invasion, but as a rule stable formations contain so few invaders that they are relatively unimportant in this connection. Invasion is most active in transition areas and in mixed formations, whether produced by juxtaposition or by succession, and its study in these places yields by far the largest number of valuable results.
As typical complete invasion, a succession is the best of all natural experiments in aggregation, migration, ecesis, and competition. This is especially true of the initial stages in which changes in the number and position are most readily followed. The methods used in studying successions have been given elsewhere. In addition, it should be pointed out that one of the first tasks in taking up the ecological investigation of a region is to make a careful search for all new and denuded areas, as well as for those in which succession is taking place. The phenomena in these areas can not be explained until the habitats and formations have been worked over critically, but the facts must be collected at the earliest possible moment, since the stages of the succession are constantly changing, while the stable formations are not.