. Research methods in ecology. Plant ecology. ALTERNATION 291 cies found in one area will be replaced in another by a different one of the same or a different genus. The controlling factors of the area render imper- ative an essential identity of vegetation and habitat form, though in system- atic position the plants may be very diverse. Such genera and species may be termed corresponding. The relation between such plants is essentially alternation; it should, perhaps, be distinguished from alternation proper as corresponsive. The prairie formation furnishes a good example of this on exposed s
. Research methods in ecology. Plant ecology. ALTERNATION 291 cies found in one area will be replaced in another by a different one of the same or a different genus. The controlling factors of the area render imper- ative an essential identity of vegetation and habitat form, though in system- atic position the plants may be very diverse. Such genera and species may be termed corresponding. The relation between such plants is essentially alternation; it should, perhaps, be distinguished from alternation proper as corresponsive. The prairie formation furnishes a good example of this on exposed sandy crests, upon which Lomatium, Comandra, and Pentstemon alternate. Formations exhibit a similar Fig. 74. Numerical alternation of Pinus and Pseudotsuga upon east and west slopes. All species that alternate show a variation in abundance from one area to another. Frequently, the difference is slight, and may be ignored, except in determining abundance. Very often, however, the variation is so great that a facies may be reduced, numerically, to the rank of a principal species, or one of the latter to a secondary This phenomenon is distinguished as numerical alternation. It arises from the fact that the similar areas are sufficiently different to affect the abundance, without producing complete suppression. It is probable that this result is due almost entirely to compe- tition. Astragalus crassicarpus grows on all the slopes of the prairie forma- tion, but on some it has the abundance of a facies, while on others it is repre-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Clements, Frederic E. (Frederic Edward), 1874-1945. Lincoln, Neb. , University Pub. Co.
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