Research methods in ecology

by Frederic E. Clements

CLASSIFICATION AND RELATIONSHIP

350. Bases. Formations may be grouped with reference to habitat or kind, development or position. Classification upon the basis of habitat places together formations which are similar in physiognomy and structure. Developmental classification is based upon the fact that the stages of a particular succession are organically connected or related, though they are normally different in both physiognomy and structure. Grouping with respect to position is made solely upon occurrence in the same division of vegetation. The formations thus brought together usually possess neither similarity of kind or structure, nor do they have any necessary developmental connection. Habitat and developmental classification are of fundamental value; regional arrangement is more superficial in character. All serve, however, to emphasize different relations, and, while the developmental system expresses the most, they should all be used to exhibit the vegetation of a region, province, or zone.

Fig. 81. Pachylophon (Pachylophus caespitosus), a family of the gravel slide formation.

351. Habitat classification. In arranging formations with reference to habitats, the direct factors, water and light, can alone be used to advantage. Such a system is fundamental, because it is founded upon similarity of habitat and of structure. Proposed groupings based upon nutrition-content, or upon the division of factors into climatic and edaphic, have elsewhere[^[42]] been shown to be altogether of secondary importance, if not actually erroneous. The basis of the habitat grouping is water-content, which is supplemented by light whenever the factor is decisive. The primary divisions thus obtained are water, forest, grassland, and desert, which are characterized respectively by associations of hydrophytes, mesophytes, hylophytes, poophytes, and xerophytes respectively. Within these, formations are arranged according to the type of habitat, i. e., pond, meadow, forest, dune, etc. These divisions comprise all formations which belong to the type by virtue of their physiognomy and structure. Such formations differ from each other very considerably or completely in the matter of floristic, i. e., component species, but they still belong to the same type. A dune formation in the interior and one on the coast may not have a single species in common, and yet they are essentially alike in habitat, development, and structure.

352. Nomenclature. The names of formations are taken from the habitats which they occupy. Each formation should have a vernacular and a scientific name. The latter is especially important since it ensures brevity and uniformity, and obviates the obscurity and confusion that arise from vernacular terms in many tongues. Scientific names have been made uniformly from Greek words of proper meaning by the addition of the suffix -ium (εῖον), which denotes place.[^[43]] The following list gives the English and the scientific name of the various habitats, and their corresponding formations, and indicates the primary divisions into which these fall.

I. Hydrophytia: water plant formations

1. ocean: oceanium: oceanad,[^[44]] oceanophilous, etc. 2. sea: thalassium surface of the sea: pelagium deep sea: pontium 3. lake: limnium, limnad 4. pond, pool, tiphium, tiphad 5. stagnant water: stasium: stasad 6. salt marsh: limnodium, limnodad 7. fresh marsh: helium 8. wet meadow: telmatium 9. river: potamium 10. creek: rhoium 11. brook: namatium 12. torrent: rhyacium 13. spring: crenium 14. warm spring: thermium 15. ditch: taphrium 16. sewer: laurium 17. swamp forest: helohylium 18. swamp open woodland: helodium 19. meadow thicket: helodrium 20. bank: ochthium rock bank: petrochthium sand bank: ammochthium mud bank: pelochthium 21. rocky seashore: actium 22. sandy seashore: agium 23. sandbar: cheradium 24. tank: phretium

a. Sciophytia: shade plant formations

26. forest: hylium 27. grove: alsium 28. orchard: dendrium 29. canyon: ancium

32. meadow: poium 33. pasture: nomium 34. culture land: agrium 35. waste place: chledium

36. desert: eremium 37. sand-hills, sandy plain: amathium 38. prairie, plains: psilium 39. dry, open woodland: hylodium 40. dry thicket: driodium 41. dry forest: xerohylium 42. gravel slide: chalicium 43. sandbar: syrtidium 44. sand draw: enaulium 45. blowout: anemium 46. strand: psamathium 47. dune: thinium 48. badlands: tirium 49. hill, ridge: lophium 50. cliff: cremnium 51. rock field: phellium 52. boulder field: petrodium 53. rock, stone: petrium 54. humus marsh: oxodium 55. alkali area: drimium 56. heath, dry meadow: xeropoium 57. moor: sterrhium 58. alpine meadow: coryphium 59. polar barrens: crymium 60. snow: chionium 61. wastes: chersium

Particular formations are indicated by means of floristic distinctions. Thus, Populus-hylium is the aspen forest as distinguished from the Picea-Pseudotsuga-hylium, or the balsam-spruce forest; and the Bulbilis-psilium, or buffalo-grass prairie, from the Bouteloua-Andropogon-psilium, or grama-bluestem prairie. Similarly, the aspen formation of the Old World and of the New may be distinguished as Populus-tremula-hylium and Populus-tremuloides-hylium, respectively. In all formational names, the facies alone should be used. Frequently, a single facies will suffice for clearness. As a rule, however, the two most important facies should be employed; in rare cases only is it necessary to use the names of three. When it is desirable to refer to two or more examples of the same formation, a geographical term is added, e. g., (1) Populus-hylium (Crystal Park), (2) Populus-hylium (Cabin Canyon).

353. Developmental classification. This is based upon succession as the record of development. Upon the basis of development, all the formations which belong to the same succession are classed together. They are arranged within each group in the sequence found in the particular succession. From its nature, developmental classification is of primary importance in exhibiting the history of vegetational changes. It has less value than the habitat system for summarizing the essential structure of a vegetation, inasmuch as it places the emphasis upon historical rather than structural features. It is evident that both deal with the same formations, and that the difference is merely one of viewpoint. The habitat classification is simpler in that it considers only those formations actually on the ground, while development has regularly to take into account stages which have disappeared. The groups of the developmental system, and the arrangement of formations within them have already been indicated under the nomenclature of succession (sections 326 and 327).

354. Regional classification. The grouping of formations with respect to the divisions of vegetations is chiefly of geographical value. It indicates a certain general relationship, but its principal use is to summarize the structure of the vegetative covering of a region. The arrangement of formations in the various divisions is made with reference to the outline of North American vegetation (section 341). This is naturally based upon the identity of altitude and latitude zones. In the study of mountain countries, it is often desirable to group formations with reference to altitude alone. In this case, the grouping is based upon the following divisions: (1) bathyphytia, lowland plant formations; (2) mesiophytia, midland formations; (3) pediophytia, upland formations; (4) pagophytia, foot-hill formations; (5) orophytia, subalpine formations; (6) acrophytia, alpine formations; (7) chionophytia, niveal formations.

355. Mixed formations. These are mixtures of two, rarely more, adjacent formations, or of two consecutive stages of the same succession. Mixed formations are really transitions in space or in time between two distinct formations. Theoretically, they are to be referred to one or the other, according to the preponderance of species. Actually, however, they often persist in an intermediate condition for many years, and it becomes necessary to devote considerable attention to them. In some cases, there is good reason to think that the species of two contiguous formations have become permanently associated, and thus constitute a new formation. This is often apparently true in succession, when the change from one stage to the next requires a long term of years, but it is really true only of the very rare cases in which a succession becomes stabilized in a transition stage. When the mixture is due to development, the formations concerned are often quite dissimilar, e. g., grassland and thicket, thicket and forest. If it is the result of position, the formations are usually similar, i. e., both are grassland, thicket, or forest, since the plants of the lower level are regularly assimilated or destroyed, when invasion occurs at two levels. The term mictium (μικτόν, mixture) is here proposed for the designation of all mixed formations, whether they arise from succession or from juxtaposition. Thus, the Mentzelia-Elymus-mictium is the transition between the Mentzelia-Pseudocymopterus-chalicium and the Elymus-Muhlenbergia-chalicium. Similarly, the Populus-Picea-mictium and the Pinus-Pseudotsuga-mictium are transition stages in the development of the Picea-hylium. On the other hand, the Andropogon-Bulbilis-mictium is a mixture produced by the mingling of two contiguous prairie formations. In the future development of this subject, it will probably become desirable to name mixed formations on the basis of origin, but at present this is unnecessary. Both in classification and in description they should be considered between the formations which give rise to them, and this will at once indicate their origin.

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.