(1) Stabilization is the universal tendency of vegetation. (2) The ultimate stage of a succession is determined by the dominant vegetation of the region. Lichen formations are often ultimate in polar and niveal zones; grassland is the final vegetation for plains and alpine stretches, and for much prairie, while forest is the last stage for mesophytic midlands and lowlands, as well as for subalpine regions. (3) Grassland or forest is the usual terminus of a succession; they predominate in lands physiographically mature. (4) The limit of a succession is determined in large part by the progressive increase in occupation, which makes the entrance of invaders more and more difficult. (5) Stabilization proceeds radiately from the pioneer plants or masses. The movement of offshoots is away from the parent mass, and the chances of ecesis are greatest near its edges, in a narrow area in which the reaction is still felt, and the occupation is not exclusive.
(1) The stages, or formations, of a succession are distinguished as initial (prodophytia), intermediate (ptenophytia), and ultimate (aiphytia). (2) Initial formations are open, ultimate formations are closed. (3) The number of species is small in the initial stages; it attains a maximum in intermediate stages; and again decreases in the ultimate formation, on account of the dominance of a few species. (4) The normal sequence of vegetation forms in succession is: (1) algae, fungi, mosses; (2) annuals and biennials; (3) perennial herbs; (4) bushes and shrubs; (5) trees. (5) The number of species and of individuals in each stage increases constantly up to a maximum, after which it gradually decreases before the forms of the next stage. The interval between two maxima is occupied by a mixed formation. (6) A secondary succession does not begin with the initial stage of the primary one which it replaces, but usually at a much later stage. (7) At present, successions are generally mesotropic, grassland and forest being the ultimate stages, though many are xerostatic or hydrostatic. If erosion continue until the sea level is reached, the ultimate vegetation of the globe will be hydrophytic. Should the heat of the sun decrease greatly before this time, the last vegetation will be xerophytic, i. e., crymophytic. (8) The operation of succession was essentially the same during the geological past as it is to-day. From the nature of their vegetation forms, the record deals largely with the ultimate stages of such successions.
CLASSIFICATION AND NOMENCLATURE
325. Basis. New or denuded habitats arise the world over by the operation of the same or similar causes, and they are revegetated in consequence of the same reactions. Similar habitats produce similar successions. The vegetation forms and their sequence are usually identical, and the genera are frequently the same, or corresponding in regions not entirely unrelated. The species are derived from the adjacent vegetation, and, except in alpine and coast regions, are normally different. The primary groups of successions are determined by essential identity of habitat or cause, e. g., aeolian successions, erosion successions, burn successions, etc. When they have been more generally investigated, it will be possible to distinguish subordinate groups of successions, in which the degree of relationship is indicated by the similarity of vegetation forms, the number of common genera, etc. For example, burn successions in the Ural and in the Rocky mountains show almost complete similarity in the matter of vegetation forms and their sequence, and have the majority of their genera in common. A natural classification of successions will divide them first of all into normal and anomalous. The former fall into two classes, primary and secondary, and these are subdivided into a number of groups, based upon the cause which initiates the succession.
Fig. 69. Aspen forest formation (Populus-hylium), the typical stage of burn successions in the Rocky mountains; it is sometimes an anomalous stage in primary successions, interpolated in place of the thicket formation.
326. Nomenclature. The need of short distinctive names of international value for plant formations is obvious; it has become imperative that successions also should be distinguished critically and designated clearly. From the very nature of the case, it is impossible to designate each formation or succession by a single Greek or Latin term, as habitats of the same character will show in different parts of the world a vegetation taxonomically very different. It may some day be possible to use a binomial or trinomial for this purpose, somewhat after the fashion of taxonomy, in which the habitat name will represent the generic idea as applied to formations, and a term drawn from the floristic impress the specific idea. Such an attempt would be futile or valueless at the present time; it could not possibly meet with success until there is more uniformity in the concept of the formation, and until there has been much accurate and thorough investigation of actual formations, a task as yet barely begun. At present, it seems most feasible as well as scientific to designate all formations occupying similar habitats by a name drawn from the character of the latter, such as a meadow formation, poium, a forest formation, hylium, a desert formation, eremium, etc. A particular formation is best designated by using the generic name of one or two of its most important species in conjunction with its habitat term, as Spartina-Elymus-poium, Picea-Pinus-hylium, Cereus-Yucca-eremium, etc. Apparently a somewhat similar nomenclature is adapted to successions. The cause which produces a new habitat may well furnish the basis for the name of the general groups of successions, as pyrium (literally, a place or a habitat burned over), a burn succession, tribium, an erosion succession, etc. A burn succession consists of a sequence of certain formations in one part of the world, and of a series of quite different ones, floristically, in another. A particular burn succession should be designated by using the names of a characteristic facies of the initial and ultimate stages in connection with the general term, e. g., Bryum-Picea-pyrium, etc. A trinomial constructed in this way represents the desirable mean between definition and brevity. Greater definiteness is possible only at the expense of brevity, while to shorten the name would entirely destroy its precision. The following classification of successions is proposed, based upon the plan outlined above. The termination -ium (εῖον) has been used throughout in the construction of names for successions, largely for reasons of euphony. If it should become desirable to distinguish the names of formations and successions by the termination, the locative suffix -on (-ών) should be used for the latter. The terms given below would then be hypson, rhyson, hedon, sphyron, prochoson, pnoon, pagon, tribon, clyson, repon, olisthon, xerasion, theron, broton, pyron, ecballon, camnon, ocheton, ardon.
I. Normal successions: cyriodochae (κύριος, regular, δοχή, ἡ, succession)
a. Primary successions: protodochae (πρῶτος, first, primary) 1. By elevation: hypsium (ὔψος, το, height, elevation, -εῖον, place) 2. By volcanic action: rhysium (ῥυσίς ἡ, flowing, especially of fire) 3. In residuary soils: hedium (ἔδος, τό, a sitting base) 4. In colluvial soils: sphyrium (σφύρον, τό, ankle, talus) 5. In alluvial soils: prochosium (πρόχωσις, ἡ, a deposition of mud) 6. In aeolian soils: pnoium (πνοή, ἡ, blowing, blast) 7. In glacial soils: pagium (πάγος, ὁ, that which becomes solid, i. e., a glacier) b. Secondary successions: hepodochae (ἕπω, to follow) 8. In eroded soils: tribium (τρίβω, wear or rub away) 9. In flooded soils: clysium (κλύσις, ὁ, a drenching, flooding) 10. By subsidence: repium (ῥέπω, incline downwards, sink) 11. In landslips: olisthium (ὄλισθος, ὁ, slip) 12. In drained and dried out soils: xerasium (ξηρασία, ἡ, drought) 13. By animal agencies: therium (θήρ, ὁ, wild animal) 14. By human agency: brotium (βροτός, ὁ, a mortal) a. Burns: pyrium (πῦρ, τό, fire) b. Lumbering: ecballium (ἐκβάλλω, cut down forests) c. Cultivation: camnium (κάμνω, cultivate) d. Drainage: ochetium (ὀχετός, ὁ, drain) e. Irrigation: ardium (ἄρδω, irrigate)
327. Illustrations. The following series will illustrate the application of this system of nomenclature to particular successions, and their stages, or formations.