Chapter IV: The Plant Formation (5)

=296. Succession in alluvial soils.= Alluvial soils are fluvial when laid down by streams and rivers, and litoral when washed up by the waves or tides. They are formed when any obstacle retards the movement of the water, decreasing its carrying power, and causing the deposit of part or all of its load. They consist of more or less rounded, finely comminuted particles, mingled with organic matter and detritus. Alluvial deposits are especially frequent at the mouth of streams and rivers, on their terraces and flood plains, and along silting banks as compared with the erosion banks of meanders. The filling of ponds by the erosion due to surface drainage, and of lakes by the deposition of the loads of streams that enter them, results in the formation of new alluvium. A similar phenomenon occurs along coasts, where bays and inlets are slowly converted into marshes in consequence of being shallowed by the material washed in by the waves and tides. Such paludal deposits are invariably salt water or brackish. Contrasted with these, which are uniformly black in consequence of the large amount of organic matter present, are the sandbars and beaches, which, though due to the same agents, are light grey or white in color, because of the constant leaching by the waves. Two kinds of alluvial deposits may accordingly be distinguished: (1) those black with organic matter, and little disturbed by water, and (2) those of a light color, which are constantly swept by the waves. The successions corresponding to these are radically different. In the first, the pioneer vegetation is hydrophytic, consisting largely of amphibious plants. The pioneer stages retard the movement of the water more and more, and correspondingly hasten the deposition of its load. The marsh bed slowly rises in consequence, and finally the marsh begins to dry out, passing first into a wet meadow, and then into a meadow of the normal type. A notable exception to this sequence occurs when the swamp contains organic matter or salts in excess, in which case the vegetation consists indefinitely of swamp xerophytes, or halophytes. The first vegetation on fresh water sandbars is xerophytic, or, properly, dissophytic, unless they remain water-swept, and the ultimate stages of their successions are mesophytic woodlands composed of water-loving genera, _Populus_, _Salix_, etc. It seems certain, however, that these will finally give way to longer-lived hardwoods. Maritime sandbars and beaches are always saline, and their successions run their short course of development entirely within the group of halophytes, unless the retreat of the sea or freshwater floods change the character of the soil. The chemical action of underground waters also produces new soils, which might be classed as alluvial. These soils are essentially rock deposits, travertine, silicious sinter, etc., made by iron and lime springs and by geysers, and they must be changed by decomposition into soils proper to be comparable with alluvial soils.

=297. Succession in aeolian soils.= The only wind-borne soils of geological importance at the present time are those which form dunes, both inland and coastal. Aeolian deposits consist largely of rounded sand particles, which are of almost uniform size in any particular dune, but vary greatly in dunes of different ages. The reaction of the pioneers on dunes plays an important part in building the latter, but the immense dunes of inland deserts, which are entirely destitute of vegetation, seem to indicate that its value has been overestimated. The first stages in dune successions are dissophytic, i. e., the plants grow in a soil of medium or high water-content, but in an atmosphere that is extremely xerophytic. The ultimate stages vary widely in accordance with the region in which they occur; they may be xerophytic heaths or mesophytic meadows and forests. Because of their striking character and economic significance, dunes have received much attention, with the result that their successions are the most thoroughly known of all. Prairie and steppe formations are probably to be regarded as the ultimate stages of successions established on wind-borne loess, and it is possible that the same is true of sand-hill vegetation in the prairie province.

=298. Succession in glacial soils.= The formation of glacial deposits is at present confined to alpine and arctic regions. Recent successions in such soils are localized in these regions, and are in consequence relatively unimportant. There can be little question, however, that the thorough investigation of succession in and near the moraines of existing glaciers will throw much light upon the successions of the glacial period. Moraines, drumlins, eskars, and alluvial cones represent the various kinds of glacial deposits. They agree in being heterogeneous in composition, and are covered to-day with ultimate stages of vegetation, except in the immediate vicinity of glaciers.

_SECONDARY SUCCESSIONS_

=299.= Generally speaking, all successions on denuded soils are secondary. When vegetation is completely removed by excessive erosion, it is an open question whether the resulting habitat is to be regarded as new or denuded. Erosion is rarely so extreme and so rapid, however, as to produce such a condition, even when it results from cultivation or deforestation. It is, moreover, especially characteristic of newly formed soils, and in studying succession in eroded habitats, it is fundamentally important to determine whether erosion has produced denudation, or has operated upon a new soil. The great majority of secondary successions owe their origin to floods, animals, or the activities of man, and they agree in occurring upon decomposed soils of medium water-content, which contain considerable organic matter, and a large number of dormant migrants. These successions consist of relatively few stages, and are rarely of extreme character.

=300. Succession in eroded soils.= Eroded soils show considerable differences, as they arise in consequence of erosion by water or by wind, though the initial stages of revegetation derive their character more from the aggregation of the soil than from the nature of the erosive agent. Eroded soils are as a rule xerophytic. In the case of erosion by water, dysgeogenous soils are readily worn away in consequence of their lack of cohesion, as in sand draws, etc., while eugeogenous soils are easily eroded only on slopes, as in the case of ravines, hillsides, etc. In the former, the extreme porosity and slight capillarity of the sand and gravel result in a low water-content. In the finer soils, the water-content is also low, on account of the excessive run-off, due to compactness of the particles and to the slope. The erosive action of winds upon soils bearing vegetation is not very general; it is found to some extent in more or less established dunes, and exists in a marked degree in buttes, mushroom rocks, and blowouts. The first two are regularly xerophytic, the last as a rule, dissophytic. The early stages of successions in eroded soils are composed of xerophytes. In loose soils, these are forms capable of binding the soil particles together, thus preventing wash, and increasing the accumulation of fine particles, especially of organic matter. In compact soils, the effect is much the same; the pioneers not only decrease erosion, but at the same time also increase the water-content by retarding the movement of the run-off.

=301. Succession in flooded soils.= The universal response of vegetation to floods is found in the amphibious plant, which is a plastic form capable of adjustment to very different water-contents. Floods are confined largely to river basins and coasts. In hilly and mountainous regions, where the slope is great, any considerable accumulation of flood waters is now impossible, although of frequent occurrence when land forms were more plastic.

In all streams that have become graded, the fall is insufficient to carry off the surplus water in the spring when snows are melting rapidly, or at times of unusual precipitation. These waters accumulate, and, overflowing the banks, spread out over the lowlands, resulting in the formation of a well-defined flood plain. This is a periodical occurrence with mature streams, and it occurs more or less regularly with all that are not torrent-like in character. The effect of the overflow is to destroy or to place at a disadvantage those plants of the flood plain that are not hydrophytes. At the same time, a thin layer of fresh silt is deposited upon the valley floor of sand or alluvium. Flooding is most frequent and of longest duration near the banks of the stream. It extends more or less uniformly over the flood plain, and disappears gradually or abruptly as the latter rises into the bench above. Floods destroy vegetation and make a place for secondary successions by drowning out mesophytic species, by washing away the aquatic forms of ponds and pools, and by the erosion of banks and sandbars. They affect the amphibious vegetation of swamp and shore to a certain extent, but, unless the period of flooding is long, they tend to emphasize such formations rather than to destroy them. The still-water formations of many cutoff and oxbow lakes owe their origin to a river which cuts across a meander in time of flood. This result is more often attained by the alternate silting and erosion of a meandering river by which it cuts across a bend in its channel. The usual successions in flooded lands are short as a rule; amphibious algae, liverworts, and mosses soon give way to ruderal plants, and these in turn to the original mesophytes of meadows, or dissophytes of sandbars. In the case of ponds and pools, the process of washing-out or silting up merely removes or destroys the vegetation, without effectively modifying the habitat, and the secondary successions that follow are extremely short.

=302. Succession by subsidence.= Subsidence is a factor of the most profound importance in changing vegetation. It operates over vast areas through immense periods of time. For these reasons, the changes are so slow as to be almost imperceptible, and the resulting successions can be studied only in the geological record. Extensive subsidence is confined to-day to coastal plains, as in Greenland, the south Atlantic coast, and the region of the Mississippi delta, where its effects are merged with the paludation of tidal rivers, and the wave and tide erosion of the sea shore. Such successions are unique, inasmuch as the denuding force operates very slowly instead of quickly, and the first pioneers of the new vegetation appear before the original formation has been destroyed. In all cases, the succession is from mesophytic or halophytic formations to paludose, and, finally, marine vegetation. In small areas of subsidence, such as shore slips along lakes and streams, sink holes, and sunken bogs, the succession is usually both short and simple, mesophytes giving place to amphibious and ultimately to aquatic forms.

=303. Successions in landslips.= Landslips occur only in montane and hilly regions, and here they are merely of local importance. In many respects, they are not unlike talus; they show essential differences, however, in that they are not sorted by gravity, and in that they destroy vegetation almost instantly. The succession arises as a rule, not upon the original soil, but upon that of the landslip, and, as pointed out elsewhere, might well be regarded as primary.

=304. Succession in drained, or dried soils.= In geological times, the subsidence of barriers must often have produced drainage and drying-out, just as elevation frequently resulted in flooding and lake formation. At the present time, the drying-out of lakes and ponds is the result of artificial drainage, or of climatic changes. The former will be considered under successions brought about by the agency of man. Climatic changes when general operate so slowly that the stages of such successions are perceptible only when recorded in strata. More locally, climate swings back and forth through a period of years, with the result that in dry years the swamps and ponds of wetter seasons are dried out, and the vegetation destroyed or changed. If the process be gradual, the succession passes from hydrophytic through amphibious to mesophytic, and, in dry regions, xerophytic conditions. When the process of drying-out occurs rapidly, as in a single summer, the original formation is destroyed, and the new vegetation consists largely of ruderal plants. A peculiar effect of climate occurs in regions with poor drainage, where the result of intense evaporation is to produce alkaline basins and salt lakes, in which the succession becomes more and more open, and is finally represented by a few stabilized halophytes, or disappears completely.

=305. Succession by animal agency.= Successions of this class are altogether of secondary importance, the instances in which animals produce denudation being relatively few. Such are the heaps of dirt thrown up by prairie dogs and other burrowing animals, upon which ruderal plants are first established, to be finally crowded out by the species of the original formation. Buffalo wallows furnish examples of similar successions in which the initial stages are subruderal, while overstocking and overgrazing frequently produce the same result with ruderal plants.

=306. Succession by human agency.= The activities of man in changing the surface of the earth are so diverse that it is impossible to fit the resulting successions in a natural system. While man does not exactly make new soils, he exposes soils in various operations: mining, irrigation, railroad building, etc. He destroys vegetation by fires, lumbering, cultivation, and drainage, and if he can not control climate, he at least modifies its natural effects by irrigation and the conservation of moisture. The operations of man extend from seacoasts and swampy lowlands through mesophytic forests and prairies to the driest uplands and inlands. Since the adjacent formations determine in large degree the course and constitution of a succession, it will be seen that the effects of any particular activity upon vegetation will differ greatly in different regions. For convenience, all classes of successions arising from the presence and activity of man will be considered in this place, though, as indicated above, some might well be regarded as producing primary successions, while others produce anomalous ones.

=307. Succession in burned areas.= It will suffice merely to point out that “burns” may arise naturally through lightning, volcanic cinders, lava flows, etc., but the chances are so slight that these causes may be ignored. The causes of fires are legion, and as they have little or no effect upon results, they need not be considered. From their nature, fires are of little significance in open vegetation, deserts, polar barrens, alpine fields, etc., since the area of the burn can never be large. In closed formations, the extent of fires is limited only by the area of the vegetation, and the effect of wind, rain, and other forces. Forest fires usually occur during the resting period, except in the case of coniferous forests. In grassland, the living parts are underground during autumn and winter, when prairie fires commonly occur. As a consequence, the repeated annual burning of meadow or prairie does not result in denudation and subsequent succession. On the contrary, it acts in part as a stabilizing agent, inasmuch as it injures the typical vegetation forms of grassland much less than it does the woody invaders. All formations with perennial parts above ground, viz., thicket, open woodland, and forest, are seriously injured by fire. A severe general fire destroys the vegetation completely; a local fire destroys the formation in restricted areas; while a slight or superficial burn removes the undergrowth and hastens the disappearance of the weaker trees. In the latter case, while the primary layer of the forest remains the same, succession takes place in the herbaceous and shrubby layers. These successions are peculiar in that they are composed almost wholly of the proper species of the forest, and that they are very short, showing only a few poorly defined stages. A local fire initiates a succession in which the pioneers are derived largely from the original formation, particularly when the latter encloses the burned area more or less completely. The constitution of the intermediate and ultimate stages will depend in a larger degree still upon the size and position of the burn. When a particular formation is destroyed wholly or in large part, the first stages of the new vegetation are made up by invaders from the adjacent formations. In the most perfect types of succession, this dissimilarity between the new and the old vegetation continues to the last stage, in which the reappearance of the facies precedes that of the subordinate layers. In many forest successions, however, the general physical similarity of the ultimate stages permits the early reappearance of the herbaceous and shrubby species, and the final stages affect the facies alone. Successions in burned areas operate usually within the water-content groups. The reconstruction of a mesophytic forest takes place by means of mesophytes; of the rarer xerophytic and hydrophytic forests, through xerophytes and hydrophytes respectively. This is due to the fact that the alteration of the soil is slight, except where the burning of the vegetation permits the entrance of erosion, as on mountain slopes.

=308. Succession in lumbered areas.= Commercial lumbering, especially where practiced for wood-pulp as well as for timber, results in complete or nearly complete destruction of the vegetation by removal and the change from diffuse light to sunlight, or by the action of erosion upon the exposed surface. In the first place, short mesophytic successions will result; in the second, the successions will be long and complex, passing through decreasingly xerophytic conditions to a stable mesophytic forest. Where a forest is cut over for certain species alone, the undisturbed trees soon take full possession, though the causes effective in the beginning will ultimately restore the original facies in many instances. Such successions are anomalous, and will be treated under that head.

=309. Succession by cultivation.= The clearing of forests and the “breaking” of grassland for cultivation destroy the original vegetation; the temporary or permanent abandonment of cultivated fields then permits the entrance of ruderal species, which are the pioneers of new successions. This phenomenon takes place annually in fields after harvest, resulting in the secondary formations of Warming, in which practically the same species reappear year after year. In fields that lie fallow for several years, or are permanently abandoned, the first ruderal plants are displaced by newcomers, or certain of them become dominant at the expense of others. In a few years, these are crowded out by invaders from the adjacent formations, and the field is ultimately reclaimed by the original vegetation, unless this has entirely disappeared from the region. The number of stages depends chiefly upon whether the final formation is to be grassland or woodland. Other activities of man, such as the construction of buildings, roads, railways, canals, etc., remove the native vegetation, and make room for the rapid development of ruderal formations. In and about cities, where the original formations have entirely disappeared, the chance for succession is remote, and the initial ruderal stages become more or less stabilized. Elsewhere the usual successions are established, and the ruderal formation finally gives way to the dominant type. In mountain and desert regions, where ruderal plants are rare or lacking, their place is taken by subruderal forms, species of the native vegetation capable of rapid movement in them. These, like ruderal plants, are gradually replaced by other native species of less mobility, but of greater persistence, resulting in a short succession operating often within a single formation. From the nature of cultivated plants, succession after cultivation generally operates within the mesophytic series.

=310. Succession by drainage.= Successions of this kind show much the same stages as are found in those due to flooding. They proceed from aquatic or swamp formations to mesophytic termini, either grassland or woodland. When drainage takes place rapidly and completely, the pioneer stages are usually xerophytic; cases of this sort, however, are infrequent.

=311. Succession by irrigation.= Irrigation produces short successions of peculiar stamp along the courses of irrigating canals and ditches, and in the vicinity of reservoirs. These are recent, as a rule, and are usually found in the midst of cultivated lands, so that their complete history is still a matter of conjecture. The original xerophytes are forced out not only by the disturbance of the soil, but also by its increased water-content. A few of them often thrive under the new conditions, and, together with the usual ruderal plants and a large number of lowland mesophytes and amphibious forms derived from the banks of the parent stream, constitute a heterogeneous association. This is doubtless to be regarded as an initial stage of a succession, but it is an open question whether the succession will early be stabilized as a new formation, or whether the original vegetation will sooner or later be reestablished under somewhat mesophytic conditions. From the number of mesophytes and from the behavior of valleys, it seems certain that the banks of such canals will ultimately be occupied by a formation more mesophytic than hydrophytic, into which some of the surrounding xerophytes of plastic nature have been adopted.

=312. Anomalous successions= are those in which the physical change in the habitat is relatively slight, resulting in a displacement of the ultimate stage, or the disturbance of the usual sequence, merely, instead of the destruction and reconstruction of a formation, or the gradual development of a new series of stages on new soil. In nature, the ultimate grass or forest stage of a normal succession is often replaced by a similar formation, especially if the facies be few or single. It is evident that certain trees naturally replace others in the last stages of a forest succession, without making the latter anomalous. The last occurs only when a normal stage is replaced by one belonging properly to an entirely different succession, as when a coniferous forest replaces a deciduous one in a hardwood region. The presence and development of such successions can be determined only after the normal types are known. The interpolation of a foreign stage in a natural succession, or a change of direction, by which a succession that is mesotropic again becomes hydrophytic, is easily explained when it is the result of artificial agents, as is often the case. In nature, anomalous successions are commonly the result of a slow backward and forward swing of climatic conditions.

=313. Perfect and imperfect successions.= A normal succession will regularly be perfect; it passes in the usual sequence from initial to ultimate conditions without interruption or omission. Imperfect succession results when one or more of the ordinary stages is omitted anywhere in the course, and a later stage appears before its turn. It will occur at any time when a new or denuded habitat becomes so surrounded by other vegetation that the formations which usually furnish the next invaders are unable to do so, or when the abundance and mobility of certain species enable them to take possession before their proper turn, and to the exclusion of the regular stage. Incomplete successions are of great significance, inasmuch as they indicate that the stages of a succession are often due more to biological than to physical causes, the proximity and mobility of the adjacent species being more determinative than the physical factors. Subalpine gravel slides regularly pass through the rosette, mat, turf, thicket, woodland, and forest stages; occasionally, however, they pass immediately from the rosette, or mat condition, to an aspen thicket which represents the next to the last stage. Such successions are by no means infrequent in hilly and montane regions; in regions physiographically more mature or stable, perfect successions are almost invariably the rule.

=314. Stabilization.= It may be stated as a general principle that vegetation moves constantly and gradually toward stabilization. Each successive stage modifies the physical factors, and dominates the habitat more and more, in such a way that the latter seems to respond to the formation rather than this to the habitat. The more advanced the succession, i. e., the degree of stabilization, the greater the climatic or physiographic change necessary to disturb it, with the result that such disturbances are much more frequent in the earlier stages than in the later development. Constant, gradual movement toward a stable formation is characteristic of continuous succession. Contrasted with this is intermittent succession, in which the succession swings for a time in one direction, from xerophytic to mesophytic for example, and then moves in the opposite direction, often passing through the same stages. This phenomenon usually is characteristic only of the less stable stages, and is generally produced by a climatic swing, in which a series of hot or dry years is followed by one of cold or wet years, or the reverse. The same effect upon a vast scale is produced by alternate elevation and subsidence, but these operate through such great periods of time that one can not trace, but can only conjecture their effects. A normal continuous succession frequently changes its direction of movement, or its type, in transition regions or in areas where the outposts of a new flora are rapidly advancing, as in wide mesophytic valleys that run down into or traverse plains. Here the change is often sudden, and grass and desert formations are replaced by thickets and forests, resulting in abrupt succession. Species guilds are typical examples of this. More rarely, a stage foreign to the succession will be interpolated, replacing a normal stage, or slipping in between two such, though finally disappearing before the next regular formation. This may be distinguished as interpolated succession.

The apparent terminus of all stabilization is the forest, on account of the thoroughness with which it controls the habitat. A close examination of vegetation, however, will show that its stable terms are dependent in the first degree upon the character of the region in which the formation is indigenous. It is obviously impossible that successions in desert lands, in polar barrens, or upon alpine stretches should terminate in forest stages. In these, grassland must be the ultimate condition, except in those extreme habitats, alpine and polar, where mosses and lichens represent the highest type of existing vegetation. Forests are ultimate for all successions in habitats belonging to a region generally wooded, while grassland represents the terminus of prairie and plains successions as well as of many arctic-alpine ones.

_CAUSES AND REACTIONS_

=315.= The initial cause of a succession must be sought in a physical change in the habitat; its continuance depends upon the reaction which each stage of vegetation exerts upon the physical factors which constitute the habitat. A single exception to this is found in anomalous successions, where the change of formation often hinges upon the appearance of remote or foreign disseminules. The causes which initiate successions have already been considered; they may be summarized as follows: (1) weathering, (2) erosion, (3) elevation, (4) subsidence, (5) climatic changes, (6) artificial changes. The effect of succeeding stages of vegetation upon a new or denuded habitat usually finds expression in a change of the habitat with respect to a particular factor, and in a definite direction. Often, there is a primary reaction, and one or more secondary ones, which are corollaries of it. Rarely, there are two or more coordinate reactions. The general ways in which vegetation reacts upon the habitat are the following: (1) by preventing weathering, (2) by binding aeolian soils, (3) by reducing run-off and preventing erosion, (4) by filling with silt and plant remains, (5) by enriching the soil, (6) by exhausting the soil, (7) by accumulating humus, (8) by modifying atmospheric factors. The direction of the movement of a succession is the immediate result of its reaction. From the fundamental nature of vegetation, it must be expressed in terms of water-content. The reaction is often so great that the habitat undergoes a profound change in the course of the succession, changing from hydrophytic to mesophytic or xerophytic, or the reverse. This is characteristic of newly formed or exposed soils. Such successions are _xerotropic_, _mesotropic_, or _hydrotropic_, according to the ultimate condition of the habitat. When the reaction is less marked, the type of habitat does not change materially, and the successions are _xerostatic_, _mesostatic_, or _hydrostatic_, depending upon the water-content. Such conditions obtain for the most part only in denuded habitats.

=316. Succession by preventing weathering.= Reactions of this nature occur especially in alpine and boreal regions, in the earlier stages of lichen-moss successions. They are typical of igneous and metamorphic rocks in which disintegration regularly precedes decomposition. The influence of the vegetation is best seen in the lichen stages, where the crustose forms make a compact layer, which diminishes the effect of the atmospheric factors producing disintegration. In alpine regions especially, this protection is so perfect that the crustose lichens may almost be regarded as the last stage of a succession. There are no recorded observations which bear upon this point, but it seems certain that the pioneer rock lichens, _Lecanora_, _Lecidea_, _Biatora_, _Buellia_, and _Acarospora_, cover alpine rocks for decades, if not for centuries. Ultimately, however, the slow decomposition of the rock surface beneath the thallus has its effect. Tiny furrows and pockets are formed, in which water accumulates to carry on its ceaseless work, and the compact crustose covering is finally ruptured, permitting the entrance of foliose forms. The latter, like the mosses, doubtless protect rock surfaces, especially those of the softer rocks, in a slight degree against the influence of weathering, but this is more than offset by their activity in hastening decomposition, and thus preparing a field for invasion. Rocks and boulders (_petria_, _petrodia_, _phellia_) furnish the best examples of this reaction; cliffs (_cremnia_) usually have a lichen covering on their faces, while the forces which produce disintegration operate from above or below.

=317. Succession by binding aeolian soils.= Dunes (_thinia_) are classic examples of the reaction of pioneer vegetation upon habitats of wind-borne sand. The initial formations in such places consist exclusively of sand-binders, plants with masses of fibrous roots, and usually also with strong rootstalks, long, erect leaves, and a vigorous apical growth. They are almost exclusively perennial grasses and sedges, possessing the unique property of pushing up rapidly through a covering of sand. They react by fixing the sand with their roots, thus preventing its blowing about, and also by catching the shifting particles among their culms and leaves, forming a tiny area of stabilization, in which the next generation can establish a foothold. The gradual accumulation of vegetable detritus serves also to enrich the soil, and makes possible the advent of species requiring better nourishment. Blowouts (_anemia_) are almost exact duplicates of dunes in so far as the steps of revegetation are concerned; while one is a hollow, and the other a hill, in both the reaction operates upon a wind-swept slope. Sand-hills (_amathia_) and deserts (_eremia_) show similar though less marked reactions, except where they exhibit typical inland dunes. Sand-binders, while usually classed as xerophytic or halophytic, are in reality dissophytes. Their roots grow more or less superficially in moist sand, and are morphologically mesophytic while their leaves bear the stamp of xerophytes. The direction of movement in successions of this kind is normally from xerophytes to mesophytes, i. e., it is mesotropic. In sand-hills and deserts, the succession operates wholly within the xerophytic (dissophytic) series. Along seacoasts, the mesophytic terminus is regularly forest, except where forests are remote, when it is grassland.

=318. Succession by reducing run-off and erosion.= All bare or denuded habitats that have an appreciable slope are subject to erosion by surface water. The rapidity and degree of erosion depend upon the amount of rainfall, the inclination of the slope, and the structure of the surface soil. Regions of excessive rainfall, even where the slope is slight, show great, though somewhat uniform erosion; hill and mountain are deeply eroded even when the rainfall is small. Slopes consisting of compact eugeogenous soils, notwithstanding the marked adhesion of the particles, are much eroded where the rainfall is great, on account of the excessive run-off. Porous dysgeogenous soils, on the contrary, absorb most of the rainfall; the run-off is small and erosion slight, except where the slope is great, a rare condition on account of the imperfect cohesion of the particles. In compact soils, the plants of the initial formations not merely break the impact of the raindrops, but, what is much more important, they delay the downward movement of the water, and produce numberless tiny streams. The delayed water is largely absorbed by the soil, and the reduction of the run-off prevents the formation of rills of sufficient size to cause erosion. As in dunes, such plants are usually perennial grasses, though composites are frequent; the root system is, however, more deeply seated, and a main or tap root is often present. On sand and gravel slopes, the loose texture of the soil results generally in the production of sand-binders with fibrous roots. Unlike dunes, such slopes exhibit a large number of mats and rosettes with tap-roots, which are effective in preventing the slipping or washing of the sand, and run little danger of being covered, as is the case with duneformers. In both instances, each pioneer plant serves as a center of comparative stabilization for the establishment of its own offspring, and of such invaders as find their way in. From the nature of these, slopes almost invariably pass through grassland stages before finding their termini in thickets or forests. Bad lands (_tiria_) furnish the most striking examples of eroded habitats. The rainfall in the bad lands of Nebraska and South Dakota is small (300 mm.); yet the steepness of the slope and the compactness of the soil render erosion so extreme that it is all but impossible for plants to obtain a foothold. Their reaction is practically negligible, and the vegetation passes the pioneer stages only in the relatively stable valleys. Mountain slopes (_ancia_), and ridges and hills (_lophia_) are readily eroded in new or denuded areas. This is especially true of hill and mountain regions which have been stripped of their forest or thicket cover by fires, lumbering, cultivation, or grazing. Where the erosion is slight, the resulting succession may show initial xerophytic stages, or it may be completely mesostatic. Excessively eroded habitats are xerostatic, as in the case of bad lands, or, more frequently, they are mesotropic, passing first through a long series of xerophytic formations. Sandbars (_cheradia_, _syrtidia_) should be considered here, though they are eroded by currents and waves, and not by run-off. They are fixed and built up by sand-binding grasses and sedges, usually of a hydrophytic nature, and pass ultimately into mesophytic forest.

=319. Succession by filling with silt and plant remains.= All aquatic habitats into which silt, wash, or other detritus is borne by streams, currents, floods, waves, or tides are slowly shallowed by the action of the water plants present. These not only check the movement of the water, thus greatly decreasing its carrying power, and causing the deposition of a part or all of its load, but they also retain and fix the particles deposited. In accordance with the rule, each plant becomes the center of a stabilizing area, which rises faster than the rest of the floor, producing the well-known hummocks of lagoons and swamps. All aquatics produce this reaction. It is more pronounced in submerged and amphibious forms than in floating ones, and it takes place more rapidly with greatly branched or dissected plants than with others. In pools (_tiphia_) and lakes (_limnia_), debouching streams and surface waters deposit their loads in consequence of the check exerted by the still water and the marginal vegetation, and delta-like marshes are quickly built up by filling. Springs (_crenia_) likewise form marshes where they gush forth in sands, the removal of which is impeded by vegetation. The flood plains and deltas of rivers show a similar reaction. The heavily laden flood waters are checked by the vegetation of meadows and marshes, and deposit most of their load. The banks of streams (_ochthia_) and of ditches (_taphria_) are often built up in the same fashion by the action of the marginal vegetation upon the current. The presence of marginal vegetation often determines the checking or deflecting of the current in such a way as to initiate meanders, while natural levees owe their origin to it, in part at least. Along low seacoasts, waves and tides hasten the deposit of river-borne detritus, causing the water to spread over the lowlands and form swamps. They often throw back also the sediment that has been deposited in the sea, the marsh vegetation acting as a filter in both cases. Successions of the kind indicated above are regularly mesotropic. Where the soil is sandy, and the filling-up process sufficiently great, or where salts or humus occur in excess, xerophytic formations result. In certain cases, these successions appear to be permanently hydrostatic, changing merely from floating or submerged to amphibious conditions, but this is probably due to the slowness of the reaction. As a rule, the accumulation of plant remains is relatively slight, and plays an unimportant part in the reaction. In peat bogs and other extensive swamps, the amount of organic matter is excessive, and plays an important role in the building up of the swamp bed.

=320. Succession by enriching the soil.= This reaction occurs to some degree in the great majority of all successions. The relatively insignificant lichens and mosses produce this result upon the most barren rocks, while the higher forms of later stages, grasses, herbs, shrubs, and trees, exhibit it in marked progression. The reaction consists chiefly in the incorporation of the decomposed remains of each generation and each stage in the soil. A very important part is played by the mechanical and chemical action of the roots in breaking up the soil particles, and in changing them into soluble substances. Mycorrhizae, bacterial nodules, and especially soil bacteria play a large part in increasing the nutrition-content of the soil, but the extent to which they are effective in succession is completely unknown. The changes in the color, texture, and food value of the soil in passing from the initial to ultimate stages of a normal succession are well known, and have led many to think them the efficient reactions of such successions. It seems almost certain, however, that this is merely a concomitant, and that, even in anomalous successions where facies replace each other without obvious reasons, the reactions are concerned more with water-content, light, and humidity than with the food-content of the soil.

=321. Succession by exhausting the soil.= This is a reaction not at all understood as yet in nature. A number of phenomena, such as the “fairy rings” of mushrooms and other fungi, the peripheral growth and central decay of lichens, _Lecanora_, _Placodium_, _Parmelia_, and of matforming grasses, such as _Muhlenbergia_, and the circular advance of the rootstalk plants, indicate that certain plants at least withdraw much of the available supply of some essential soil element, and are forced to move away from the exhausted area. It is probable that the constant shifting of the individuals of a formation year after year, a phenomenon to be discussed under alternation, has some connection with this. It will be impossible to establish such a relation, however, until the facts are exactly determined by the method of quadrat statistics. So far as native formations are concerned, there can not be the slightest question that prairies and forests have existed over the same area for centuries without impoverishing the soil in the least degree, a conclusion which is even more certain for the open vegetation of deserts and plains. With culture formations, the case is quite different. The exhaustion of the soil by continuous or intensive cultivation is a matter of common experience in all lands settled for a long period. Calcium, phosphorus, and nitrogen compounds especially are used up by crops, and must be supplied artificially. The reason for this difference in reaction between native and culture formations seems evident. In harvesting, not merely the grain, but the stems and leaves, and in gardening often the root also, are removed, so that the plant makes little or no return to the soil. In nature, annual plants return to the ground every year all the solid matter of roots, stems, leaves, and fruits, with the exception of the relatively small number of seeds that germinate. Perennial herbs return everything but the persistent underground parts. Shrubs and trees replace annually an immense amount of material used in leaves and fruits, and sooner or later, by the gradual decay of the individuals or by the destruction of the whole formation, they restore all that they have taken from the soil. This balance is further maintained to an important degree by the activity of the roots, which take from the deep-seated layers of the soil the crude materials necessary for the formation of leaves and fruits. Upon the fall and decay of these, their materials are incorporated with the upper layers of the formation floor, from which they may be absorbed by the undergrowth, or find their way again into the layers permeated by the tree roots. From the universal occurrence of weeds in cultivated regions, the pioneers in impoverished or exhausted fields are uniformly ruderal plants. As is well known, the seed production and ecesis of these forms are such that they take possession quickly and completely, while their demands upon the soil are of such a nature that the most sterile field can rapidly be covered by a vigorous growth of weeds. As indicated elsewhere, ruderal formations ultimately yield to the native vegetation, though in regions so completely given over to culture that native formations are lacking or remote, it is probable that successions reach their final stage within the group of ruderal plants.

=322. Succession by the accumulation of humus.= This is the characteristic reaction of peat bogs and cypress swamps (_oxodia_), in which the accumulation of vegetable matter is enormous. The plant remains decompose slowly and incompletely under the water, giving rise to the various humic acids. These possess remarkable antiseptic qualities, and have an injurious effect upon protoplasm. They affect the absorption of water by the root-hairs, though this is also influenced by poor aeration. The same acids are found in practically all inland marshes and swamps, but the quantity of decomposing vegetation in many is not great enough to produce an efficient reaction. Formations of this type usually start as freshwater swamps. The succession is apparently hydrostatic, but no thorough study of its stages has as yet been made.

=323. Succession by modifying atmospheric factors.= All layered formations, forests, thickets, many meadows and wastes, etc., show reactions of this nature, and are in fact largely or exclusively determined by them. The reaction is a complex one, though it is clear that light is the most efficient of the modified factors, and that humidity, temperature, and wind, while strongly affected, play subordinate parts. In normal successions, the effect of shade, i. e., diffuse light, enters with the appearance of bushes or shrubs, and becomes more and more pronounced in the ultimate forest stages. The reaction is exerted chiefly by the facies, but the effect of this is to cause increasing diffuseness in each successively lower layer, in direct ratio with the increased branching and leaf expansion of the plants in the layer just above. In the ultimate stage of many forests, especially where the facies are reduced to one, the reaction of the primary layer is so intense as to preclude all undergrowth. Anomalous successions often owe their origin to the fact that certain trees react in such a way as to cause conditions in which they produce seedlings with increasing difficulty, and thus offer a field favorable to the ecesis of those species capable of enduring the dense shade. Successions of this kind are almost invariably mesostatic, as it is altogether exceptional that layered formations are either xerophytic or hydrophytic.

_LAWS OF SUCCESSION_

=324.= The investigation of succession has so far been neither sufficiently thorough nor systematic to permit the postulation of definite laws. Enough has been done, however, to warrant the formulation of a number of rules, which apply to the successions studied, and afford a convenient method for the critical investigation of all successions upon the basis of initial causes, and reactions. Warming has already brought together a few such rules, and an attempt is here made to reduce the phenomena of succession, including its causes and effects, to a tentative system. At present it is difficult to make a thoroughly satisfactory classification of such rules, and they are here arranged in general conformity with the procedure in succession.

I. Causation. The initial cause of a succession is the formation
or appearance of a new habitat, or the efficient change of an
existing one.

II. Reaction. Each stage reacts upon the habitat in such a way as
to produce physical conditions more or less unfavorable to its
permanence, but advantageous to the invaders of the next stage.

III. Proximity and mobility.

(1) The pioneers of a succession are those species nearest at
hand that are the most mobile.

(2) The number of migrants from any formation into a habitat
varies inversely as the square of the distance.

(3) The pioneer species are regularly derived from different
formations, as the latter nearly always contain permobile
species capable of effective ecesis.

(4) The plants of the initial stages are normally algae and
fungi, with minute spores, composites, and grasses, which
possess permobile fruits, or ruderal plants, on account of
their great seed production.

IV. Ecesis.

(1) All the migrants into a new, denuded, or greatly modified
habitat are sorted by ecesis into three groups: (1) those that
are unable to germinate or grow, and soon die; (2) those that
grow normally under the conditions present; (3) those that pass
through one or more of the earlier stages in a dormant state to
appear at a later stage of the succession.

(2) Wherever ruderal vegetation is present, it contributes a
large number of the pioneer species of each succession, on
account of the thorough ecesis. In other regions this part is
played by subruderal native species.

(3) Annuals and biennials are characteristic of the early stages
of secondary successions, on account of their great seed
production and ready ecesis.

(4) In layered formations, heliophytes appear before sciophytes;
they ultimately yield to the latter, except where they are able
to maintain a position in the primary layer.

(5) Excessive seed production and slight mobility lead to the
imperfect ecesis of individuals in dense stands, and in
consequence usually produce great instability.

(6) Each pioneer produces about itself a tiny area of ecesis and
stabilization for its own offspring, for the disseminules of
its fellows, or of invaders.

(7) Species propagating by offshoots, or producing relatively
immobile disseminules in small number, usually show effective
ecesis, as the offspring appear within the area of the reaction
of the parent forms.

V. Stabilization.

(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.

VI. General laws.

(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.

=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)