E. CLASSIFICATION OF SUCCESSIONS AND OF THEIR TERMINAL STAGES

12 TYPES OF SUCCESSION

P. DANSEREAU

Contents

12.1 Introduction. 125

12.2 Gradients: Smooth and Bumpy 126

12.3 Sere and Ecosystem. 127

12.4 Trends and Inhibitions of Succession 131

12.5 Autogeny, Allogeny, Biogeny 134

R. Knapp (ed.), Vegetation Dynamics© Dr. W. Junk b.v. — Publishers, The Hague 1974

12 TYPES OF SUCCESSION

12.1 Introduction

The phenomenon of succession cannot readily be separated from the whole of vegetation dynamics, which is the object of this section of the handbook. The complexity of vegetation change has been revealed to us in ever-greater detail since the turn of the century when COWLES (1899) and later CLEMENTS (1936) formally defined the phenomenon of community replacement and suggested that the combined forces of the environment were conducive to a sort of convergence which they called the climax.

Detailed critical research oriented towards a verification of this hypothesis was not forthcoming for some time. Dogmatic incor­poration of the climax theory in many textbooks served the purpose, for awhile, of deadening the issue on the one hand, and of lending excessive vulnerability to a workable theory that had not been sufficiently challenged. There followed a period when, in the eyes of some ecologists, succession could be witnessed everywhere and, in the eyes of others, virtually nowhere (EGLER 1951, 1967). It had long been observed by such management-minded men as foresters (SPURR 1952). Studies on rightsofway, "abandoned" land, exploited forest revealed how often an expected stage could be "skipped" or cornered, and how surprisingly stable many pioneer or consolidation communities could be (KENFIELD 1966).

It does not seem to be the purpose assigned to me in the present context to discuss the climax hypothesis. I shall therefore attempt to separate the issues involved in succession from their possible cul­mination in a state of equilibrium. I am nevertheless somehow bound to state a personal position on this still surprisingly con­troversial topic by referring to my former paper (DANSEREAU 1956b). In the intervening fifteen years I have had occasion to revise my judgment of many ecological phenomena which I had witnessed and then interpreted within a hopefully coherent frame­work (see 1957), and I trust this has been apparent in my pub­lications of recent years and that it will be evident in the present text. However, I am bound to say that I honestly see little reason to doubt that the allogenic and autogenic forces of the ecosystem tend towards change on the one hand and are subject to a great variety of ear(y or late inhibitions, on the other hand.

125

This means that the double task of the investigating ecologist still is:

I) to decompose the forces that promote succession (or change), and to identify their strength and orientation;

2) to locate the counter-forces of inhibition, arrest, and de­viation that re-orient change, set it back, or induce intermediate and often durable equilibria.

2.2 Gradients: SlD.ooth and BulD.py

The first line of approach is geared to the continuity-discon­tinuity phenomenon. Change in vegetation, whatever its tempo, is continuous, but certain turnovers are more likely self-perpetuating than others because of built-in forces or of repeatedly active outside pressures.

It is inevitably a subjective urge and possibly a cultural hang­up that gives greater emphasis to continuity (e.g., the Wisconsin School) or discontinuity (the S.I.G.M.A. School). The "mind and the eye" (ARBER 1954) are congenitally geared to each other very differently in the body and spirit of different investigators and this inner matrix is in turn variously influenced, not to say impregnated, by the schools. The requirements of synecological study do not preclude the assumption of this subjective approach, even in highly sophisticated research (WILLIAMS 1967a, b). They do, however, imply a constant recognition of the duality of alternating continuity and discontinuity.

The ordination of plant-communities along' a gradient is a constat of the first order. Many shorelines with a regularly pulsating flood-level have belt after belt of vegetation dominated by different species (typical example in DANSEREAU 1956a, Fig. 16). Whereas we know that all zonation is not indicative of succession, we can also point to many instances where it is, and where, precisely, it is the displacement of the underlying gradient itself that induces it. Thus, the gradual progress of sedimentation on a shoreline displaces the quantities and periods of water-availability in such a way as to make the ecological conditions of the position originally occupied by belt A so very similar to those of belt B that the typical (and eventually the dominant) species of A yield to the invasion of B­adapted species.

Smooth gradients lend themselves to an investment by a gamut of species (and communities), but this may occur in four different ways.

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1) Where each of the segments of the gradient is large enough, this gamut fully develops into well-marked belts.

2) When the critical bands ABCDEFG either contract or expand, the gradient can be reduced to ACDFG, where the Band E positions are too narrow to allow minimal Lebensraum (they have been cornered).

3) Where the values undergo a reversal, the spatial arrange­ment will favour an ordination such as ABCBABCDEBEF.

4) Where there occurs an intrusion of a quite different in­fluence, it will cause a spatial sequence such as AB'CDE'FG.

These four kinds of gradient are, therefore: 1) smooth, open, continuous; 2) contracted, somewhat discontinuous; 3) sequentially reversed; 4) bumpy and heterogeneous.

12.3 Sere and Ecosystem.

A sere is a group of plant-communities related in a predictable linear order that replace one another in time on a given site. Thus, on a dry sandy plain in the Montreal Lowlands (see DANSEREAU 1956a, Fig. 8), the following succession of communities is observed: Oenotheretum, Dan thonietum, F estuc etum, Solidagine­tum, Crataegetum, Pinetum strobi. (Fuller description of these plant-communities, their structure and dynamics, will be found in DANSEREAU 1959.) In the course of this replacement, organic content and moisture-retaining capacity of the substratum have increased; so has the differentiation of soil horizons; the veg­etation has become more stratified and this has buffered the macro­climate considerably. On the other hand (see also DANSEREAU 1956a, Fig. 8), on a silting floodplain in the St. Lawrence River Valley, from the permanently submerged to the regularly flooded land the following are apparent: Nupharetum, Scirpetum, Calamagroste­tum, Spiraeetum, Alnetum, Acereto-Ulmetum. In the course of this movement the water-content of the substratum decreases; exposure to air increases; likewise, vegetation mass stratifies and increases.

Virtually all landscapes, all over the world, can be mapped as to their driest and wettest parts, leaving the well-drained (mesic) areas in-between. Commonly, therefore, one reads of a hydrosere and of a x eros ere, with a mesosere in-between (where water avail­ability in the soil is not critical).

However, this recognition of three main segments in the land­scape is not enough since it points only to one factor. Important as water availability may be, it frequently happens that other forces in the landscape are more critical because of the stress induced by

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TABLE I. Huguet del VILLAR'S (1929) ecological, non-geographical classification of the physiological regimes that characterize ecosystems

Sub- Habitat Relative Nature Quality Regime Typical stratum harmony of of eco-

control control systems

Totally Harmony Constant 1. Limno- Lakes, or partly of phytia ponds, aquatic factors streams

Sub- 2. Helo- Marshes, constant phytia tem-

porary ponds

Chem- Alcalini- 3. Halohy- Sea, salt ical ty drophytia lakes

Acidity 4.0xyhy- Acid drophytia lakes

Hydro- Domi- Thermic Excess 5. Hydro- Warm phytia nant thermo- springs

discrep- phytia ancy of one Defi- 6. Cryo- Arctic factor ciency phytia seas, ice,

snow

geophysical Biotic Mephitic * (Hydro-accumu- sapro-lations phytia)

emerged Har- Meso- Constant 7. Hygro- Tropical mony phytia phytia rain-of forest factors

Ecophytia Pezo- Sub- 8. Subhy- Sub-

phytia constant grophytia tropical and temper-ate rain-forest

Discon- 9. Tropo- Monsoon tinuous phytia and de-

ciduous forest

Domi- Water Mod- 10. Mesoxero- Medi-nant scarce: erately phytia terranean discrep- forest ancy of one Xero- Ex- 11. Hyper- Desert factor phytia tremely xerophytia

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geophysical:

Ecophytia

organic:

Sapro­phytia (dead)

Bio­phytia (living)

emerged

Pezophytia

TABLE I (continued)

Temper- Excess ature extreme Defi­

ciency

12. Sub- Savana xerophytia

13. Psychro- Tundra phytia

Reaction Alcalinity 14. Halo- Seashore diverging phytia

from A . d· 15 0 h· B d neutral Cl lty . xyp rytta ogs an

Domi- Edapho- Loose nant phytia

discrep- Physical Dry ancy condition of one

needle­leaf forest

16. Psammo- Dunes phytia

1 7. Cherso- Shallow phytia gravels

factor un-favorable Petro­sub- phytia stratum exces­sively:

Pertur­bing biotic factor

18. Chasmo- Crevices phytia

Compact 19. Litho­phytia

Putres­cib1e accumu­lations

** (Pezosapro­phytia)

Rocks

Biogeno- General 20. Biogeno- Bird phytia trans- phytia cliffs

for- (s.str.) mati on of the environ­ment

21. Paranthro- Buildings pophytia yards,

railways

Aquatic Har- Texture Con- 22. * Hydrosa- Logs prophytia under

water Emerged mony of sub- stant

offactors stratum

Sup- Har-porting mony Har- of boring factors

Texture Con­of sub- stant stratum

23. **Pezosa- Rotting prophytia logs

24. Ectobio­phytia

25. Endobio­phytia

Bark of trees, sheaths of brome­liads Intestines of animals, living wood

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their scarcity or excessive abundance, or immediate impact. Thus the prevalence of salt induces a halosere, of acidity, an oxysere, of hard rock, a lithosere, of moving sand, a psammosere, etc.

If we were to propose a classification of "types of succession," should not these determining factors serve well? Are they not the basic cause of any observed discontinuities between ecosystems?

A classification of types if succession could therefore be delin­eated on types of ecosystems. In several publications from 1952 on­wards (see especially 1956a, 1957, 1959, 1966b, 1970), I have suggested that HUGUET DEL VILLAR'S (1929) scheme was very ap­propriate for this purpose. In a very slightly modified way I have used it as a frame for regional studies in the St. Lawrence Valley (1959), in Puerto Rico (1966a), in the Azores (1970).

DEL VILLAR'S classification (Table I) is based upon physio­logical regime and therefore upon the equipment that all forms of plant life which are present in a given ecosystem must have to meet a difinable kind of stress: hardness or acidity of substratum, flooding, extreme per­meability, etc. Admittedly the 25 resulting categories may be con­tested, and I shall not discuss their inclusiveness or exclusiveness at this time, but I do believe this kind of scheme to be highly useful, both as a means of description and as a functional framework.

Thus, successions will be: (1) limnophytic, (2) helophytic, (3) halohydrophytic, etc. (see Table I, sixth column). In any given landscape, the mosaic of plant-communities will be such that eco­systems will border upon each other. Thus, a dune (psammophy­tia), a bog (oxyphytia), and a marsh (helophytia). As long as veg­etational change progresses within each ecosystem (dune, bog, marsh), the physiological regime is invariable: the various plant­communities (Caricetum, Chamaedaphnetum, Ledetum, Piceetum ericaceum in the bog, for instance) essentially effect a similar cycling of similar resources, similarly accessible and trans­formable. The boundary between the dune and the bog may be permanent or shifting, but it rather breaks the continuity of succession. The physiology of dune plants is so utterly different from that of bog plants that virtually no binding species will mark the transition. This hiatus is definitely lesser in the transition from bog to marsh where several species are likely to carryover, although in a reduced role. However, the relay in quality, quantity, and availability of resources involves a major qualitative change and a new ecological background has developed and a potential new succession has begun.

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12.4 Trends and Inhibitions of Succession

The causes of succession are numerous and very unevenly in­fluential under different ecosystematic regimes. It is easier to detect the effects of a progressive succession and to list them, as follows.

In the soil:

1) moderation of drainage (deficient or excessive to regular); 2) addition of organic material; 3) redistribution of minerals; 4) improvement of structure (loosening or cohesion); 5) tapping or liberation of buried or bound elements.

On the microclimate:

6) attenuation of temperature extremes; 7) reduction of temperature and humidity fluctuations; 8) decrease of radiation and evaporation, at least near soil surface; 9) increase of shade;

10) buffering of wind.

In the vegetation:

11) increase of coverage and of biomass; 12) changes in light exposure with stratification; 13) greater utilization of airmass and soil layers; 14) changes in composition and diversity of exploiting species; 15) structural complexification; 16) niche multiplication; 17) fuller utilization.

In a field study of the strategies involved in succession, it will appear that critical factors undergo a regular shift that taxes the physiology of the participating plants, first in one way and then in another. Thus, such plants as Oenothera biennis and Danthonia spicata are excellently adapted to great temperature and humidity fluc­tuations and to low water and organic content of the soil, but poorly equipped to compete with Festuca rubra or Poa pratensis once the above-mentioned conditions have "improved", and not at all able to withstand either the crowding or the shade created by invading Solidago.

What is it, then, that favours the progress of succession? What stops it? What sets it in reverse?

These questions should be posed by looking at the cycling of resources in the ecosystem and by testing the efficiency of cycling. Figure 1 shows the now classic triangle (or pyramid) which has served to illustrate the relative position of the agents and the re­sources in the ecosystem.

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REAU 1969) as I have done in another context. It will serve the present purpose well enough, however, since our concern is with primary productivity.

Three principal regimes, involving three basically different metabolisms, are involved in the cycling of resources:

minerotrophy takes place through several pedogenetic processes that transform parent-rock under the impact of climate and veg­etation and make available certain qualities and quantities of resources (light, energy, heat, gases, liquids, solids in the air and soil) ;

phytotrophy involves the metabolism of plants which operate a transformation of mineral products and transform them into veg­etable tissue;

zootrophy results from the further elaboration of mineral and vegetable products by animals.

Each turnover, at each level, makes for storage, reduction, reinvestment, and, of course, loss. It is largely the balance between loss and storage that controls succession. Considerable loss of water (through erosion or excessive evaporation) at the minerotrophic level, of plant-exploiters (through a sudden, widespread disease) at the phytotrophic level, of animal consumers (through emigration) at the zootrophic level may very well set back the successional trend and thereby cause a retrogression.

On the other hand, an accumulation of a large capital of resources, creating a surplus, invites invasion. The latter may be temporary or cyclic, such as the passage of migrating birds in the tundra or in the tropical savana (see MOREL & BODRLrERE 1962), or it may be permanent, such as the colonizing of an "old field" by birches or pines.

What is it, then, that stops succession? One force is the capacity of many species, especially dominants of a community, to maintain their spatial position after their habitat and even the climate itself have ceased to favour full vitality ("law of persistence", DANSEREAU 1966b). Another is the impact of allelopathy (MOLISCH 1937, EVENARI 1961, MULLER 1966, KNAPP 1954, 1967), the chemical power of some plants to exert an antibiotic effect. These are auto­genic forces, inherent to the flora.

Among the inhibitions caused by a permanent or recurring feature of the site are numerous edaphic and physiographic con­ditions affecting pedogenic or microclimatic processes, such as frost-pockets, abruptness and hardness of substratum, in fact any of the 25 ecological situations enumerated in Table 1.

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12.5 Autogeny, Allogeny, Biogeny

These considerations lead naturally to TANSLEY'S (1935) recog­nition of three basic types of succession: autogenic, allogenic, and bio­genzc.

Autogenic succession is the replacement of one plant-community by another which is primarily due to the better adaptation of the invaders to ecological conditions that result from the residents' accumulation of resources and "improvement" of the site. The residents, therefore, cannot fully utilize conditions that are essen­tially "of their own making" or at least cannot compete with better­equipped invaders, which could not have withstood the adversities that prevailed at an earlier time. It thus appears that it is the pre­vailing resident agents, through their metabolic power, that have induced a significant change.

Allogenic succession, on the contrary, implies only a minimum of resource accumulation by the resident community of plants and animals, whereas materials from other ecosystems more or less drastic­ally altef· the resource-basis and thereby allow new agents to prevail, inasmuch as the residents are poorly adapted to the new conditions.

Biogenic succession supposes the rather catastrophic, or at least sudden, interference of a living agent which is capable either of altering some significant resource (light, heat, and evaporation in the grassland, on the tracks of buffalo herds) or of destroying or reducing the mass of a major agent (chestnut blight in Eastern North American forests).

Thus, the three "types of succession" differ fundamentally in their ecosystematic strategy:

autogenic succession results from the accumulation of excessive resources that permit the access of new primary-producing agents;

allogenic succession results from the neutralization (sedimen­tation) or the ablation (erosion) of existing resources and the liberation (erosion) or substitution (sedimentation) of new resources which lend themselves to a new occupancy;

biogenic succession is the result of the excessive utilization by a primary consumer that changes the composition and/or structUl e of the community and thereby the existing balance.

Succession, therefore, as an ecological phenomenon, involves a shift in the interlocking mineral, vegetable, and animal cycles. It obeys the uneven pressures of more or less rapid turnovers, more or less abundant storages, and is at the mercy of both resources and agents that originate in other ecosystems.

There are many types of succession, and they imply different

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kinds and degrees of primary productivity. They may be classified as:

a) showing greater or lesser continuity where they are geared to coordinated environmental gradients;

b) being characteristic of certain identifiable ecosystematic controls (or physiological regimes);

c) showing discernible trends towards progression, retrogres­sion, stagnation, inhibition;

d) as being essentially inner-controlled (autogenic) or outer­controlled by incoming resources (allogenic) or by primary con­sumers (biogenic).

135