Plant Succession and Climax Notes
Stabilization and Climax
Stabilization
Stabilization is the outcome of increased occupation due to aggregation and migration, leading to control of the habitat by the population. It is essentially the increase of dominance, culminating in a stable climax. This process involves a mutual and progressive interaction between the habitat and the community.
Stabilization can be seen as a synonym for succession, emphasizing the final adult stage of development.
Causes of Stabilization
The essential cause of stabilization is dominance. Dominance results from:
Increasing occupation of a bare area.
The life-form of the species.
Effective dominance occurs when the prevailing life-form exerts a significant reaction that holds the population in a certain stage until the reaction becomes unfavorable or a superior life-form invades. Dominance is the ability of a characteristic life-form to control the community for a period by controlling soil factors (water-content), air factors (light), or both.
Initial life-forms like algae, lichens, and mosses are characteristic but not dominant because their reactions favor invaders rather than prevent them. Intermediate stages show both effects: initially favoring aggregation, then favoring invasion. Each stage is a minor process of stabilization.
Reaction causes dominance and the loss of dominance, explaining why communities develop and dominate temporarily before being replaced. Reaction explains both stabilization and successive invasions.
Relation to the Climax
The end of stabilization is a climax. Each successional stage reduces extreme conditions, creating better growing conditions or conditions favorable to a wider range of species. This reduces excess or insufficient water content, bringing the habitat closer to mesophytic conditions.
Exceptions occur in desert regions or water areas with alternating deposition and erosion. The effect on the plant population is that the number of species and individuals increases until light becomes restrictive. Life-forms change from those with minimal requirements (lichens, submerged plants) to those with increasingly high, balanced needs (perennials, shrubs, trees). The climax is permanent due to its harmony with a stable habitat, persisting as long as the climate remains unchanged and no new dominant migrates in.
Degree of Stabilization
The final stabilization of a sere varies in permanence. Water seres transitioning to moor, heath, and then forest illustrate this. Soil reactions can create subclimaxes of unusual duration. Human activities can cause artificial climaxes.
A similar effect occurs in the Rocky Mountains where springs keep the soil too moist for pines, allowing aspens to remain dominant. Repeated fire also promotes the persistence of aspen and is a large factor in prairie communities. Complete stabilization occurs only when the climax is controlled by trees. Other final communities are subclimaxes, which may be due to:
Climatic control
Reaction upon the soil
Interference by man
Exclusion by later dominants
The removal of checks allows complete development. The evolution of new vegetation leads to new climax formations and seres. All possible degrees of stabilization have occurred, except for complete developmental stability, which is impossible as long as plants evolve or conditions change.
Life-History Stages
The movement from the initial stage to the climax is continuous, with periods of apparent stabilization corresponding to population or invasion maxima.
Nature of Stages
Real stages exist because each dominant holds its place until replaced. Demarcation is sharper when the life-form changes (e.g., grassland to scrub or forest). In some secondary seres, there is little change of life-form, resulting in few and indistinct stages. Sometimes, all dominants are present from the first year after a burn, and stages are due solely to the rate of growth.
Kinds of Stages
Stages can be distinguished based on:
Change of population
Change of life-form
Change of habitat
Dominance with reaction includes all these bases and is the best method for analysis. Stages are marked by a dominant or group of dominants, with substages recognized for changes in character, especially in early seres.
Stages can be grouped as:
Initial, medial, or final
Temporary or migratory vs. permanent, stable, ultimate, or climax
One primary sere may show rock, gravel, grassland, and woodland stages, while another may show water, sedgeland, grassland, and woodland. Corresponding life-form stages would be lichen, moss, herb, grass, scrub, forest and algae, herb, sedge, grass, scrub, forest.
Role of Life-Forms
Dominance and reaction result from the life-form, making life-forms the main markers of development stages. Life-form includes vegetation form, habitat forms, and reproduction form, marking the species as an ecological agent.
Life-forms can be seen as lower or higher based on demands on the habitat and their reaction. Pioneers in primary seres show taxonomic development as well. Plankton algae are not pioneers in a particular water sere but are in the geosere. Pioneers in a water sere are charads, submerged mosses, and flowering plants. Floating forms mark the division into water and air media, transitioning to reed forms.
Sedge forms can be distinguished from reed, and the change from grassland to woodland emphasizes the reaction upon light and air factors. While woody forms are distinct, subordinate forms like bushes, shrubs, and trees differ primarily in size. Sphagnum is essentially a submerged moss that can cause swamping. Shrubs characteristic of heath are due to acid soil, deficient aeration, or winter conditions.
In rock seres, algae are pioneers on wet rocks, and lichens are pioneers on dry rocks. Algae such as Pleurococcus may establish on rocks during wet periods before lichens. On moist rocks, algae may be followed by lichens, especially Collemaceae. Mosses may also be pioneers. On dry rocks, there is a successional difference between crustose and foliose lichen forms. The moss form quickly follows lichen stages.
The pioneer herb form on exposed rock usually has a mat habit. Grasses and herbs in rock clefts grow in soil and are only apparent rock plants, belonging to the next stage, similar to those in the water sere.
The sequence of life-forms in secondary seres is similar to primary seres, but pioneer life-forms are rarely the same. Mosses may appear after a burn, but flowering plants usually develop the same year, preventing mosses from forming a characteristic stage.
Reasons Why Plants Disappear
Stages result from the disappearance of occupants and the appearance of invaders. Plants disappear due to:
Unfavorable conditions due to reaction
Competition
Unfavorable conditions from parasites, animals, or man
Old age
Reaction and competition are universal causes. Complete destruction results in secondary succession. Influence of old age is not evident, but may decide competition between short-lived and long-lived trees like aspens and conifers.
Reasons Why Plants Appear at Certain Stages
Migrules are carried into an area continuously, with appearance depending on mobility and distance. Habitat factors exert real control through selective action in ecesis. Conditions in secondary areas are due to reactions of original vegetation. Stages appear because certain species develop more rapidly and become dominant. Premature appearance is due to migration overcoming unfavorable conditions or local variations.
The premature development of an entire stage is caused by agencies that rapidly change the habitat, such as animals or man. Accelerated reaction may also be retarded, delaying stages. In secondary successions, the time of appearance of shrub and tree stages depends on the denuding agent.
Initial Stages
No sharp line exists between initial and medial stages. Initial stages of primary seres are marked by extreme physical conditions and specialized life-forms. Primary areas like open water, rock, dune sand, etc., occur worldwide. Pioneer aquatics, lichens, mosses, and xerophytic grasses are universal and mobile. Initial stages may be identical in widely separated regions.
Initial stages persist for a long time due to slow reaction and incomplete occupation. Duration is greatest in rock seres. The general limit of initial stages is indicated by a marked change in habitat and the accumulation of humus.
In water, initial stages are the submerged, floating, reed, and sedge stages. On rock, the number of stages to a closed community is usually five: crustose lichens, foliose lichens, mosses, cushion plants, herbs, and grasses. Early stages of water or rock seres are excluded in dunes because soil formation has already taken place.
The first initial stage is the pioneer stage. Lichens and submerged plants are pioneers for rock and water seres, respectively. Initial conditions are rarely extreme in secondary seres, and invasion is rapid. The first stage is short, lasting a year or so, with high occupation and few stages.
Medial Stages
Medial stages are characterized by:
Uniform density
Well-developed dominance
Increasing humus
Medium water amounts
They consist of well-developed communities with layers, where the most characteristic life-forms are grasses and shrubs. Medial stages include all stages after the initial ones but the last. In seres ending in forest, all successive forest communities are climax stages.
The Climax
Concept
Every complete sere ends in a climax, when the occupation and reaction of a dominant exclude the invasion of another dominant. The climax marks the close of general development, recognizable through careful scrutiny of the whole process. Duration is not a guide, as even pioneer stages may persist. The test of development is necessary in climax stages where dominants belong to the same life-forms as the climax dominant.
Nature
The climax is the mature stage of vegetation and the fully developed community. The climax formation resembles a complex organism, making recognition and limitation indispensable for developmental study. Succession should begin with an intensive study of the adult organism.
Relation to Succession
A climax occurs because succession is reproduction. The reproductive process terminates in the adult form. Natural or artificial factors may hold it in an imperfect initial or medial stage. The underlying causes of complete development are found in the habitat.
Kinds of Climaxes
The climatic formation is the real climax. Agents may prevent complete development, producing apparent climaxes or subclimaxes which depend on the continuation of the inhibiting action. A potential climax is the actual climax of an adjacent region, replacing the climax of the region if its climate changes. Potential climaxes are in a sonal relation to a particular formation.
Subclimaxes
Various causes produce subclimaxes:
Soil
Reaction
Competition
Migration barriers
Man
These prevent development by hindering or destroying some stage. Apart from plant reactions, excess salt in the soil prevents development. Reactions that retard succession include Sphagnum accumulating water and moor and heath producing acids. Competition maintains a grassland subclimax. Barriers to immigration prevent the spread of final dominants like Picea or Fagus. Human activities such as burning, clearing, and grazing produce subclimaxes by disturbance and destruction.
Potential Climaxes
Zones of vegetation indicate possible changes due to a change of climate. Regional zones are produced by cumulative change, while local zones are due to gradual changes in water content. Climate sets the usual climax limit but can fix an earlier or later limit. Increased rainfall continues development from prairie to woodland, while increased temperature leads to desert.
Changes of Climate
A change of climate continues a sere or brings it to a close earlier than the climax. Indirectly, changes may result in new areas being produced. Over long periods, profound changes in flora occur. The ecotone between two climatic associations records small variations of climate. Under mesophytic conditions, existing seres are continued by the addition of stages dominated by higher life-forms. Potential climaxes are deciduous forest in the east and scrub and pine woodland in the west.
A swing of climate resulting in decreased rainfall gives rise to the areas with vegetation one stage more xerophytic than the existing climax. The corresponding communities of Aristida or of Gutierrezia-Artemisia become the climax vegetation.
Preclimax and Postclimax
The significance of potential climaxes is best seen in mountain ranges. On north exposures, mesophytic associations descend, while on south exposures, xerophytic communities ascend. The postclimax continues the development by replacing the climax with increased water content. The preclimax foreshadows conditions reducing water content, ceasing development before reaching the climax. Subclimaxes are practically always preclimaxes.
Changes of Climax
The climax may change due to climate or the development of a new flora. Climate may exhibit great alternations. A gradual invasion of ice produces preclimaxes, while a recession changes seral climaxes into postclimaxes. Alternating wet and dry climates give rise to a sequence of climaxes derived from postclimaxes. The original arctic climax, the Dryas association, was succeeded by aspen, pine, oak, and beech climaxes.