Plant Succession
Introduction
Succession is a directional non-seasonal cumulative change in the types of
plant species that occupy a given area through time. It involves the processes
of colonization, establishment, and extinction which act on the participating
plant species. Most successions contain a number of stages that can be
recognized by the collection of species that dominate at that point in the
succession. Succession begin when an area is made partially or completely
devoid of vegetation because of a disturbance. Some common mechanisms of
disturbance are fires, wind storms, volcanic eruptions, logging, climate change,
severe flooding, disease, and pest infestation. Succession stops when species
composition changes no longer occur with time, and this community is said to
be a climax community.
The concept of a climax community assumes that the plants colonizing and
establishing themselves in a given region can achieve stable equilibrium. The
idea that succession ends in the development of a climax community has had a
long history in the fields of biogeography and ecology. One of the earliest
proponents of this idea was Frederic Clements who studied succession at the
beginning of the 20th century. However, beginning in the 1920s scientists
began refuting the notion of a climax state. By 1950, many scientists began
viewing succession as a phenomenon that rarely attains equilibrium. The
reason why equilibrium is not reached is related to the nature of disturbance.
Disturbance acts on communities at a variety of spatial and temporal scales.
Further, the effect of disturbance is not always 100 percent. Many disturbances
remove only a part of the previous plant community. As a result of these new
ideas, plant communities are now generally seen as being composed of
numerous patches of various size at different stages of successional
development.
Abandoned Field to Oak Forest
One of the earliest studies of plant succession was done by Dwight Billings in
the 1930s (see 1938, Ecological Monographs 8: 437-499). In this
investigation, Billings examined the succession of plant species that occurred
on abandoned agricultural fields in North Carolina. Billings studied a number
of fields that had been deserted from just a few years to a maximum of about
150 years. From observations of the plant communities that existed in these
sites, Billings was able to construct a detailed successional sequence.
The first stage of succession was characterized by the pioneering colonization
of annual plant species on bare ground and nutrient poor soils (Figure 9i-1).
These annual species had short lifespans (one growing season), rapid maturity,
and produce numerous small easily dispersed seeds. The annuals were then
quickly replaced in dominance in the next year by biennial plants and grasses.
After about 3 to 4 years, the biennial and grass species gave way to perennial
herbs and shrubs. These plants live for many years and have the ability to
reproduce several times over their lifespans.
Figure 9i-1: Succession of plant species on abandoned
fields in North Carolina. Pioneer species consist of a
variety of annual plants. This successional stage is then
followed by communities of perennials and grasses,
shrubs, softwood trees and shrubs, and finally hardwood
trees and shrubs. This succession takes about 120 years to
go from the pioneer stage to the climax community.
After about 5 to 15 years, the sites were then colonized by a number of
different softwood tree species including loblolly pine (Pinus taeda), shortleaf
pine (Pinus echinata), Virginia pine (Pinus virginiana), and sweetgum. As the
softwoods increased in their numbers and grew in height, they began forming a
forest canopy. This canopy reduces the amount of light reaching the forest
floor. The resulting shaded understory conditions caused the exclusion of many
light loving perennial herb and shrub species. Low light conditions also
inhibited the germination of pine seedlings. Perennial herb and shrub species
that were adapted to low light conditions now began to take over the ground
cover. The canopy also changed the microclimate of habitat near ground level.
It was now more humid, has moderated temperatures, and less wind. These
conditions, plus the development of a soil litter layer, allowed for the
germination of hardwood species, like oak (Quercus spp.) and various species
of hickory (Carya spp). The seedlings of these tree species also tolerate low
light levels.
By about 50 to 75 years after the initial colonization of the pioneer species, the
hardwoods started to replace the softwood species in the developing forest. At
this stage in the succession, the pines had maximum heights of about 25
meters, while the oaks and hickories were on average about 10 meters tall.
Because of their shorter lifespans (50 years), many of the softwood species
were beginning to die out and the gap that was created was then filled by a
subdominant hardwood tree. Hardwood species, like oak and hickory, can live
for more than 100 years. Sites more than 100 years old were found to be
dominated by mature oak forests.
Organismic and Individualistic Views of Succession
In the first quarter of this century there was considerable debate about the
nature of the community. F.E. Clements (1916) conceived of the community
as a sort of superorganism whose member species were tightly bound together
both now and in their common evolutionary history. Thus, individuals,
populations, and communities have a relationship to each other that resembles
the associations found between cells, tissues, and organs.
Clements' theories on communities were also linked to succession. His
successional concept suggested that communities of organisms are subject to
special laws in which the action of the whole is greater than the sum of the
parts, and that this action results in a directional change in the species
composition of the community to a climax state controlled primarily by
climate.
The main processes acting to produce the various successional stages of
species dominance, and finally climax, are competition and plant modification
of the abiotic environment. Environmental modification, however, is
detrimental to the plants doing the modifying. Modification changes the
environment allowing the establishment of new colonists, and then results in
the subsequent competitive exclusion of the former inhabitants by these
colonists. This facilitative process stops when the climax community is
reached.
Clements presented a deterministic unidirectional view of succession where
the present pattern is governed by the past pattern. The philosophical structure
of Clements' holistic approach is quite similar to advances in other sciences of
that time. In geology and geomorphology theorists presented views that
contained various stages of maturity, and compared landscape evolution
metaphorically to a developing organism. The parallel in theoretical approach
of these sciences with the work of Clements may be the result of attitudes
prevalent in science as a whole at that time.
In 1926, H. Gleason (1926) devised a new theory to explain the nature of
communities. Gleason's individualistic concept saw the relationship between
coexisting species as simply the result of similarities in their requirements and
tolerances (and partly the result of chance). Taking this view, community
boundaries need not be sharp, and associations of species would be much less
predictable than one would expect from the superorganism concept.
Gleason argued that the holistic view point of Clements was inadequate in
explaining the mechanism of succession. For example, Gleason suggested that
Clementsian concepts could not properly explain the occurrence of such
phenomena as retrogressive successions. In reference to his view of
succession, Gleason stated that ".... every species of plant is a law unto itself,
the distribution of which in space depends upon its individual peculiarities of
migration and environmental requirements". Thus, associations of plants, or
communities, were not highly organized, but aggregations of independent plant
species, each specialized to survive on habitats they were adapted for.
Retrogressive successions were possible in Gleason's model if environmental
variables deteriorated with time, changing the pattern of establishment, growth
and reproduction of plants in a habitat. Clements' model, however, assumed
long term climatic stability, and this assumption does not allow for short term
retrogressive community change.
Clements and Gleason presented two diametrically opposed opinions on
community organization and structure. Further investigation in this discussion
will show that these views are still present in the hypotheses of later theorists,
but in a somewhat modified form. Many of these modified hypotheses involve
a synthesis of the early ideas of Clements and Gleason. This synthesis is the
result of the addition of new ecological information or the re-analysis of old
information on how ecosystems function over time. The synthetic evolution of
successional hypotheses must be expected, as investigation finds new
mechanisms responsible for temporal vegetation change in a relatively
unexplored world. The early presence of simple diametrically opposed
successional hypotheses in the early years is probably the result of the
immature state of understanding of turn of the century ecology.
Central to Gleason's succession model is the notion of abiotic and biotic
heterogeneity in space and time. This concept is a characteristic view of much
of modern ecology. Recently, several scientists have examined the role of
disturbance on community structure. These researchers suggest that
disturbance is a common process in most communities that shapes the nature
and structure of biotic interactions and processes. These ideas follow directly
from Gleason's early observations of pattern and process in the plant
community.
The individualistic concept of succession outlined by Gleason was ignored by
the scientific community for some twenty to thirty years. Important papers and
books citing this work did not appear until the late 1940s and early 1950s. It
was the ideas of Clements that dominated ecological thought in one way or
another up to this period.
Our current view on the nature of community structure is close to the
individualistic concept. Results of many studies indicate that a given location,
by virtue mainly of its physical characteristics, possesses a reasonable
predictable association of species. However, a given species that occurs in one
predictable association is also quite likely to occur with another group of
species under different conditions elsewhere.
Types of Succession
Primary succession - is the establishment of plants on land that has not been
previously vegetated - Mount Saint Helens. Begins with colonization and
establishment of pioneer species.
Secondary succession - is the invasion of a habitat by plants on land that was
previously vegetated. Removal of past vegetation may be caused by natural or
human disturbances such as fire, logging, cultivation, or hurricanes.
Allogenic succession - is caused by a change in environmental conditions
which in turn influences the composition of the plant community. In Cornwall
England, observations on the estuary of the Fal river suggest that the
deposition of silt may be causing an allogenic succession from salt marsh to
woodland. Measurements indicate sedimentation rates of about 1 cm per year
on the mud flats that are found 15 kilometers (9 miles) into the estuary. Over
the last 100 years, this salt marsh has increased its elevation and has extended
itself seaward by 800 meters (2600 feet). The adjacent woodland has followed
the salt marsh by invading its landward limit.
Autogenic succession - is a succession where both the plant community and
environment change, and this change is caused by the activities of the plants
over time. Mt. St. Helens after the last volcanic eruption.
Progressive succession - is a succession where the community becomes
complex and contains more species and biomass over time.
Retrogressive succession - is a succession where the community becomes
simplistic and contains fewer species and less biomass over time. Some
retrogressive successions are allogenic in nature. For example, the introduction
of grazing animals result in degenerated rangeland.
Table 9i-1 describes some of the plant, community, and ecosystem attributes
that change with succession.
Table 9i-1: Comparison of plant, community, and
ecosystem characteristics between early and late stages of
succession
Attribute
Plant Biomass
Plant Longevity
Early Stages of
Succession
Small
Short
Late Stages of
Succession
Large
Long
Seed Dispersal Characteristics of
Dominant Plants
Plant Morphology and Physiology
Photosynthetic Efficiency of
Dominant Plants at Low Light
Rate of Soil Nutrient Resource
Consumption by Plants
Plant Recovery Rate from Resource
Limitation
Plant Leaf Canopy Structure
Site of Nutrient Storage
Role of Decomposers in Cycling
Nutrients to Plants
Biogeochemical Cycling
Rate of Net Primary Productivity
Community Site Characteristics
Importance of Macroenvironment on
Plant Success
Ecosystem Stability
Plant Species Diversity
Life-History Type
Seed Longevity
Well dispersed
Poorly dispersed
Simple
Complex
Low
High
Fast
Slow
Fast
Slow
Multilayered
Monolayer
Living Biomass
and Litter
Litter and Soil
Minor
Great
Open and Rapid
High
Extreme
Closed and Slow
Low
Moderate (Mesic)
Great
Moderate
Low
Low
r
Long
High
High
K
Short
Succession Mechanisms
An overview of the mechanisms of succession has been produced by Connell
and Slatyer (1977, American Naturalist 111: 1119-1144). Connell and Slatyer
propose three models, of which the first (facilitation) is the classical
explanation most often invoked in the past, while the other two (tolerance and
inhibition) may be equally important but have frequently been overlooked.
The essential feature of facilitation succession, in contrast with either the
tolerance or inhibition models, is that changes in the abiotic environment are
imposed by the developing plant community. Thus, the entry and growth of
the later species depends on earlier species preparing the ground.
The tolerance model suggests that a predictable sequence is produced because
different species have different strategies for exploiting resources. Later
species are able to tolerate lower resource levels due to competition and can
grow to maturity in the presence of early species, eventually out competing
them.
The inhibition model applies when all species resist invasions of competitors.
Later species gradually accumulate by replacing early individuals when they
die. An important distinction between models is the cause of death of the early
colonists. In the case of facilitation and tolerance, they are killed in
competition for resources, notably light and nutrients. In the case of the
inhibition model, however, the early species are killed by very local
disturbances caused by extreme physical conditions or the action of
predators.