Download Succession Among the Ocean Tides

9.1 RESTORATION OF DAMAGED LANDS
Restoration of ecologically damaged lands is an important practical problem.
Throughout history, people have altered the landscape, often with undesirable
effects; forests have been cleared and soil eroded.5 These destructive effects on
the land are not new. During the height of the ancient Greek civilization, for
example, Plato wrote that the hills of Attica in Greece were a "skeleton" of their
former selves, i As people sought more agricultural and pasture land, marginal
areas of forests and grasslands were converted to crops and pasture and later
abandoned when they were no longer productive.
A major problem in the twentieth century is that toxic pollutants have
damaged ecosystems. For example, in an area known as the "black triangle," an
industrial zone that spans the Czech Republic, Poland and eastern Germany, air
pollution damage to forests is extensive. It is estimated that by the beginning of
the 21st century, 76% of the forests in the Czech Republic will be changed by air
pollutants.5 Mining has ruined many areas. In eastern Germany, open-pit mining
has destroyed about 60,000 ha (about 148,000 acres); in northern Bohemia, an
estimated 155,000 ha (more than 380,000 acres) has been destroyed.6 People are
becoming increasingly active in trying to restore some of these damaged
landscapes. However, restoration is not a simple task. For example, in 1991 the
Black Triangle Environmental Programme was established and although air
pollution emissions have decreased, contribution of emissions to forest damage in
the region still remains considerable. We are finding that one of the important
components of restoration efforts is to learn the patterns of succession in nature.7
9.2 ECOLOGICAL SUCCESSION
Recovery of ecosystems occurs naturally through a process called ecological
succession. This natural recovery can occur if the damage is not too great.
Sometimes, the rate of recovery is long in comparison to human desires. Natural
areas are subject to disturbances of many kinds. These disturbances are not
usually human induced; natural disturbances such as storms and fires have always
been a part of the environment.8 Such disturbances have existed for so long that
animals and plants have adapted to them and benefit from their occurrence.9
If fundamental requirements for life are available, areas on Earth without life are
soon filled with living things. Over time, ecosystems undergo patterns of
development called ecological succession. There are two kinds of ecological
succession: primary and secondary. Primary succession is initial establishment
and development of an ecosystem; secondary succession is reestablishment of an
ecosystem. In secondary succession, there are remnants of a previous biological
community, including such things as organic matter and seeds. By contrast, in
primary succession such remnants are nonexistent or negligible. Forests that
develop on new lava flows (Figure 9.2) or at the edge of a retreating glacier
(Figure 9.3) are examples of primary succession. A forest that develops on an
abandoned pasture (Figure 9.4) or one that grows after a hurricane, flood, or fire
is an example of secondary succession.
One of the best documented examples of natural disturbance is the role of fire in
the northern woods of North America. The Boundary Waters Canoe area is an
example of nature relatively undisturbed by people. It consists of a million acres
in northern Minnesota designated as wilderness under the U. S. Wilderness Act.
With that protection, the area is no longer open to logging or other direct humaninduced disturbances. In tlie early days of European exploration and settlement of
North America, French voyagers traveled through this region hunting and trading
for furs. In some places, logging and farming was common in the nineteenth and
early twentieth centuries, hut for the most part the land has been relatively
untouched. In spite of the lack of human influence, the forests show a persistent
history of fire. Fires occur somewhere in this forest almost every year, and on the
average the entire area burns once in a century. Fires cover areas large enough to
be visible by satellite remote sensing (Figure 9.5). When fires occur at natural
rates and natural intensities, there are some beneficial effects. For example, trees
in unhurned forests appear more susceptible to insect outbreaks and disease. Thus
recent ecological research suggests that wilderness depends on change and that
succession and disturbance are continual processes. The landscape is dynamic.10
Succession is one of the most important ecological processes, and tlie patterns of
succession have many management implications. We see examples of succession
all around us. When a house lot is abandoned in a city, weeds begin to grow.
After a few years, shrubs and trees can lie found; secondary succession is taking
place. A farmer weeding a crop and a homeowner weeding a lawn are both
fighting against the natural processes of secondary succession. Succession may
involve large areas, such as that affected by the eruption of Mount Saint Helens
(see A Closer Look 9.1, "Reforestation of Mount Saint Helens"). Even more
common is succession on a fairly local level. For example, one tree may he blown
over, opening up a gap for early successional plants.
Stages in Succession
Succession involves recognizable, repeated patterns of change. The pattern is
familiar in areas of the United States such as New England, Michigan, Wisconsin, and Minnesota, where much land was cleared for farming and logging in
the nineteenth century and later abandoned. Redevelopment of forests after farm
fields have been abandoned is a typical and classic pattern of secondary
succession (see A Closer Look 9.2, "An Example of Succession in an Abandoned
Farm Field").
General Features of Succession
Succession takes place not only in abandoned farm fields hut also in areas of New
England ravaged in 1938 by a hurricane that destroyed large areas of forest.
Similar patterns occur elsewhere after fire, illustrated in tlie table in the
Environmental Issue for lodgepole [line forests in tlie American West. In both
cases, the forests grew back and recovered. From this we can describe several
general features of succession.
First, the set of species that are present changes during succession.1' In
forested areas, plants that are rapid growing and short lived do well in the bright
light and higher nutrient conditions that often occur after disturbances; these
species tend to have widely and rapidly dispersing seeds. Such species are called
pioneers, or early successional species. Plant species that dominate later stages
of succession tend to lie slower growing and longer lived. These plants do
comparatively well in shade, and they have seeds that, while not as widely dispersing, can persist a rather long time. These arc called late successional species.
In early stages of succession, hiomass and biological diversity increase
(Figure 9.7). In middle stages of succession, there are many species of trees and
many sizes of trees. Gross and net production (see Chapter 8) change during
succession: gross production increases and net production decreases. Chemical
cycling also changes. The organic material in tlie soil increases, as does the
amount of chemical elements stored in the soils and the trees.12 Some classic
cases of succession are discussed in A Closer Look 9.3. "Some Classic Cases of
Ecological Succession."
9.3 PATTERNS OF SPECIES CHANGE DURING SUCCESSION
During succession one species replaces another. In this section, we consider the
causes of these changes. Do earlier species influence the timing ot when other
species are able to enter an ecosystem? The answer has practical consec[iiences.
White pine. for example, is an economically important tree in New England.
White pine regenerates naturally following a disturbance; after tlie hurricane of
193H. white pine grew readily in abandoned pastures. But attempts to grow white
pine commercially by planting it on bare soil did not succeed. Why is this so?
Must grasses or other herbaceous plants lie present before white pine can grow?
Patterns of Interaction
We find that there are at least three patterns of interaction among earlier and later
species in succession (Figure 9.10).".14
1. Facilitation: One species can prepare the way for the next (and may even be
necessary for the occurrence of the next).
2. Interference: Early successional species may in some way prevent the
entrance of later successional species.
3. Life History Differences; One species may not affect the time of entrance of
another; two species may appear at different times during succession because
of differences in transport, germination, growth, and longevity of seeds.
CLOSER LOOK 9.2
AN EXAMPLE OF FOREST SUCCESSION
Within a few years after a field is abandoned, seeds of many kinds sprout, some of shortlived weedy plants and some of trees (Figure 9.4fl). After a few years have passed,
certain species, generally referred to as pioneer species, become established. In the
Poconos of Pennsylvania, red cedar is such a species (Figure 9.4a). In New England,
white pine, pin cherry, white hirch, and yellow birch are especially abundant. These trees
are fast growing in bright light, and have widely distributed seeds. For example, red
cedar seeds are eaten by birds and dispersed; birches have very light seeds that are
dispersed
widely by the wind. After several decades, forests of the pioneer species are well
established, forming a dense stand of trees (Figure 9.6c).
Once the initial forest is established, other species begin to grow and become
important. Typical dominants in the Northeastern United States are sugar maple
and beech. These later successions of trees are slower growing than the ones that
came into the forest first, but they have other characteristics that make them well
adapted to the later stages in succession. These species are what foresters call
shade tolerant; they grow relatively well in
the deep shade of the redeveloping forest.
After three or four decades, most of the short-lived species have matured,
borne fruit, and died: these trees cannot grow in the shade ot the forest and so do
not regenerate in a forest that has been reestablished. For example, after five or
six decades a New England forest is a rich mixture of birches, maples, beeches,
and other species. The trees are a variety of sizes, although dominant trees are
generally taller than those dominating earlier stages. After 1 or 2 centuries, such a
forest will be composed mainly ot shade-tolerant species.
CLOSER LOOK 9.3
SOME CLASSIC CASES OF ECOLOGICAL SUCCESSIOIM
Succession Among the Ocean Tides
In the ocean, succession occurs where there is relative constancy of the
environment such as between the high and low tidemarks along a rocky shore or
in a coral reef. Where the ocean environment is constantly changing, succession
does not appear to occur. For example, there is no perceptible succession in the
middle of the ocean, where open ocean waters are continually stirred by winds,
waves, and currents.
Wetland Succession
From a geologic point of view, a pond is a temporary feature of a landscape,
eventually filling in with sediments. Ponds are common in areas subject to largescale geologic disturbances, such as glaciation, that shift the drainage patterns of
streams and create depressions and dams. This is why Minnesota, which lies
within the heavily glaciated area of North America, is called the Land of 10,000
Lakes and why natural ponds are rare in the southern Great Plains
FIGURE 9.8
(a) Diagram of bog succession.
south of the glacial moraines, such as in Oklahoma
(«»)
FIGURE 9.8
(a) Diagram of bog succession.
south of the glacial moraines, such as in Oklahoma.
When a pond is first created, it tends to have clear water with little sediment, low
concentrations of chemical elements, and little organic matter. Over time these constituents
increase. Streams carry sediments that are deposited in the comparatively quiet waters of
the pond. The streams bring chemical elements to the pond, suspended in the particles and
dissolved in the water. These enrich the pond, adding nutrients necessary for life. This
increase in chemical elements is called the eu-trophication of the pond. The young,
nutrient-poor pond is called oligotrophic; the old, nutrient-rich pond is called eutrophic.
The input of sediments and nutrients is sometimes referred to as the loading of the pond.
The increase in chemical elements in the pond allows a greater production of plants and
animals, leading to an increase in live organic matter and an increase in the organic content
of the sediments. This process is usually very gradual, and a pond may shift back and forth
over a long period, from oligotrophic to eutrophic to oligotrophic, as climate and land
vegeta tion upstream in the watershed supplying the pond's waters change.
Bog Succession
A bog is a body of water with acid waters and little if any surface outflow, so that the
waters have little current. Often, a bog has sphagnum moss and inlets but no surface outlets. (This is a general description of typical bogs, as a technical definition may vary from
expert to expert.) Succession in a bog is a process that begins with open water and ends
with a forest (Figure 9.8.).30 Bog succession can be observed easily because the pond fills
in from the edges toward the center. The center is successionally the youngest, and the
bog's original edge is the oldest. In the quiet waters of the open part of a bog, sedge plants
form floating mats that grow out over the water's surface. These short-lived shrubs are the
pioneers. Their mat of thick, organic matter forms a primitive soil into which seeds of other
plant species fall and germinate. Meanwhile, sediments build up on the bog bottom made
up of dead organic matter from aquatic animals and plants as well as organic material that
flows in from surface streams or is blown in by the wind.
The bog slowly fills in from the bottom to the top. Eventually the floating mat and
sediments meet to form a base firm enough to support trees. The first trees that can survive
under these conditions are adapted to wet grounds. For example, cedars and larches grow
on these floating mats in northern forests, such as those of Minnesota, Michigan, and New
England. These floating mats are called quaking bogs for a good reason. A visitor to a bog
can easily demonstrate that the mat of sedges, shrubs, and trees is floating. If you jump up
and down, the mat moves with you, like a water mattress.
If the process continues undisturbed, the entire bog fills in and a raised, heavily organic soil
forms, in which other trees can survive. In some cases, the bog disappears and the area is
taken over by tree species that are characteristic of mature forests on the well-drained soils
of the region. In other cases, open water or a moss-covered wetland with some open water
can persist. Wetlands can persist for very long periods without completely filling in. For
example, in some Alaskan forests, a tree stage is replaced by a sphagnum moss stage,
which can persist for a long time.
Sand Dune Succession
Plant succession on sand dunes along beaches is a worldwide phenomenon. important along the shores of the Great Lakes in North
America (l-'igure 9.9), in Cape Cod, Massachusetts, where Henry David Thoreau observed it in the nineteenth century, along the
coasts of Australia, as well as in the famous Donana National Park in southern Spain. Dunes along tile shores of Lake Michigan were
among the first sites where succession was studied early in the twentieth century.^'
Sand dunes are geologically unstable. They are continually formed, destroyed, and reformed by the action ot winds, tides, and
storms. The first pioneer plants that survive on a newly formed dune are dune grasses: they have long runners that anchor the plants in
tile sand. The runners have sharp ends that force their way through (lie sand as they grow (Figure 9.9). Dune grasses help stahili/e the
dune. making it possible for other plants to become established. Then seeds of shrubs and small trees germinate and grow on the
stabili/.ed dune. Eventually a small forest develops, including (lines and oaks.
There are limits to tlie stability of tlie dune. Wind and water from a major storm can break through the dune. redistributing the
sand and starting tlie process of succession over again. Often, as on the shores of Lake Michigan and the coast of Australia, series of
dunes are visible, extending a considerable distance inland. The dunes closest to tlie water are in an early stage of succession, whereas
the interior dunes were deposited earlier and have an older forest. It is possible to study tlie history of dune succession simply by
walking inland.
From these examples, we can begin to see some general patterns of succession in tlie change in tlie physical structure of
vegetation. Typically, succession begins with small plants (the dune's grasses, the bog's Boating mats) and proceeds to larger and
larger plants.
9.9 Dune succession on the shores of Lake Michigan. Dune grass shoots appear
scattered on the slope where they emerge from underground runners.
FIGURE
There is actually a fourth possibility: Succession never occurs and the species that
enters first remains until tlie next disturbance. This fourth case is called chronic
patchiness.
Each of these processes occurs in nature. Which occurs depends in part on the
environmental conditions and in part on the pool of species that are available to take part
in succession.
Facilitation
This pattern has been found to take place in tropical rain forests.^ Early
succes.sional species speed the reappearance of the microclimatic condi tions that occur
in a mature forest. In tropical forests, temperature, relative humidity, and light intensity at
the soil surface can reach conditions similar to those of a mature rain forest after only l4
years.16 Once these conditions are established. species that are adapted to deep forest
shade can germinate and persist.
Sand dunes and bogs also illustrate facilitation. Dune grasses anchor the sandy soil so
that seeds of plants that fall on the ground have a chance to germinate before they are
buried too deep or blown away again (see A Closer Look 9.3). Sedges that form floating
mats on the waters of a bog create a substrate where seeds of other species can lodge,
germinate, and grow. (See "A Closer Look 9.3".)
Interference
Examples of interference can be found in certain tropical rain forests. When a rain
forest is cleared, used for agriculture, and then abandoned, perennial grasses grow that
form dense mats. For example, in parts of Asia, these include bamboo and Imperata, as
well as thick-leaved small trees and shrubs. Together, these form stands so dense that
seeds of other, later successional species cannot reach the ground, germinate, or obtain
enough light, water, and nutrients to survive. Imperata either replaces itself or is replaced
by bamboo, which then replaces itself.17 Once established, Imperata and bamboo appear
able to persist for a long time.
Life History Differences
An example of life history difference is illustrated by seed disbursal. When early
successional species produce seeds, the seeds are readily transported by wind or animals,
and so reach a clearing sooner and grow faster than seeds of late successional species
(Figure 9.10). In many forested areas of eastern North America, birds eat the fruit of
cherries and red cedar and their droppings contain the seeds that are spread widely.
Species typical of old-growth forest, such as sugar maple, can grow in open areas, but
these seeds take longer to get there. In this case the early succession of species might be
neutral or interfere. Old-growth forest species typically have seeds with an adequate food
supply for a seedling. Therefore, the seeds (maple, beechnut, acorns) tend to be heavy
and not transported easily.
Chronic Patchiness
Whether a change in species occurs during succession depends on the complex
interplay between life and its environment. Life tends to build up, to aggrade, whereas
nonbiological processes in the environment tend to erode and degrade. In harsh
environments, where energy or chemical elements required for life are limited and where
disturbances are frequent, the physical, degrading environment dominates, and succession
does not occur. Deserts exemplify the chronic patchiness that results. For example, in the
warm deserts of California, Arizona, and Mexico, the major shrub species grow in
patches that are often composed of mature individuals with few seedlings. These patches
tend to persist for long periods until there is a disturbance.18 Similarly, in highly polluted
environments, a sequence of species replacement may not occur.
Knowledge of the causes of the succession of species can be useful in the restoration
of damaged areas.19 Plants that facilitate the presence of others should be planted first, as
on sandy areas, where dune grasses can help hold the soil before attempts are made to
plant larger shrubs or trees.
This discussion suggests that species of plants show adaptations to specific stages in
succession. When we attempt to restore landscapes, we can make use of our knowledge
of these adaptations, just as the ecologists in our opening case study did when they used
grasses adapted to early stages in succession in the restoration of mined lands.
Facilitation
Interference
Grass continues to remain grass
9.10 Several ways species might affect one another during ecological succession, (a) Facilitation. As Henry David Thoreau
observed in Massachusetts, pines provide shade and act as "nurse trees" for oaks. Pines do well in openings. If there are no pines, few
or no oaks will grow. The pines facilitate the entrance of oaks. (&) Interference. Some grasses that grow in open areas form dense
mats that prevent seeds of trees from reaching the soil and germinating. The grasses' interference in the addition of tree species, (c)
Chronic patchiness. The null condition, neither positive nor negative interactions. One species does not aid or hinder another. The
physical environment tends to be dominant.
FIGURE
9.4 SUCCESSION AND CHEMICAL CYCLING
On the kind, there is generally an increase in the storage of chemical elements (nitrogen,
phosphorus, potassium, and calcium, essential for plant growth and function) during the
progression from the earliest stages of succession to middle or late succession. There are
two reasons for this. First, organic matter stores chemical elements; as long as there is an
increase in organic matter within the ecosystem, there will lie an increase in the storage
of chemical elements. This is true for live and dead organic matter. Additionally, many
plants have root modules containing bacteria that can assimilate atmospheric nitrogen,
which is then used by the plant in a process known as nitrogen fixation. The second
reason is indirect: The presence of live and dead organic matter helps retard erosion. Both
organic and inorganic soil can lie lost to erosion because of the effects of wind and water.
Vegetation tends to prevent such losses and therefore causes an increase in total stored
material.
Organic matter in soil contributes to the storage of chemical elements in two ways. First,
it contains chemical elements itself. Second, dead organic matter functions as an ion
exchange column that holds onto metallic ions that would otherwise be transported in the
groundwater as dissolved ions and lost to the ecosystem. As a general rule. the greater the
volume of soil and the greater the percentage of organic matter in the soil, the more
chemical elements will lie retained.
The amount of chemical elements stored in a soil depends on the total volume of soil and
its storage capacity for each element. Chemical storage capacity of soils varies with the
average size of the soil particles. Large, coarse particles, like sand, have a smaller surface
area per unit volume and can store a smaller quantity of chemical elements. Clay, which
is made up of the smallest particles, stores the greatest quantity of chemical elements.
Soils contain greater quantities of chemical elements than do live organisms. However,
much of what is stored in a soil may he relatively unavailable. or it may onlv become
available slowly, lie-cause the elements are tied up in complex compounds that decay
slowly. In contrast, the elements stored in living tissues are readily available to other
organisms through tood chains.
The rates of cycling and average storage times are system charcteristics (as discussed in
Chapter 3). Soils store more elements than live tissue, hut cycle them at a slower rate.
The increase in chemical elements that occurs in the early and middle stages of
succession does not continue indefinitely. If an ecosystem persists on the landscape for a
very long period with no disturbance, there will lie a slow but definite loss of stored
chemical elements. Thus the ecosystem will slowly run downhill and become depauperate, that is, less able to support rapid growth. high biomass density, and high
biological diversity (Figure 9. ID.2"
Changes in Chemical Cycling During a Disturbance
When an ecosystem is disturbed by fire. storms, or human actions, changes occur in
chemical cycling. For example, when a forest is burned, complex organic compounds
(such as wood) are converted to smaller inorganic compounds (including carbon dioxide,
nitrogen oxides, and sulfur oxides). Some of the inorganic compounds from the wood are
lost to tlie ecosystem during the fire. as particles of ash that are blown away or as vapors
that escape into the atmosphere and are distributed widely. Other compounds are
deposited on the soil surface as ash. These are comparatively highly soluble in water and
readily available for vegetation uptake. Therefore, immediately after a fire there is an
increase in the availability of chemical elements. This is true even if the ecosystem as a
whole has undergone a net loss in total stored chemical elements.
If sufficient live vegetation remains after a fire. then the sudden, temporary increase,
or pulse, of newly available elements is taken up rapidly, especially if there is a moderate
amount of rainfall (enough for good vegetation growth but not so much as to cause
excessive erosion). The pulse of inorganic nutrients can then lead to a pulse in growth of
vegetation and an increase in the amount of stored chemical elements in the vegetation.
This in turn provides an increase in nutritious food for herbivores, which can
subsequently undergo a population increase. The pulse in chemical elements in the soil
can therefore have effects that extend through the food chain.
Other disturbances on the land have effects similar to those of fire. For example,
severe storms, such as hurricanes and tornadoes, knock down and kill vegetation, which
decays, increasing the concentration of chemical elements in the soil. which are then
available for vegetation growth. Storms have another effect in forests: when trees are
uprooted, the soil can lie turned over, and chemical elements that were near the bottom of
the root /one are brought to the surface, where they are more readily available.
Knowledge ot the changes in chemical cycling and the availability of chemical
elements from the soil during succession can be useful to us in restoring damaged lands.
We know that nutrients must he available within the rooting depth of the vegetation.
There must lie sufficient organic matter in the soil to hold onto nutrients. Restoration will
lie more difficult on lands where tlie soil has lost its organic matter and lias been leached
than on lands that retain organic matter and have been little leached. Soils subject to acid
rain and acid mine drainage pose special challenges to the process of land restoration.
9.5 SUCCESSION AND THE BALANCE OF NATURE
Succession on dunes and in wetlands and forests helps us think about one of the oldest
and most intriguing questions people ask about wilderness:
What are the characteristics of nature undisturbed? That is, what is tile pristine, virgin
state of nature unaffected by human beings? As we asked in Chapter 1: Is there a balance
of nature, an ecological stability in an ecosystem undisturbed by humans? The answers to
such questions tell us about the baseline state of nature, against which we can compare
the effects of our actions. Therefore these questions have much practical importance in
the management of our natural resources and our entire planet and are also important to
recreation, aesthetics, and philosophy.
As mentioned in Chapter 1. there lias long been a belief that an undisturbed ecosystem
lias a kind of constancy and permanencv. But recent evi-dence disputes this: the
examples we have just discussed of very long successional patterns cast doubt on this
idea. A forest in cciuilibrium must by definition have a zero net production: No addition
ot organic matter takes place. Such a forest can lose nutrients through geologic processes
lint lias little means to accumulate them. Thus tlie ultimate fate ol a never-disturbed
forest is to go downhill biologically.
For much of the twentieth century, ecologists believed that succession proceeded to a
fixed, classic climax state, defined as a steady-state stage tliat would persist indefinitely
and have maximum organic matter, maximum storage of chemical elements. and
maximum biological diversity.21 This is an imaginary condition, however, never attained
in nature. Evidence suggests that the greatest amount of organic matter and the greatest
storage of chemical elements occur in the middle of succession (Figure 9.7).22 There are
two primary reasons lor this situation. First, as organisms and populations grow. they
must increase the amount of stored material (otherwise they could not grow). Thus. in a
growing or aggrading ecosystem, inputs of chemical elements to the system will he
greater tlian outputs, and there will be a net increase in ecosystem chemical storage.
Second, at tlie same time that biological processes of grow'-th are leading to an increase
in stored chemical elements, nonhiological processes of erosion function to decrease this
storage. Elements are dissolved in water and transported out of the ecosystem by ground
and surface waters. Winds lift panicles and move them away. In terms of chemical
cycling, the process of succession is a continual conflict betw een aggrading forces of the
biota and degrading forces of the physical environment.23 Thus we cannot expect even
those forests we protect from human interference to last forever,
In an old forest with a complex and diverse community and with trees of many species,
(lie rate of loss from these physical processes may lie small. In some ecosystems, such as
certain tropical rain forests, there are many specific adaptations ot vegetation to retain
chemical elements. In such rain forests, only a small fraction of their chemical elements
are stored in the soil: most chemical elements are held in the live biota and are recycled
rapidly when trees die. In general, however, land ecosystems slowly lose some fraction of
their stored chemical elements every year by the erosive effects of wind and water. This
loss can lie extremely slow in terms of human lifetimes. Whether physical erosion and
loss dominate or whether biological aggradation and storage dominate depends on the
relative rates of growth and erosion.
In an area of very heavy rainfall, erosion tends to override the biological effects. As
an example, the west coast of New Zealand's southern island is famous for beautiful
glaciers, glaciated valleys and harbors. and temperate rain forests. The glaciers are still
melting and retreating as part of long-term trends that began at the end of the last ice age.
Rainfall in this area of New Zealand is very high, as much as 700 cm/year (275 in./year).
Erosional processes that might take much longer in areas with low rainfall occur rapidly
here. The processes of succession and erosion can lie observed on a walk from the
present edge of a glacier toward tlie ocean shore. First, you pass over hare rock, then over
rock covered by lichens, then over areas vegetated by grasses and lichens, then to a shrub
and perennial grass stage. and on to several stages in the development of a temperate rain
forest, which has great species diversity, high biomass, and large trees. This is the state
that ecologists classically (throughout most of the twentieth century) have called tlie
climax state. They assumed that this stage would persist indefinitely, as long as tlie forest
was not disturbed.
But beyond the rain forest, farther from the glaciers and free of ice for thousands of years,
is an even later stage of succession, composed of low shrubs and grasses with low species
diversity and low biomass. Succession has not led to constancy or to maximum biomass
and diversity.
Inspection of the soil shows a deeply leached layer. Over the centuries, rain has
dissolved, transported, and deposited chemical elements from tlie upper soil, where plant
roots can reach, to a depth below the reach of most plant roots. Without an abundance of
chemical elements, the magnificent rain forests cannot persist.21 Although tlie rate at
which eroded elements are transported is unusually rapid on the west coast of New
Zealand's south island, this process occurs in other places and is part of a general pattern.
Another example of the pattern occurs on a sequence of dunes in eastern Australia,
which provides information about more than 100.000 years of succession.2^ Just as with
the rain forests of New Zealand, the pattern of succession at first follows tile-classic idea.
As one walks inland to older and older dunes, the stature of the forest first increases, as
do total soil organic matter and the richness of tlie species. But then these factors
decrease, and the very oldest dunes are depauperate. Gone are tlie large trees and the
great diversity of species. On tlie oldest dunes one finds shrubs of low stature and a
comparatively barren landscape. Tlie soil lias been slowly leached of its nutrients, which
have been washed so far below the surface tliat tree roots can no longer reach them.