Download Chapter 20 Succession and Stability In 1794, Captain George

Chapter 20 Succession and Stability
In 1794, Captain George Vancouver visited
the inlet to what is today called Glacier Bay,
Alaska (fig. 20.1). He could not pass beyond the
inlet to the bay, however, because his way was
blocked by a mountain of ice. Vancouver (1798)
described the scene as follows: "The shores of
the continent form two large open bays which
were terminated by compact solid mountains of
ice, rising perpendicularly from the water's edge,
and bounded to the north by a continuation of
the united lofty frozen mountains that extend
eastward from Mount Fairweather."
FIGURE 20.1 Glacier Bay, Alaska.
In 1879, John Muir explored the coast of
Alaska, relying heavily on Vancouver's earlier descriptions. Muir (1915 ) commented in his journal that Vancouver's
descriptions were excellent guides except for the area within Glacier Bay. Where Vancouver had met "mountains of
ice," Muir found open water. He and his guides from the Hoona tribe paddled their canoe through Glacier Bay in rain
and mist, feeling their way through uncharted territory. They eventually found the glaciers, which Muir estimated
had retreated 30 to 40 km up the glacial valley since Vancouver's visit 85 years earlier.
Muir found no forests at the upper portions of the bay. He and his party had to build their campfires with the
stumps and trunks of long-dead trees exposed by the retreating glaciers. Muir recognized that this "fossil wood" was
a remnant of a forest that had been covered by advancing glaciers centuries earlier. He also saw that plants quickly
colonized the areas uncovered by glaciers and that the oldest exposed areas, where Vancouver had met his
mountains of ice, already supported forests.
Muir's observations in Glacier Bay were published in 1915 and read the same year by the ecologist William S.
Cooper Encouraged by Muir's descriptions, Cooper visited Glacier Bay in 1916 in what was the beginning of a lifetime
of study. Cooper saw Glacier Bay as the ideal laboratory for the study of ecological succession, the gradual change in
plant and animal communities in an area following disturbance or the creation of flew substrate. Glacier Bay was
ideal for the study of succession because the history of glacial retreat could be accurately traced back to 1794 and
perhaps farther.
Cooper ultimately made four expeditions to Glacier, Bay. His work and that of later ecologists produced a
detailed picture of succession there. Several species of plants colonize an area during the first 20 years after it is
expose by the retreating glacier. These plants, the first in a successional sequence, form a pioneer community. The
most common members of the pioneer community are horsetail, willow herb, willows, cottonwood seedlings, and
Sitka spruce.
About 30 years after an area is exposed, the pioneer community gradually grades into a community dominated
by mats of Dryas, a dwarf shrub. These Dryas mats also contain scattered alder, willow, cottonwood and spruce.
Then, about 40 years after glacial retreat, the community changes into a shrub-thicket dominated by cottonwood.
Soon after the closure of the cottonwood thicket, however, spruce will grow above it, covering about 50% of the
area on sites 50 to 70 years old.
In 75 to I00 years, succession leads to a forest community dominated by spruce. Mosses carpet the understory of
this spruce forest and with them there grow seedlings of western hemlock, Tsuga heterophylla, and mountain
hemlock, Tsuga mertensiana. Eventually, the population of spruce declines and the forests are dominated by
Hemlock. On landscapes with shallow slopes these hemlock forests eventually give way to muskeg, a landscape of
peat bogs and scattered tussock meadows.
Because succession around Glacier Bay occurs on newly exposed geological substrates, not significantly modified
by organisms, ecologists refer to this process as primary succession, Primary succession also occurs on newly formed
volcanic surfaces such as lava flows. In areas where disturbance destroys a community without destroying the soil,
the subsequent succession is called secondary succession. For instance, secondary succession occurs after
agricultural lands are abandoned or after a forest fire.
Succession generally ends with a community whose populations remain stable until disrupted by disturbance.
This late successional community is called the climax community. The nature of the climax community depends upon
environmental circumstances. The communities we discussed in chapter 2—temperate forests, grasslands, etc.—
were essentially the climax communities for each of the climatic regimes that we considered. The climax community
around Glacier Bay is determined by the prevailing climate and local topography. On well-drained, steep slopes the
climax community is hemlock forest. In poorly drained soil on shallow slopes the climax community is muskeg.
Studies of succession show that communities and ecosystems are not static but constantly change in response to
disturbance, environmental change, and their own internal dynamics. In many cases, the general direction of change
in community structure and ecosystem processes is predictable, at least over the short term. The patterns of change
in community and ecosystem properties during succession and the mechanisms responsible for those changes are
subjects covered in this chapter. We also consider a companion topic, community and ecosystem stability.
CONCEPTS
Community changes during succession include increases in species diversity and changes in species composition.
Ecosystem changes during succession include increases in biomass, primary production, respiration, and nutrient
retention.
Mechanisms that drive ecological succession include facilitation, tolerance, and inhibition.
Community stability may be due to lack of disturbance or community resistance or resilience in the face of
disturbance.
CAST HISTORIES: community change during succession.
Community changes during succession include increases in species diversity and changes in species composition.
Some of the most detailed studies of ecological succession have focused on succession leading to a forest climax.
Though primary and secondary forest succession requires different amounts of time, the changes in species diversity
that occur in each appear remarkably similar. Primary Succession at Glacier Bay We have already reviewed the basic
patterns of primary succession around Glacier Bay. Now we return to Glacier Bay to examine successional changes in
species diversity and composition. William Reiners, Ian Worley, and Donald Lawrence (1971) studied changes in plant
diversity during succession at Glacier Bay. They worked at sites carefully chosen for similarity in physical features but
differing substantially in age. Their eight study sites were below 100 m elevation, were on glacial till, an unstratified
and unsorted material deposited by a glacier, and all had moderate slopes. The study sites ranged in age, that is,
time since glacial retreat, from 10 to 1,500 years.
Their youngest site, which was approximately 10 years old, supported a pioneer community of scattered
Epilobium, Equisetum, and Salix. Site 2 was about 23 years old and supported a mix of pioneer species and clumps of
Populus and Dryas. Site 3, which was approximately 33 years old, supported a mat of Dryas enclosing clumps of Salix,
Populus, and Alnus. Site 4 was 44 years old and was dominated by a mat of Dryas with few open patches. Site 5,
which was approximately 108 years old, was dominated by a thicket of AInus and Salix with enough emergent
Populus and Picea to form a parrial canopy. Site 6 was a 200-year-old forest of Picea. Using geological methods,
Reiners and his colleagues dated site 7 at 500 years and site 8 at 1,500 years. Both sites were located on Pleasant
Island, which because it is located outside the mouth of Glacier Bay had escaped the most recent glaciation, which
had destroyed the forests along the bay. Site 7 was an old forest of Tsuga that contained a few Picea. Site 8 was a
muskeg with scattered lodgepole pines, Pinus contorta.
The total number of plant species in the eight study sites increased with plot age. As you can see in figure 20.2,
species richness increased rapidly in the early years of succession at Glacier Bay and then more slowly during the
later stages, approaching a possible plateau in species richness.
FIGURE 20.2 Change in plant species richness during primary succession at Glacier Bay, Alaska (data from
Reiners. Worley, and Lawrence 1971).
Not all groups of plants increased in diversity
throughout succession. Figure 20.3 shows that while the
species richness of mosses, liverworts, and lichens
reached a plateau after about a century of succession,
the diversity of low shrubs and herbs continued to
increase throughout succession. In contrast, the
diversity of tall shrubs and trees increased until the
middle stages of succession and then declined in later
stages.
FIGURE 20.3 Succession of plant growth forms at Glacier
Bay, Alaska (data from Reiners, Worley, and Lawrence
1971).
The pattern of increased species richness withstand
age that Reiners and his colleagues described for the
successional sequence around Glacier Bay is one that we
will see several times in the examples that follow. However, the tempo of succession is far different. The late
successional climax community at Glacier Bay was 1,500 years old. In the following example of secondary succession,
the climax forest community was 150 to 200 years old, approximately one-tenth the age of the climax community
studied at Glacier Bay.
CASE HISTORIES: ecosystem changes during succession
Ecosystem changes during succession include increases in biomass, primary production, respiration, and nutrient.
As succession changes the diversity and
composition of communities, ecosystem
properties change as well. In the last section,
we saw how plant and animal community
structure changes during primary and
secondary succession. In this section, we
review evidence that many ecosystem
properties also change during succession. For
instance, many properties of soils, such as the
nutrient and organic matter content, change
during the course of succession.
Ecosystem Changes at Glacier Bay
Stuart Chapin and his colleagues (1994)
documented substantial changes in ecosystem
structure during succession at Glacier Bay. They
focused their studies in four areas of
approximately 2 km2 each. Their first site had
been deglaciated about 5 to 10 years and was
in the pioneer stage. Their second site had
been deglaciated 35 to 45 years and was
dominated by a mat of Dryas. Dryas was just beginning to invade this site when it was studied by Reiners' group
more than 20 years earlier. The third site had been deglaciated about 60 to 70 years and was in the Alnus stage. This
site had been studied by Reiners when it was a young thicket of Alnus and by Cooper when it was in the pioneer
stage. The fourth site studied by Chapin and his colleagues had been deglaciated 200 to 225 years earlier and was a
forest of spruce, Picea, as it was when studied by
Reiners and Cooper.
Chapin and his research team measured
changes in several ecosystem characteristics
across these study sites. One of the most
fundamental characteristics was the quantity of
soil. Total soil depth and the depth of all major soil
horizons all show significant increases from the
pioneer community to the spruce stage (fig.
20.10).
FIGURE 20.10 Soil building during primary
succession at Glacier Bay, Alaska (data from
Chapin et al. 1994).
Several other ecologically important soil
properties also changed during succession at the
Glacier Bay study sites. As figure 20.11 shows, the
organic content, moisture, and nitrogen
concentrations of the soil all increased
substantially. Over the same successional
sequence, soil bulk density pH, and phosphorus concentration all decreased. Why are these changes in soil
properties important? They demonstrate that succession involves more than just changes in the composition and
diversity of species. Terrestrial succession changes key ecosystem properties. Changes in soil properties are
important because soils are the foundation upon which terrestrial ecosystems are built.
FIGURE 20.11Changesin soil properties during
succession at GlacierBay, Alaska (data from
Chapinetal.1994).
We can also see from these ecological studies
that the physical and biological properties of
ecosystems are inseparable. Organisms acting upon
mineral substrates contribute to the building of soils
upon which spruce forests eventually grow around
Glacier Bay. Soils, in mm, strongly influence the
kinds of organisms that grow in a place.
As we saw in chapter 19, disturbance of
vegetation significantly increases the loss of
nutrients from forest soils. As we shall see in the
next section, succession appears to increase the
retention of nutrients by forest ecosystems.
A Model of Ecosystem Recovery
As a result of their observations on the Hubbard Brook Experimental Forest, Bormann and Likens proposed a model
for recovery of ecosystems from disturbance (fig. 20.13). Their "biomass accumulation model" divides the recovery
of a forest ecosystem from disturbance into four phases: (1) a reorganization phase of 10 to 20 years, during which
the forest loses biomass and nutrients, despite accumulation of living biomass; (2) an aggradation phase of more
than a century, when the ecosystem accumulates biomass, eventually reaching peak biomass; (3) a transition phase,
during which biomass declines somewhat from the peak reached during the aggradation phase; and (4) a steady
state phase, when biomass fluctuates around a mean level.
FIGURE 20.13 The biomass accumulation model of forest
succession (data from Bormann and Likens 1981).
How well does the biomass accumulation model represent the
process of forest succession? Does a similar sequence of stages
occur during succession in other ecosystems? For instance, do
ecosystems eventually reach a steady state? The generality of the
biomass accumulation model can be tested on ecosystems, such as
Sycamore Creek, Arizona, that undergo rapid succession. Such
ecosystems give the ecologist the chance to study multiple
successional sequences. As we will see in the following section, the
patterns of ecosystem change during succession on Sycamore
Creek suggest that several ecosystem features eventually reach a steady state.
Mechanisms of Primary Succession Following Deglaciation
The complex mechanisms underlying succession were well demonstrated by the detailed studies of Chapin's
research team (1994). They combined field observations, field experiments, and greenhouse experiments to explore
the mechanisms underlying primary succession at Glacier Bay, Alaska. Like Morris and Wood, they found that no
single factor or mechanism determines the pattern of primary succession at Glacier Bay, Alaska.
Figure 20.22 summarizes the complex influences of four successional stages on establishment and growth of
spruce seedlings. During the pioneer stage, then is some inhibition of spruce germination. Any spruce seedlings that
become established, however, have high survivorship but low growth rates. Spruce seedling growth rates and
nitrogen supplies are increased somewhat during the Dryas stage. However, this facilitation during the Dryas stage is
offset by poor germination and survivorship, along with increased seed predation and mortality.
FIGURE 20.22 Inhibition and facilitation of spruce during the major successional stages at Glacier Bay, Alaska
(data frorn Chapin et al. 1994).
Strong facilitation of spruce seedlings first occurs in the Alnus stage. During this stage, germination and
survivorship remain low and seed mortality, root competition, and light competition are significant. However, these
inhibitory effects are offset by increased soil organic matter, nitrogen, mycorrhizal activity, and growth rates. The net
effect of the Alnus stage on spruce seedlings is facilitation.
In the spruce stage, the net influence on spruce seedlings is inhibitory. Germination is high during the spruce
stage but this is counterbalanced by several inhibitory effects. Growth rates and survivorship are low and nitrogen
availability is reduced. In addition, seed predation and mortality, Not competition, and light competition are all high.
These results combined with those of Morris and Wood on Mount St. Helens remind us that nature is far more
complex and subtle than models such as that proposed by Connell and Slatyen However, the Connell and Slatyer
model challenged ecologists to think more broadly about succession and to go out and conduct field tests of
alternative successional mechanisms. Their response produced today's improved understanding of the process of
ecological succession.
In this and the previous two sections, we have discussed community and ecosystem change and the mechanisms
producing that change. In the next Case History section, we consider a companion topic: community and ecosystem
stability.
SUMMARY CONCEPTS
Succession is the gradual change in plant and animal communities in an area following disturbance or the creation of
new substrate. Primary succession occurs on newly exposed geological substrates not significantly modified by
organisms. Secondary succession occurs in areas where disturbance destroys a community without destroying the
soil. Succession generally ends with a climax community whose populations remain stable.
Community changes during succession include increases in species diversity and changes in species
composition. Primary forest succession around Glacier Bay may require about 1,500 years, while secondary forest
succession on the Piedmont Plateau takes about 150 years. Meanwhile, succession in the intertidal zone requires 1
to 3 years and succession within a desert stream occurs in less than 2 months. Despite the great differences in the
time required, all these successional sequences show increased species diversity over time.
Ecosystem changes during succession include increases in biomass, primary production, respiration, and
nutrient retention. Succession at Glacier Bay produces changes in several ecosystem properties, including increased
soil depth, organic content, moisture, and nitrogen. Over the same successional sequence, several soil properties
show decreases, including soil bulk density, pH, and phosphorus concentration. Succession at the Hubbard Brook
Experimental Forest increased nutrient retention by the forest ecosystem. Several ecosystem properties change
predictably during succession in Sycamore Creek, Arizona, including biomass, primary production, respiration, and
nitrogen retention.
Mechanisms that drive ecological succession include facilitation, tolerance, and inhibition. Most studies of
succession support the facilitation model, the inhibition model, or some combination of the two. Both facilitation
and inhibition occur during intertidal succession. Facilitation and inhibition also occur during secondary and primary
forest succession.
Community stability may be due to lack of disturbance or community resistance or resilience in the face of
disturbance. Ecologists generally define stability as the persistence of a community or ecosystem in the face of
disturbance. Resistance is the ability of a community or ecosystem to maintain structure and/or function in the face
of potential disturbance. The ability to bounce back after disturbance is called resilience. A resilient community or
ecosystem may be completely disrupted by disturbance but quickly return to its former state. Studies of the Park
Grass Experiment suggest that our perception of stability is affected by the scale of measurement. Studies in
Sycamore Creek indicate that resilience is sometimes influenced by resource availability and that resistance may
result from landscape-level phenomena.
Repeat photography can be used to detect long-term ecological change. Most successional sequences and most
community and ecosystem responses to climatic change take place over very long periods of time. Repeat
photography has become a valuable tool to help ecologists study these long-term changes.