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Transcript
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.