We have developed the foundations of a theory of plant community ecology. In the discussion of Gradients (Chapter 9), we covered gradient analysis in considerable detail. The point was to illustrate the dynamics of the gradient. This dynamics is the mechanism inherent in communities which allows the communities of the Landscape to persist over long time intervals, adjusting as needed to changes in the environment. In this, the final chapter of this text, we shall look in greater detail at the nature of Succession which is defined as an [orderly] transition of community structure over time on a single site. Unlike the previous chapter which used actual data, this chapter will focus on theoretical considerations.
When succession was first described by H. C. Cowles in 1899 (Cowles. H. C. 1899. “The Ecological Relations of the Vegetation on the Sand Dunes of Lake Michigan” Botanical Gazette 27: 95-117, 167-175. reprinted, in part, in Kormundy, E. J., ed Reading in Ecology, Prentice-Hall, 1965), and analyzed by F. C. Clements (Clements, F. C. 1916. Plant Succession, an Analysis of the Development of Vegetation Carnegie Institution of Washington, Publication 242, pp. 1-512. reprinted, in part, in Kormundy 1965), it was treated as the community analog of development of an individual from fertilized egg to adult organism. Clements noted that “The developmental study of vegetation necessarily rests upon the assumption [emphasis added] that the unit or climax formation [a classification unit] is an organic entity, As an organism the formation arises, grows, matures, and dies. …Furthermore, each climax formation is able to reproduce itself, repeating with essential fidelity the stages of its development.” Although the community as a ‘super-organism’ has largely fallen out of favor among plant ecologists, the “climax community” concept has lingered, and drives most considerations of succession.
The ‘development’ of the climax community was seen as a series of discreet steps. If the
site is ‘new’ (never previously occupied, such as after a volcano eruption, major landslide,
…), the succession will be “primary succession.”
(1) the arrival of the pioneer species and their “ecesis” or establishment as a viable population.
(2) the vegetation undergoes a series of seral stages as early successional species arrive and become established after the ‘harsh’ environment of the site is modified by the pioneer species populations on it. The pioneer species will shade the soil, reducing the diurnal cycle in soil temperature (cooler by day, warmer by night), thereby increasing soil moisture; and will contribute organic material into the soil by decomposition of dead leaves and plants. The typical early successional species is “r-selected” [recall that r is reproductive rate], exhibiting high reproductive potential, dispersal capabilities, and limited self-regulation of population density.
As the site is further modified by the early successional species, the late successional species arriving at the site can become established, and out-compete the early successional species. The typical late successional species is “K-selected” [recall that K is carrying capacity (definition 2 in the glossary)], exhibiting lower reproductive potential, reduced dispersal capabilities, higher competitive capabilities, and higher self-regulation of population density.
(3) The ‘climax community’ becomes established, and is self-maintaining, thus effectively terminating the changes in species composition.
Eventually, the climax community becomes ‘over mature’ and begins to die off, leaving a site which is suitable for re-invasion by early successional communities which will develop into a new climax commmunity similar to the previous climax. Since many of the modifications to the site by the previous Vegetation will remain, this will be “secondary succession.”
I proposed this hypothesis (as a study of succession as a Markov process) to my major professor
after the research for my PhD dissertation was complete, and my dissertation was in prepration. As it
did not appear that post-doctoral funding would become available, I accepted a teaching position at a
community college, where research was not encouraged, so I did not pursue this line of research as
such. As I, over the intervening decades, have thought more about this hypothesis, I came to realize
that Vegetation dynamics does not conform well to the assumptions of a Markov process, so I developed
a concept of succession as a stochastic (controlled by probabilites) without the constraints of any
particular model of stochastic processes. In this context, the Theory of Island Biogeography can be
modified to account for the dynamics of Plant Communities.
The “Island” is whatever site(s) is ‘interesting.’ Offshore from the island [off site] there are sites which serve as the source region for emigrants which will become immigrants onshore. Disperal agents (diaspores in Botanical terms, disseminules in general terms; such as seeds, fruits, spores for Plants and mating pairs or gravid females for Animals) must arrive on site and undergo ecesis in order to become immigrants. According to the Theory of Island Biogeography, the probability of arriving is dependent upon the distance from the source to the island. It has been shown that seed dispersal around isolated trees is roughly Normally distributed (but should exhibit different variances depending on prevailing wind direction and speed). For most potential immigrants (any, and all, species which are in immigration range of the site), the immigrants may have emigrated from any of several source sites, so the probability of arriving is the sum of the probabilities of arriving from each potential emigration site.
As soon as dispersal agents arrive on the site, the probability of those species which have arrived jumps to 100%. Now, the probability of undergoing ecesis depends on arriving in an environment within the tolerance range of the immigrating species. If the environment is is the marginal ranges, the species will have a low, but not zero, probability of ecesis. In the suboptimal and optimal ranges, the probability of ecesis is much higher. As the immigrant species undergo ecesis, their probabilities jump to 100%. Populations are now on the site, and should grow to their local carrying capacity on a first come first served basis. As this growth occurs the remaining ‘unused resources’ dwindle, and later arriving immigrants will have to compete with the early immigrants for these resources. This part of the process does not end as in classic succession, but continues indefinitely as individuals of the established populations die and renewed competition for the newly available resources begins. Even without an individual dying, a sronger competitor can crowd an existing population to at least a niche displacement during the start of the next growing season, before the populations of the community re-establish their claim on the resources of the site. This leads to ‘cyclic replacement’ of the community composition as the populations on site change relative frequencies in the annual start of growing season competition for resources. This has also been called a ‘shifting-mosaic steady state.’
When we add to this process the complex of cyclic changes observed in climate (see Chapter 8. Bioclimatology), these cyclic changes cause a shift of the species response curves (see Chapter 6. Competition and Chapter 9. Gradients), so every species may have a different carrying capacity from one year to the next. This again would influence the outcome (probability of attaining full potential) of the annual start of growing season competition. I observed this over several years in the roadside weeds of rural northwest Indiana: during dry years, the chicory (Chicorium intybus) had higher population density and more robust plants with more inflorescences than did the asters (Aster umbellatus?); while during wet years, the asters had higher population densities and more robust plants with more inflorescensces than did the chicory. The two species were clearly exhibiting cyclic replacement, with neither species able to assume permanent dominance due to the climate cycling between dry and wet years.
|End of PERMIAN||245 Myr|
|End of DEVONIAN||360 Myr|
There are several longer climate cycles which are associated with the Milankovitch cycles (downloaded
16 Mar 2011 from http://www.homepage.montana.edu/~geol445/hyperglac/time1/milankov.htm
(1) eccentricity of Earth orbit (about 3% difference between nearest and farther distance from the Sun today to 20 - 30% difference), 100,000 years;
(2) axial tilt (varying from 21.5° to 24.5°, currently 23.5°), 41,000 years; and
(3) precession of the Earth axis (currently pointing at Polaris and rotating to Vega), 23,000 years.
The combined effects of these three cycles produces a compound cycle which roughly matches the glacial history of the Earth.
Glaciers are known to have occurred in the Precambrian and the Cambrian, the worst of which may have produced a “Snowball Earth” with sea level glaciers at the equator. The Snowball Earth may be relevant to biology in the sense that multicellular organisms seem to have first appeared after the extreme glaciation during Snowball Earth. However, we can ignore as irrelevant all glacial times of the Precambrian and Cambrian because we are here concerned only with terrestrial ecology, and the earliest known fossils of terrestrial plants occur following the Silurian glaciers of about 425 Myr BP (Before Present; the U.S. Bureau of Standards, [Standard Reference Material (SRM) 4990B] established that the ‘Present’ began in 1950, as the date from which radiocarbon, C14, dating is to be calculated). The earliest land plants known are Rhynia circa 400 Myr BP. These plants grew on tidal mud flats, and probaly looked like a bunch of green £ 2 pencils (up to 3mm [1/8 inch] in diameter, 20 cm [almost 8 inches] tall) stuck in the mud point down, with the erasers (brown or tan, not pink) as the reproductive structures (producing spores). It has been suggested that the emergence of land plants was delayed by the glacial climate prior to 400 BP, because shortly after the first Rhynia plants there was a proliferation of other similar plants, plus the Phyla Equisophyta and Lycophyta, in the Silurian and Devonian. Some of the Equisophyta and Lycophyta became large trees up to an estimated 1 m (39 inches) diameter, 20 to 30 m (70 to 98 ft) tall. The ferns (Polypodiophyta) also appeared during the Devonian.
The next glaciation, 360 Myr BP, at the end of the Devonian, coincides with a major extinction event affecting marine invertebrate Animals. Most of the land dwelling organisms survived this extinct event. It was in the Carboniferous that some of the ferns became trees rivaling the Equisetophte and Lycophyte trees all of which became the co-dominants in the great swamp forests which produced most of the coal beds currently being mined.
Following the Carboniferous, during the Permian, Conifers began to appear apparently as the dominant species of the drier uplands [the “Petrified Forest” consists of Conifer trees uprooted, transported and buried in mud from major river flooding]. Another glaciation occurred at the end of the Permian, 245 Myr BP. Most of the Psilophytes (such as Rhynia), Lycophytes, and Equisophytes became extinct during this glacial event. The surviving ferns became non-woody ground flora in moist to swampy areas, as they are today.
The Triassic forests were dominated by Conifers. The Ginkophyta, Cycadophyta, and Gnetophyta first began to appear during the Triassic as small trees, shrubs, and non-woody ground flora. This was the Vegetation during the first Age of the Dinosaurs (mostly four legged) with the bipedal (two legged) Raptors as small, chicken to domestic turkey sized, Animals.
After the glaciation (207 Myr BP), and accompanying extinction event at the end of the Triassic, the Cycads became the dominant species of tropical and temperate zone Jurassic forests, and the Conifers shifted to their current distribution patterns. This, the Cretaceous, was the second Age of the Dinosaurs, dominated by the bipedal Dinosaurs. Then two new major taxa appeared - the Flowering Plants and the Insects. This is particularly significant because for the first time Animals were recruited by the Plants to assist in reproduction by pollination in exchange for food, rather than relying on wind alone. This gave both groups a tremendous competitive advantage, and they began to take over the world. Mammals and Birds also became more numerous.
Then came 65 Myr BP. An asteroid impact followed by Cosmic Winter and yet another glaciation led to the end of the Age of Dinosaurs and the beginning of modern times. The Tertiary was here with all of the modern Plant and Animal groups. It was the grasses and the Primates which appeared early in the Tertiary and completed the modern groups. New Biomes also began around this time - Grasslands and Deserts. Finally the arid regions of the Earth supported ecosystems, and the large Mammals appeared. The rather lengthy account of the glacial history of the Earth reveals that, as long as there have been terrestrial Plants and Animals, the terrestrial communities have persisted continuously in spite of five (5) major extinction events!
About 2 Myr BP, another extinction event began, coinciding with the start of the Pleistocene glaciation. It is not completely clear whether or not either the glacial or the extinction events are over. We also know that, unlike the glaciations described on television science channels where glaciers advance and remain so for the duration of the glacial period, during real glaciations the glaciers advance and retreat repeatedly. Thus far the Pleistocene has advanced and retreated at least five times, most recently in the Wisconsin (North America), the Weichselian (Europe) and the Devensian (Britain). This glaciation is sufficiently recent that we can document its advances and retreats fairly accurately, and can document the effects on the Vegetation at a regional scale. Plant pollen and spores fall continually from the air onto any exposed surface. When they land on lakes, the pollen grains settle to the bottom of the lake in the same order as they fell on the water surface. Periodic flooding upstream from the lake adds silt layers between the pollen. The silt layers document the glacier melts plus local precipitation, and the pollen profile from top to bottom of the lake sediments documents the changes in the Vegetation over the life of the lake [some of these lakes have been completely filled with sediments and now support terrestrial communities]. And the time line is short enough for fairly accurate dating of the sediments by radiocarbon dating. There is now extensive data covering the most recent Pleistocene glacier retreat. These pollen profiles reveal that entire communities shifted southward with glacier advances and northward with glacier retreats. The shifting of an entire commmunity in response to changing climate regimes is called a “cliseral shift.” One example of a post-Wisconsin advance, without radiocarbon dating, is Griffin, Charles D, 1950 “A pollen profile from Reed bog, Randolph County, Indiana” Butler University Botanical Studies Vol 9 (issue 1) Article 13 (downloaded 16 March 2011 from digitalcommons.butler.edu/botanical/vol9/iss1/13/). Reed Bog is just south of the Wisconsin moraine marking the maximum advance of the Wisconsin ice sheet in east-central Indiana (East of Muncie, IN).
Although Griffin provided graphic and narrative interpretations of this profile, I will provide my own interpretations tailored to the context of this discussion. I have graphed the 16 species plus “other” from warm dry species at the bottom to cool wet species at the top, and with ‘time’ from old (deep) at the left to recent (shallow) at the right. Although I included all 16 species in the data plus the “other” (which groups all other pollen and spores together), I will discuss only a few of them. Since there is no C14 dating information, I will describe time as tdepth.
|dry Woodland||white Pine|
This discussion will use the Biomes (as described in Chapter 2. Life Zones & Biomes) represented
in pollen profiles in sites south of the western (upper) Great Lakes region of North America (see Table).
We are concerned here only with cliseral shifts. The most striking feature of this profile (and others equally close to the maxima of ice advances) is the apparent cliseral shift of Tundra as far south as about 40° N in eastern Indiana, based on Willow [Salix spp.] pollen from depth 28 to 23 feet [8.53 to 7.01 m]. The regional importance of Willow remains low even at its maximum, suggesting that the actual Tundra is close to the site, but never at the site itself. The glacier re-advanced toward the region until t28, then retreated slightly and re-advanced reaching in maximum at t25. There is geological evidence including nearby moraines to confirm these glacier movements. This probably means that t25 is around 18,000 - 12,000 yrs BP, but this would need confirmation from C14 dating.
During the glacier advance from t34 to t25, the Vegetation was mostly Tiaga (Fir [Abies spp.], Spruce [Picea spp.]). The Woodlands (Oak [Quercus spp.] and Pines [Pinus spp.]) were probably on the moraines left south of the site by the previous advances of the Erie Lobe of the Wisconsin Glacial episode [remember, the pollen profile collects pollen from the region, not just the site]. As the glacier retreated for final time before the Present, The Woodlands expanded and the Taiga shifted northward declining in important in the region. Species of the Temperate Forests exhibit the beginning of (northward) shifts to the region. The importance of the moist Woodlands (Oak [Quercus spp.], Hickory [Carya spp.]) increased until t13. There are several references to an unusually warm period (called the “hypsithermal”) around 8,500 to 5,000 yr BP, so we can tentatively take t13 to be 8,500 to 5,000 yr BP (again needing C14 dating to confirm this).
Following the hypsithermal, the Vegetation of Present has occupied the region, predominantly Oak [Quercus spp.] - Hickory [Carya spp.] moist Woodlands, with Pine [Pinus spp.] Barrens (dry Woodlands) on the driest sites, and Temperate Forests (Elm [Ulmus spp.] - Walnut [Juglans spp.] on exposed uplands and Beech [Fagus - Maple [Acer spp.] on sheltered uplands.
This Vegetation exhibits clear cyclic shifts southward, with cool periods at depths 33, 26, 21, 16, 10, possibly 3; and northward, with warm periods at depths 28, 24, 18, 13, and 6. We can tentatively identify the most recent cool period based on the southern Taiga species by comparing these data to actual data on climate in Peoria IL (1856 to 1945). The most recent increase in black Spruce pollen was at depth 5 feet, and the coolest recorded year was 46.9&$176 F in 1875. The most recent warm period based on the white Pine and Jack Pine pollen was at depth 3 feet, and the warmest recorded year was 55.6° F in 1931.
© 2010 TwoOldGuys
revised: 14 Mar 2011