Distributions of

Communities in time:

Disturbance and

Ecological

Succession

Outline

1. Definitions

2. Mechanisms of succession

3. Successional theory

Disturbance – any relatively discrete event that disrupts the stand structure or changes resource availability (Pickett and White 1985)

Endogenous – originate within the community (e.g. tree death due to old age, fungal disease in soil, grazing)

Exogenous – originate outside the community (e.g. volcanic ash deposition, floods)

Is wildfire endogenous or exogenous?

Denver Post

Succession – process by which a series of different plant communities and associated animals and microbes successively occupy and replace each other over time in a particular ecosystem or landscape location following a disturbance (Kimmins 2004)

Primary succession – ecological succession in an area that lacks organic matter and has not been altered by living organisms

– Examples: establishment after a glacial retreat or volcanic activity

Secondary succession – ecological succession in an environment already modified by biota

– Example: post-fire establishment, establishment after clearcut

Sere – sequence of communities that develop over the course of succession

Seral stage – stages in the development of a sere

Climax – (controversial idea) community that has reached a steady state or equilibrium. Species present are those best adapted to the climate.

But is succession directional?

Mechanism of succession – process that contributes to successional change; “proximate cause”

Autogenic succession - changes in the environment caused by the species themselves which drive successional change

Allogenic succession – changes in the environment caused by physical processes which are relatively independent of the species

1. Facilitation - a limited group of “early successional species” modify the environment so that it is less suitable for new recruitment of “early successional species” and more suitable for “late successional species”

Possible modes of facilitation: Amelioration of environmental stress (e.g. nurse shrub) Increase in resource availability (nitrogen-fixers)

Mechanism of succession

Three-pathway model of succession (Connell and Slatyer 1977)

Nitrogen fixation: atmospheric nitrogen (N2) is converted to ammonia (NH3) which is available to plants Importance : Fixed nitrogen is required to create nucleotides (DNA, RNA) and amino acids (proteins) Plants with the ability to fix nitrogen can often survive in nutrient poor soil and thus are often early successional species and are thought to facilitate establishment of later successional species

2. Inhibition - early occupants modify the site and inhibit both early and late successional species. Common modes of inhibition: Deep litter Reduction in light availability Reduction in soil nutrients and moisture Changes in soil pH Allelopathy

Black walnut

Three-pathway model of succession (Connell and Slatyer 1977)

Three-pathway model of succession (Connell and Slatyer 1977)

3. Tolerance - plants modify environment so that it becomes less suitable for early successional species but has little or no effect on subsequent recruitment of late successional species

Possible modes of tolerance: Passive tolerance – succession occurs because of differences in the species traits such as growth rate and longevity Active tolerance – ability to endure low resource levels (shade-tolerance)

Generalizations about Connell and Slatyer’s 3-mechanisms of succession model

1. Most successional sequences involve a mixture of mechanisms (facilitation at early stages can be replaced by inhibition at later stages)

2. More than one mechanism may operate simultaneously in same community

3. The same species may be involved in more than one mechanism at different life stages

Classical Successional Theories

Frederic Clements 1916 - Monoclimax theory

Arthur Tansley – Polyclimax theory

Henry Gleason – Individual concept of the plant association

Handout on classical successional theories

Frederic Clements

1874-1945

Clements, 1916 – Monoclimax Theory “The developmental study of vegetation necessarily rests upon the assumption that the unit or climax formation is an organic entity. As an organism the formation arises, grows, matures, and dies.”

Clements, 1916 – Monoclimax Theory “The developmental study of vegetation necessarily rests upon the assumption that the unit or climax formation is an organic entity. As an organism the formation arises, grows, matures, and dies.” Bottom line - plant community is an organism (Organismal View)

Clements, 1916 – Monoclimax Theory Reaction – modification of site by plants that guides or drives succession What mechanism of succession does this sound like? “Reaction continues the development of the organism”- “reaction is final and critical” "succession is due to biotic reactions only, and is always progressive . . . succession being developmental in nature, the process must and can be progressive only”

Clements, 1916 – Monoclimax Theory Reaction – modification of site by plants that guides or drives succession What mechanism of succession does this sound like? “Reaction continues the development of the organism”- “reaction is final and critical” "succession is due to biotic reactions only, and is always progressive . . . succession being developmental in nature, the process must and can be progressive only” Bottom line - Pioneer species colonize site and modify it to make it better for later successional species and worse for itself. Reaction (facilitation) drives succession forward

Clements, 1916 – Monoclimax Theory “A sere is a unit of succession. It comprises the development of a formation from the appearance of the first pioneers through the final or climax stage. Its normal course is from nudation to stabilization.”

Clements, 1916 – Monoclimax Theory “A sere is a unit of succession. It comprises the development of a formation from the appearance of the first pioneers through the final or climax stage. Its normal course is from nudation to stabilization.” Bottom line - Climax stage is deterministic (climate), predictable, repeatable

Climax is in

equilibrium

with climate

Arthur Tansley

1871-1955

English botanist

Tansley, 1935 – Polyclimax Theory “Mature well-integrated plant communities (which I identified with plant associations) had enough of the characters of organisms to be considered as quasi-organism, in the same way that human societies are habitually so considered. The comparison with an organism is not merely a loose analogy but is firmly based. For Clements the plant community is an organism, and he who does not believe it departs from the true faith. There is no need to weary the reader with a list of the points in which the biotic community does not resemble the single animal or plant. They are so obvious and so numerous that the dissent expressed and even the ridicule poured on the proposition that vegetation is an organism are easily understood.”

Tansley, 1935 – Polyclimax Theory “Mature well-integrated plant communities (which I identified with plant associations) had enough of the characters of organisms to be considered as quasi-organism, in the same way that human societies are habitually so considered. The comparison with an organism is not merely a loose analogy but is firmly based. For Clements the plant community is an organism, and he who does not believe it departs from the true faith. There is no need to weary the reader with a list of the points in which the biotic community does not resemble the single animal or plant. They are so obvious and so numerous that the dissent expressed and even the ridicule poured on the proposition that vegetation is an organism are easily understood.” Bottom line - plant communities have some similarities to organisms, so an analogy may be appropriate (quasi-organism) – modification of Clements’ ideas

Tansley, 1935 – Polyclimax Theory “It is the fact that autogenic and allogenic factors are present in all successions”

Tansley, 1935 – Polyclimax Theory “It is the fact that autogenic and allogenic factors are present in all successions” Bottom line - believed succession was not always driven by the plant community (disagreed with Clements)

Tansley, 1935 – Polyclimax Theory “The usual view is that under the " typical " climatic conditions of the region and on the most favorable soils the climatic climax is reached by the succession [Clement’s climax community]; but that on less favorable soils of special character different kinds of stable vegetation are developed and remain in possession of the ground, to all appearance as permanently as the climatic climax. These are called edaphic climaxes, because the differentiating factor is a special soil type. Similarly special local climates determined by topography (i.e., land relief) determine physiographic climaxes. But we may go farther than this and say that the incidence and maintenance of a decisive " biotic factor " such as the continuous grazing of animals may determine a biotic climax. And again we may speak of a fire climax when a region swept by constantly recurrent fires shows a vegetation consisting only of species able to survive under these trying conditions of life... In each case the vegetation appears to be in equilibrium with all the effective factors present, including of course the climatic factors, and the climax is named from the special factor differentiating the vegetation from the climatic climax.”

Tansley, 1935 – Polyclimax Theory “The usual view is that under the " typical " climatic conditions of the region and on the most favorable soils the climatic climax is reached by the succession [Clement’s climax community]; but that on less favorable soils of special character different kinds of stable vegetation are developed and remain in possession of the ground, to all appearance as permanently as the climatic climax. These are called edaphic climaxes, because the differentiating factor is a special soil type. Similarly special local climates determined by topography (i.e., land relief) determine physiographic climaxes. But we may go farther than this and say that the incidence and maintenance of a decisive " biotic factor " such as the continuous grazing of animals may determine a biotic climax. And again we may speak of a fire climax when a region swept by constantly recurrent fires shows a vegetation consisting only of species able to survive under these trying conditions of life... In each case the vegetation appears to be in equilibrium with all the effective factors present, including of course the climatic factors, and the climax is named from the special factor differentiating the vegetation from the climatic climax.” Bottom line - There are many possible climaxes under the same climate conditions (e.g. also depends on soil, topography, animals, disturbances)

Henry Gleason

1882-1975

American ecologist

Gleason, 1939 – Individualistic Concept of the Plant Association “The vegetation unit is a temporary and fluctuating phenomena, dependent in its origin, its structure, and its disappearance on the selective action of the environment and on the nature of the surrounding vegetation. Under this view, the association has no similarity to an organism and is scarcely comparable to a species.”

Gleason, 1939 – Individualistic Concept of the Plant Association “The vegetation unit is a temporary and fluctuating phenomena, dependent in its origin, its structure, and its disappearance on the selective action of the environment and on the nature of the surrounding vegetation. Under this view, the association has no similarity to an organism and is scarcely comparable to a species.” Bottom line - A plant community (association) is not even comparable to an organism (disagrees with Clements and Tansley). Plant associations are temporary, fluctuating. Stochastic processes important.

Gleason, 1939 – Individualistic Concept of the Plant Association “Of the various species which reach one spot of ground, the local environment determines which may live, depending on the individual physiological demands of each species separately.” “In summary it may be stated that environment varies constantly in time and continuously in space; environment selects from all available immigrants those species which constitute the present vegetation, and as a result vegetation varies constantly in time and continuously in space.”

Gleason, 1939 – Individualistic Concept of the Plant Association “Of the various species which reach one spot of ground, the local environment determines which may live, depending on the individual physiological demands of each species separately.” “In summary it may be stated that environment varies constantly in time and continuously in space; environment selects from all available immigrants those species which constitute the present vegetation, and as a result vegetation varies constantly in time and continuously in space.” Bottom line –Site acts as a filter to determine which species that have dispersed to the site can survive (element of chance – which species were able to disperse?). Communities change in space and time.

Gleason, 1939 – Individualistic Concept of the Plant Association “A piece of vegetation which maintains a reasonable degree of homogeneity over an appreciable area and a reasonable permanence over a considerable time may be designated as a unit community. Within such an area and during such a period similarity in environmental selection tends toward similarity in vegetation.” “Since every community varies in structure, and since no two communities are precisely alike, or have genetic or dynamic connection, a precisely logical classification of communities is not possible.”

Gleason, 1939 – Individualistic Concept of the Plant Association “A piece of vegetation which maintains a reasonable degree of homogeneity over an appreciable area and a reasonable permanence over a considerable time may be designated as a unit community. Within such an area and during such a period similarity in environmental selection tends toward similarity in vegetation.” “Since every community varies in structure, and since no two communities are precisely alike, or have genetic or dynamic connection, a precisely logical classification of communities is not possible.” Bottom line – It is possible for communities to persist over a long time period due to stable environmental conditions (no climax/equilibrium language!). Since communities vary in time and space and cannot be classified.

Pollen records show communities change over time, species migrate independently of each other, and they form new and different associations.

Who’s theory does this support?

Although climate is important in determining the types of vegetation,

disturbances can also determine community distribution

Example: Bond et al. (2004) used models to estimate ecological change

in the absence of fires.

Results - Without fire, savannas in Africa will eventually develop into

forests (savannas are fire-mediated ecosystems)

Who’s theory does this disprove?

Successional views - late 1970s – mid-1980s • Clements’ ideas persisted through the 1970s

– Climax/equilibrium model - ignores importance of disturbances

– Deterministic model - left little room for influences of chance (e.g. dispersal)

• “Recent” recognition of role of disturbances – Gleason supported non-equilibrium viewpoint

• Shift from climax/equilibrium to non-equilibrium ideas

National Geographic

Pumice Plain

Pumice Plain

Pumice Plain

Spirit Lake

Spirit Lake

Toutle River

Toutle River

Zhou Quan

Pyroclastic Flow

Pyroclastic Flow

USGS

Blowdown

USGS

Blowdown

Fawn Lake

Blowdown

Will primary or secondary succession occur?

What biota could survive the pycroclastic flow?

Aug 29, 1979 NASA

Sept 24, 1980 NASA

Aug 23, 1981 NASA

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May 22, 1983 NASA

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June 22, 1985 NASA

Sept 11, 1986 NASA

Aug 29, 1987 NASA

Aug 31, 1988 NASA

Sept 3, 1989 NASA

Sept 22, 1990 NASA

Sept 9, 1991 NASA

Aug 26, 1992 NASA

Aug 29, 1993 NASA

Set 1, 1994 NASA

Aug 19, 1995 NASA

Aug 21, 1996 NASA

July 23, 1997 NASA

Aug 27, 1998 NASA

July 29, 1999 NASA

Aug 16, 2000 NASA

Aug 19, 2001 NASA

July 21, 2002 NASA

Aug 25, 2003 NASA

Aug 11, 2004 NASA

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Sept 2, 2006 NASA

Sept 21, 2007 NASA

Sept 7, 2008 NASA

Sept 10, 2009

Aug 12, 2010 NASA

Succession usually studied using a chronosequence (several sites with similar characteristics except time since disturbance – different sites represent different successional stages)

Space for time substitution (data can be collected during a short time period instead of having to study one plot over many hundreds of years)

Wood and del Morals (1987) • Unique opportunity to study primary succession in real time

• Observation – limited invasion of plants onto barren surfaces – Limited by lack of seed rain?

– Limited by lack of tolerance of physical conditions?

– Is facilitation occurring?

Who argued facilitation is

the most important mechanism that drives

succession?

Data collection:

Seed Rain (seed traps)

Determined location of seedlings

– Measured survival over 2 years

– Distance to adults species (seed source)

– Existing plant cover (facilitation?)

Experiment: assess species tolerances to conditions

– Planted seed from 22 subalpine plant species in 264 plots on volcano

– Monitored survival over 2 years

Wood and del Morals (1987)

Estimated seed-dispersal

distance for each species

with distance of seedlings

to nearest adults

How far can most species

disperse?

Pappus

Species have different dispersal capabilities and thus may colonize site at different rates

Some species such as Hieracium gracile and Antennaria were usually present in abundance in the seed traps, but not established in plots. Why not?

Which successional theory do these results support?

Distribution of plots into

percent adult cover

classes (open bars) and

percent 1st year seedlings

in each adult cover class

(black bars; 3 year

means).

0 = no veg cover

7 = high veg cover

Why are there more seedlings in areas with

intermediate adult cover and few in open areas?

Don’t know if tolerance or inhibition were occurring for certain species

since they were not tested.