MODERN FRAMEWORK FOR STUDYING SUCCESSION AND FOREST DYNAMICS

Outline:

1. Some key terms.

2. Overview of a modern “hierarchical patch dynamics paradigm of vegetation dynamics.”

3. Mechanisms vegetation dynamics.

4. Synthesizing mechanisms into models of forest dynamics (forest growth model, Oliver’s model of whole stand replacement, gap processes, regeneration niche, regeneration mode).

Some Key Terms Related to Succession

a. Definitions of Succession or Ecological Succession:

The temporal development of and change in ecosystem structure and function. (Kimmins).

Changes in the types, numbers, and groupings of organisms that occupy an area and concomitant changes in certain features of the physical microenvironment. (Kimmins)

A cumulative change in the species composition of a plant community resulting in a shift in relative dominance of species.

Notes: 1) Succession implies a directional change as opposed to fluctuating conditions around an average condition. Yet, sometimes it is useful to recognize “cyclical succession” and “species alternation.” 2) Long-term (millenial scale) changes in the physical environment such as the shift to a dramatically different climate are usually not considered succession. Yet, fluctuating climate clearly is a determinant of the changes we typically describe as succession.

b. Primary succession occurs on sites that did not previously support a plant cover.

c. Secondary succession occurs on sites that previously supported a plant cover, and some remains and “legacies” of that plant cover persist at the site.

d. Autogenic mechanisms of succession are changes in the environment caused by the plants themselves which drive or trigger successional change.

e. Allogenic mechanisms of succession are changes in the environment caused by physical processes which are relatively independent of the vegetation itself.

f. Biogenic mechanisms of succession refer to a sudden change in the biota which has a major influence on succession (e.g., introduction of a plant disease which removes a major plant species, introduction of an herbivore which significantly affects plant populations).

g. A sere is a sequence of plant communities that develop over the course of succession; seral stages are sometimes recognized. Sere is the “product” of succession whereas the term “succession” is usually used to refer to the process of the change.

Is it appropriate to describe a sere as being “autogenic succession,” “allogenic succession,” or “biogenic succession?”

A Hierarchical Patch Dynamics Paradigm of Vegetation Dynamics

A. Disturbance and the Patch Dynamics Perspective

Disturbance: "any relatively discrete event in time that disrupts ecosystem, community, or population structure and changes resources, substrate availability or the physical environment." (Pickett and White 1985)

Or, "any mechanisms which limit the plant biomass by causing its partial or total destruction." (Grime 1979).

Disturbances are often described as being “exogenous” (originating outside of the plant community or ecosystem) as opposed to “endogenous” (created by the plant community itself.

1. Patch dynamics perspective: the patchiness in vegetation is due to both patchiness in the underlying physical environment and vegetation responses to disturbances.

2. A disturbance regime is a description of the spatial and temporal characteristics of disturbances affecting a particular landscape over a particular time period.

Descriptors (or parameters) of a disturbance regime are for each kind of disturbance (e.g. fire, blowdown, insect outbreaks, etc.):

a. spatial distribution (in relation to environmental gradients and also in terms of patchiness as fine resolution vs. coarse resolution, large patch size vs. small patch size, etc.)

b. frequency (mean number of events per time period)

c. mean return interval (mean number of years between successive events)

d. size of area disturbed per event

e. predictability (inversely related to the variance of the mean return interval; i.e. do events occur regularly or sporadically?

f. rotation period (also called turnover time or disturbance cycle; it is the mean time required to disturb an area equivalent to the size of the study area once.

g. magnitude measured as:

1. intensity (physical force), or as

2. severity (impact on the vegetation)

h. disturbance interactions:

1. synergism (combined effects are greater than the sum of independently occurring disturbance events; e.g. insect outbreak during a drought)

2. timing or sequence of occurrence (e.g. browsing following fire)

B. A Broad-Scale Causal Hierarchy of Vegetation Dynamics

see Pickett et al’s “hierarchy of causes of succession” in Table 17-1 (p. 476) in Kimmins.

Mechanisms of Vegetation Dynamics

a. A mechanism of succession is a process of interaction that contributes to successional change; it is a "proximate cause" of vegetation change.

What is termed a "mechanism" varies according to the level of organization (e.g. landscape/ecosystem, community, and individual plant levels of organization). E.g. competition is a mechanism of succession at the community level, and physiological processes are mechanisms at the level of individual plants.

b. A pathway of succession is a temporal pattern of ecosystem change (i.e. the sequence of plant communities that develop and change during succession).

c. A model of succession is a conceptual construct to explain successional pathway by combining various mechanisms and specifying the relationships among the mechanisms and the various phases or stages of a successional pathway.

Models may be verbal, diagrammatic, or quantitative.

Models may be specific to a system or they may be general.

3. Clements' (1916) mechanisms of successional change:

a. nudation

b. migration

c. ecesis

d. competition

e. reaction

4. Connell and Slatyer's (1977) mechanisms of successional change

They recognize three general mechanisms of successional change (see row C in the diagram):

a. Facilitation-- (similar to Clements' reaction)

amelioration of environmental stress

increase in resource availability

enhanced invasion

b. Tolerance -- (similar to Egler's "Initial Floristic Composition" concept)

passive tolerance (differences in life history traits)

active tolerance (ability to endure low resource levels)

c. Inhibition -- early occupants inhibit both early and late successional species.

Generalizations about Connell and Slatyer mechanisms:

a. More than one mechanism may operate in a sere.

b. More than one mechanism may operate simultaneously in the same stand.

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

d. Discrimination among mechanisms requires understanding of the demographic and/or ecophysiological causes of species turnover.

Some Simple Conceptual Models of Forest Dynamics

Goal: to synthesize the various mechanisms of vegetation dynamics into simple models that help us conceptualize the complex process of forest dynamics

Synthesizing mechanisms into models of forest dynamics (forest growth model, Oliver’s model of whole stand replacement, gap processes, regeneration niche, regeneration mode).

The Forest Growth Cycle (Whitmore)

This stresses the importance of tree-fall gaps and the regeneration niche. The regeneration niche concept stresses the importance of differential survival and growth of juvenile plants in response to fine-scale environmental heterogeneity. At the scale of the adult plant these differences in microsite may not be apparent.

Formalized by T. Whitmore in c. 1976 but based on the ideas of A.S. Watt about "pattern and process" developed prior to the 1940s. Similar to "shifting mosaic" idea of Aubreville and "shifting mosaic steady-state" of Borman and Likens.

Phasic development of forests:

a. The gap phase -- focus on the role of gaps in filtering which species establish in the gap; importance of gap traits.

b. The building phase -- patch of rapid growth and self-thinning.

c. The mature phase -- long period of continued dominance of the site (i.e. inhibition)

Through the forest growth cycle it is possible that "global compositional equilibrium" may be maintained throughout the whole stand, whereas at any point in the stand the species composition may be unstable.

Oliver’s Model of Whole-Stand Replacement (see p. Kimmins p. 401).

a. Stand initiation stage

b. Stem exclusion stage

c. Understory reinitiation stage

d. Old growth stage

In this model the emphasis is on coarse-scale, severe disturbances that initiate even-aged tree populations. Over time, differential tree growth rates and differences in species’ abilities to tolerate understory conditions result in the pattern of stand development. In the structurally complex, multi-age stage of old growth the dominant mechanisms of forest change are quite similar to those described by Whitmore’s Forest Growth Cycle.

The forest growth cycle has been applied both to fine-scale treefall gap dynamics and to "whole-stand replacement" following coarse scale disturbances that destroy entire stands.

The concept of regeneration mode (Veblen 1992) is a useful way of describing broad patterns of tree regeneration in relation to both The Forest Growth Cycle and Oliver’s Model of Whole-Stand Replacement.

Regeneration mode describes a species' behavior in relation to disturbances of different spatial scales. It can be inferred from data on tree age structures and spatial patterns.

There is a continuum of regeneration modes described as:

1. Catastrophic mode

2. Continuous mode

3. Fine-scale gap phase mode (including reorganization and the new recruitment responses)