FOREST SYNECOLOGY

Definitions and objectives:

1. Synecology is the study of interactions of groups of organisms with their abiotic and biotic environments.

2. Objectives of plant synecology include the study of how plant communities originate, develop, and maintain themselves.

Emphasis in the Forest Synecology section is on:

1. Understanding forest dynamics -- why and how forests change in their structures and species composition (vocabulary, data collection, data analyses, interpretation of data).

2. Critical evaluation of alternative ideas (theories, models) that influence how the evidence of forest dynamics is evaluated.

Note that in this course in contrast to treatment of autecology, in discussing forest synecology differences in philosophical approaches and in interpretations of results often lead to major debates. One of the goals of this course, is to consider the backgrounds to controversial issues so that students can be informed participants in such debates.

TREE POPULATION ECOLOGY

Spatial Arrangement of Populations (Kimmins pp. 367-68):

Random vs. Clustered (clumped, contagious, aggregated) vs. Even (regular, homogeneous) spatial patterns. How can tree spatial patterns be used to infer the origin of the population?

Population Growth (Kimmins pp. 373-376)

What is the difference between geometric (exponential) growth and logistic growth curve?

In qualitative terms, what do the variables “r” and “k” mean in the geometric and logistic equations of population growth?

Plant Population Ecology

What is the unit of a population?

Genet = a unit (plant) in the population arising from a seed; from sexual reproduction.

Ramet = a unit (plant) in the population arising from vegetative reproduction; ramets (even after separated from parent plants) are genetically identical. Group of ramets from a single parent = a clone.

Population concept: simple for discrete individuals, but complicated for clones (e.g. aspen).

A plant can be conceptualized as a population of growth modules (e.g., stem, branches, leaves, roots).

1. Tree seedling recruitment/establishment

a. Seed rain

b. Seed bank = seed pool

c. Bud bank

d. Seedling bank (“advance regeneration”).

e. Seedling recruitment and “safe sites”--

Environment acts as a “filter”-- therefore, absence of seedlings may reflect lack of seed rain or lack of “safe sites”.

2. Seedling survival/tree population demography

Cohort = all the population units that originate in the same time period. Time period may be a single year or defined to be multiple years (the latter is typical in the study of the demography of long-lived trees).

A. Cohort life table = a “fixed” cohort life table (a dynamic life table).

Recruitment rate and mortality rate can be measured by monitoring cohorts in long-term studies. Result is a cohort life table. Age (x) refers to a time interval (e.g., number of days , one year, multiple years). “Age-specific mortality rate” = q_x = the % of the population dying during a particular time interval.

The number of individuals that die during the age interval = d_x. The number surviving to the beginning of the next age interval = N_x.

The proportion of the population surviving at the beginning of the age interval is the survivorship (l_x).

B. Survivorship curves

Deevey’s (1947) classification is:

Type I: low mortality, then collapse of the population.

Type II: constant percentage of survivors die per unit of time.

Type III: high death rate of juveniles followed by low and relatively constant mortality rate.

Note: Kimmins in Fig. 14.8 has added a new Type II curve (constant absolute number of deaths); Deevey’s Type II = Kimmins’ Type III (constant %) and Deevey’s Type III = Kimmins’ Type IV (declining % mortality). Ignore Kimmins’ classification.

“Stable age distribution” = constant shape to the survivorship curve (but total number of individuals may vary); the population may be steadily declining or increasing.

Constant population size means natality = mortality, and has a “stationary age distribution” (i.e., neither growing nor declining). This is equivalent to the surviroship curve for a population that is neither growing nor decreasing and that is at equilibrium with its environment.

C. Static life tables (stationary age distribution; instantaneous life table)

These are “time-specific life tables” because they are based on determining plant ages at one point in time rather than following cohorts. Tree population age structures are sometimes assumed to be true cohort life tables from which survivorship curves can be constructed and age-specific mortality rates derived.

What are the assumptions required to derive tree mortality rates from static age structure data?

D. Density-Independent Regulation (mortality) and Density-Dependent Regulation

Is there a predictable way that self-thinning is expressed in terms of stand density and tree size?

E. Oliver’s simple model of stand development.

Stand initiation

Stem exclusion

Understory reinitiation

Old growth

F. Are there character syndromes that allow predictions of how a tree species will respond to resource availability and disturbance?

r- AND K-SELECTION

1. Continuum.

r-selection: emphasizes rapid (geometric) population growth instead of competitive ability; typical of patchy and unpredictable habitats (disturbance). Synonyms: opportunists, pioneers, weedy spp.

K-selection: emphasizes competitive ability rather than rapid population growth; typical of stable, uniform and predictable habitats (less disturbance). Synonyms: equilibrium spp., climax spp.

2. Application to single or range of life forms (e.g. just consider trees or compare trees, shrubs, herbs, epiphytes, etc).

3. Application to trees:

Trait habitat competitive ability mortality pests and pathogens dispersal growth of individuals population growth sexual maturity colonizing ability population size life span size of individual periodicity of reproduction vegetative reproduction

r-strategist unstable, unpredictable low density independent, often catastrophic not resistant highly effective rapid rapid earlier (precocious) high highly variable but usually much less than carrying capacity short smaller often highly periodic with large % of energy allocated to sexual reproduction usually low

K-strategist stable, predictable high density dependent resistant ineffective, slow slow slow later low often near the carrying capacity long larger usually regular but allocating small % of energy to sexual reproduction usually high