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Classification of

Stand Structure and Development

4-Stage Model of Stand Development• Stand initiation, stem exclusion, understory re-initiation, old-growth

8-Stage Model of Stand Development• Including biological legacies & multiple old-growth processes

Are climax forests the same as old-growth forests?

Stand

initiation

Stem

exclusion

Understory

re-initiation Old-growth

4-Stage Model of Stand Development Structure and Process

(Oliver 1981, Oliver and Larson 1990)

Stand initiating

disturbance

Fine-scale,

tree mortality

I. Stand Initiation

• Colonization and establishment– depending on site availability and conditions

• Duration depends on:– site attributes

– disturbance size and severity

– source of propagules

– life histories of establishing vegetation

– interactions among species

– stochasticity

I. Stand Initiation

• Colonization and

establishment

• Allocation to growth

– competitive advantage

among individuals

• Environmental factors

become limiting

Stand

initiation

Stem

exclusion

Understory

re-initiation Old-growth

4-Stage Model of Stand Development Structure and Process

(Oliver 1981, Oliver and Larson 1990)

Stand initiating

disturbance

Fine-scale,

tree mortality

II. Stem Exclusion

• Growing space occupied

• No new tree establishment

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II. Stem Exclusion

• Growing space occupied

• No new tree establishment

• Resources become limiting

– canopy closes and foliage layer rises

• Intense competition

– self-thinning = “density-dependent” mortality

= “crowding-dependent” mortality

II. Stem Exclusion

Self-Thinning Rule

Log (Number of individuals)

Lo

g (

Bio

ma

ss)

Yoda et al. 1963

B = CN – 1/2

slope = -1/2

II. Stem Exclusion

Self-Thinning Rule

Log (Number of individuals)

Lo

g (

Bio

ma

ss)

Yoda et al. 1963

B = CN – 1/2

slope = -1/2

II. Stem Exclusion

Self-Thinning Rule

Log (Number of individuals)

Lo

g (

Bio

ma

ss)

Yoda et al. 1963

B = CN – 1/2

slope = -1/2

Given points “x” and “y”,

explain two scenarios how crowding

and self-thing affect tree populations

according to the self-thinning rule,

B = CN -1/2

II. Stem Exclusion

Size hierarchy develops

• Trees similar in age (single cohort)

• Variable survival and growth rates

– site conditions, density, genetics

• Size differentiation

• Vertical strata develop

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II. Stem Exclusion

Pioneers = grow quickly, good competitors

suppress other trees

II. Stem Exclusion

Size differentiation + mortality

A

B

C

ove

rsto

ryu

nde

rsto

ry

Stand

initiation

Stem

exclusion

Understory

re-initiation Old-growth

4-Stage Model of Stand Development Structure and Process

(Oliver 1981, Oliver and Larson 1990)

Stand initiating

disturbance

Fine-scale,

tree mortality

III. Understory Reinitiation

Changes in forest structure:

• Canopy structure differentiation

• Overstory tree mortality

• Light reaches ground level

• Herbs, shrubs, seedlings establish

• Second cohort of trees establishes

Structural

diversity of

the canopy:

multiple layers

Seedling and sapling bank in the understory

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III. Understory Reinitiation

A

B

C

ove

rsto

ryu

nde

rsto

ry

D

Stand

initiation

Stem

exclusion

Understory

re-initiation Old-growth

4-Stage Model of Stand Development Structure and Process

(Oliver 1981, Oliver and Larson 1990)

Stand initiating

disturbance

Fine-scale,

tree mortality

IV. Old-Growth StageA

B

C

ove

rsto

ryu

nde

rsto

ry

D

IV. Old-Growth Stage

• Gaps form in canopy

– mortality due to multiple factors

• Understory trees and regeneration

– release and recruit

• New seedlings establish

– multiple cohorts become uneven-aged

• Structural variation

IV. Old

Growth:

gaps form

IV. Old Growth:

regeneration

releases and

establishes

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IV. Old Growth:

structurally

diverse

IV. Transition Old-GrowthA

B

C

ove

rsto

ryu

nde

rsto

ry

D

Regeneration of

canopy trees from

the understory

IV. True Old-GrowthA

B

C

ove

rsto

ryu

nde

rsto

ry

D

Stand

initiation

Stem

exclusion

Understory

re-initiation Old-growth

4-Stage Model of Stand Development Structure and Process

(Oliver 1981, Oliver and Larson 1990)

Stand initiating

disturbance

Fine-scale,

tree mortality

Limitations of the Four-Stage Model:

1. Initial disturbance – severe, no legacy

2. Focus on living trees, assumes no subsequent

disturbances

3. Emphasis on early stages of development

SI + SE + UR = 80-100 years

= 10% of lifespan of Douglas-fir

or western redcedar

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Model 2: Eight-Stages of Development in

Natural Forests with Long-lived Trees

(Franklin et al. 2002)

• Important additional considerations

– severity of initial disturbance

– deadwood and structural legacies

– spatial variation

– complexities and variation in late stages of

development

Cohort

establishment

Biomass

accumulation &

competitive

exclusion Maturation

8-Stage Model of Stand Development

Structure and ProcessOld-growth processes:

Vertical diversification

Horizontal diversification

Loss of pioneer cohort

Disturbance and

biological legacies

Canopy

closure

Fine-scale tree morality,

disturbance and canopy gaps

wide range

of tree sizes

snags

presentdeep canopy

with multiple

layers

large trees

widely spaced

2+ species

present

abundant logs or

coarse woody debris

large-diameter

trees

canopy gaps

Horizontal

diversification

Vertical diversification

Are “climax” forests the same as “old-growth” forests?

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Stand

initiation

Stem

exclusion

Understory

re-initiation Old-growth

“Climax”

4-Stage Model of Stand Development Structure and Process

Species Composition & Succession

Stand initiating

disturbance

Douglas-fir

Western redcedar

Western hemlock

Pacific silver fir

Stand

initiation

Stem

exclusion

Understory

re-initiation Old-growth

“Climax”

Traditional Model of Forest SuccessionCoastal Western Hemlock Zone, BC

Stand initiating

disturbance

Douglas-fir

Western redcedar

Western hemlock

Pacific silver fir

Stand-initiating

disturbance

Revisit the traditional paradigm,

“Climax” forests develop under stable conditions in absence of disturbance.

This assumes disturbance history is a secondary influence and

the impacts decrease with time since the last severe event.

“Old-growth” forests develop as trees establish and recruit

under the influence of fine-scale canopy gaps.

Old-growth is a process driven by disturbance.

Disturbance and

biological legaciesFine-scale tree morality,

disturbance and canopy gaps

Alternatively,

Cohort

establishment

Biomass

accumulation &

competitive

exclusion Maturation

Contemporary Succession & Development Models

Coastal Western Hemlock Zone, BCOld-growth processes:

Vertical diversification

Horizontal diversification

Loss of pioneer cohort

Disturbance and

biological legacies

Canopy

closure

Douglas-fir

Western redcedar

Western hemlock

Pacific silver fir

Why is this subtle difference important?Because theory underpins

forest management and conservation.