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Transcript of PowerPoint Presentationfrst351.sites.olt.ubc.ca › files › 2019 › 08 ›...
4/1/2018
1
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.