The Importance of Disturbances in Earth System Dynamics

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The Importance of Disturbances in Earth System
Dynamics

• Mike Apps,
• mapps_at_nrcan.gc.caCanadian Forest Service

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Burgess Shales, 100 km West of Banff
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200 km West of Banff
Mountain Pine Beetle outbreak
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Disturbances system phenomena that occur at all
scales
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Disturbances as Discrete Events

• Common characteristics
• abrupt and locally catastrophic event
• evoke both immediate and long-term responses
• highly non-linear (not easily approximated by low
  order power series)

Delayed response Smooth change
Immediate response Discontinuous change
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Disturbances as Discrete Events

• Common characteristics
• abrupt and locally catastrophic event
• evoke both immediate and long-term responses
• highly non-linear (not easily approximated by low
  order power series)

• scale dependent (at lower resolution we treat as
  noise, at higher resolution look like smooth
  transitions)
• Involve factors both endogenous and exogenous
• Not closed system (involves critical transfers of
  mass, energy or information across boundaries)
• despite importance and ubiquitous nature, we
  often ignore or treat as surprises

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Highly nonlinear dynamics

• Equilibrium approaches are inadequate
• Disturbance causes an abrupt reorganization of
  system components
• Reorganizes, redistributes resources
• Begins a new development cycle (restarts
  system clock)
• Restructures population of objects in the system
• Provides opportunity formode change of system
  dynamics

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Forest Ecosystem Example 1
Redistribution of resources carbon, light,
nutrients Reorganizaiton - system components ,
clock reset (stand age) New population - begin a
new cycle of stand development Note
potential for change in system dynamics
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Forest Ecosystem Example 1
The new state may be a different mode
disturbance type disturbance frequency
cumulative changes in climate/resources (e.g.,N,
H2O)
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Disturbance and the adaptive cycle

• Birth-growth-death-renewal, an adaptive cycle,
  repetition with difference (a concept borrowed
  from Darwin)
• Stylized representation in system dynamics
  (Holling 1986)

Mature resources locked up in successful
individuals
Regeneration new individuals struggle to get
started, reorganization of system components
disturbance
Competitive phase- with individuals attempting to
capture as much of resource as they can
survival of the fittest
Resources released as mature individuals disappear

• Disturbances seen as agents of creative
  destruction

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Disturbance and Resilience

• Subtle shifts in environment easily incorporated
  in the adaptive cycle

Successive cycles may not repeat exactly e.g.
changed conditions, more or less productive
If accumulated changes are large enough, the
disturbance may trigger move to a new state
-e.g., a new vegetation assemblage
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Self Organizing Systems

• New spatial patterns emerge as new quasi stable
  populations form and dissipate under new
  conditions, triggered by disturbances
• Examples
• Forest mosaic of steppe and treed stands
• Gyres in ocean systems (?)
• Human population centres

Self organized criticality is a new way of
viewing nature perpetually out-of-balance, but
organized in a poised state Per Bak, How Nature
Works, 1996
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Extension across scales

• New assemblages form under changed environmental
  conditions and, in turn, modify their environment
  through competition
• Propogates influence up and down scales of time
  and space

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Canadian Boreal Forests

• Example role of disturbances in the dynamics of
  boreal forests, using ecosystem carbon as the
  indicator

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Major disturbance types

• Forest inventory gt90 is even-aged
• i.e. stands recovering from stand-replacing
  disturbances within the age of the dominant trees
• Stand replacing (crown) fire
• 97 of the area burned is in the 3 of the fires
  that exceed 200 ha in extent

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Disturbances vary temporally
10
8
ha)
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Area (Million
4
2
0
1920
1940
1960
1980
2000
ClearCut
Fire
Insects
Total
Kurz and Apps 1999
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Disturbances vary spatially
New trends emerging? Changing precipitation
regimes? Changing management regimes? New
disturbance regimes?
different precipitation regimes different
management regimes different disturbance regimes
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Forest Carbon Stocks DynamicsStand level
Change in C stock is removal from or addition to
atmosphere
Significant time before C released during/after
disturbance is recaptured
Biomass detritus soils
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Forest demographics

• Disturbance regimes determine the age-class
  distribution in forests
• Age-class distribution is a record of the past
  birth and death statistics a snapshot of
  disturbance history of the forest

• Age-class distribution useful for scaling stand
  level C dynamics to the whole forest

• Most useful for forest composed of even-aged
  stands (most of Canada)

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Forest Carbon Budget Sum over all stands in
landscape

• Sinks or sources when changes occur to
• age-class distribution
• or
• C accumulation curves

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older forests have more C so age distribution is
critical .....
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Changing disturbance regime cause sources or sinks
Source
Sink
Older forests contain more carbon.
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What do the changes in disturbances in Canadas
forests do to Carbon?
Note apparent increase after 1970
10
8
ha)
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Area (Million
4
2
0
1920
1940
1960
1980
2000
ClearCut
Fire
Insects
Total
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Canadian Forest C fluxes 1920 - 1995
Note change after 1970
400
Sink
300
200
Tg C / yr
100
0
-100
Source
-200
1920
1940
1960
1980
2000
Variable Temp
Constant Temp
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Lessons learned

• Disturbances fluctuate over time not
  necessarily periodic
• Long-term consequences, legacy effects
• system memory, historical contingencies
• Within a geographical region, synchronicity
  between events is important
• If steady average, effects average out (as much
  die as are reborn)
• If synchronous change, potential for resonances
  and enhanced effect on dynamics
• Factors inducing synchronicity
• Nature of disturbance e.g. contagious processes
  such as fire, insects, logging, storm tracks
• Mesoscale phenomena e.g., weather, circulation
  patterns, pollution patterns .
• Human population demographics
• Climate change.
• i.e., the forces of global change

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Challenges for Future

• What factors influence the disturbance regime?
• Synchronies in time and space
• Human drivers
• Human modifications of natural regimes
• How do global change forces modify the system
  susceptibility to disturbance?
• Alteration of system response
• Stresses and memory of past events

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Challenges for Future

• How can we incorporate these disturbances into
  our projective models?
• At regional to global scales
• Impacts on goods and services
• Guide our management and mitigation strategies
• How have disturbances change over time at all
  scales of interest?
• Spatial extent and frequency
• Intensity, and nature of disturbance
• Data to guide our development of projective
  models

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Learn From the Past?
Predicting the future solely as an
extrapolation of the past is like driving on a
mountain road at night using only the rear view
mirror Kimmins 1989
Yes - historical changes leave a legacy that can
dominate the present dynamics!
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Mapps_at_nrcan.gc.ca
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