Overview of Historical Range of Variability

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Overview of Historical Range of Variability

• How HRV is determined
• How HRV is used in management, especially in
  restoration
• Limitations

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• "Ecosystems are not defined so much by the
  objects they contain as by the processes that
  regulate them" -- Christensen et al. 1989
• Human-generated changes must be constrained
  because nature has functional, historical and
  evolutionary limits. Nature has a range of ways
  to be, but there is a limit to those ways, and
  therefore, human changes must be within those
  limits. -- Christensen et al. 1996
•  
• Management should strive to retain critical
  types and ranges of natural variation in resource
  systems to maintain their resiliency. -- Holling
  and Meffe 1996

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Developed By Managers

• Searching for a legally defensible approach to
  conservation of biological diversity
• Premised on current ecological understanding
• Now central to sustainability, ecological
  integrity, and ecological restoration
• Now included in USFS planning regulations

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Ponderosa Pine Restoration Example
A) Presettlement StructureLive crowns (shaded)
and dead tree evidence (circle with X). On this
plot, total forest density was 72 trees/ha, 47
pines/ha and 25 oaks/ha, in 1870.
B) Forest Structure 1995Live tree crowns are
shaded on this demonstration plot. On this plot
in 1995 there were 1396 live trees/ha, 1223
pines/ha and 173 oaks/ha.
C) Restoration TreatmentThe restoration
treatment marked on this plot conserves the
patchy presettlement forest structure. All
presettlement trees and some replacement trees
were retained. The treated plot has 104 trees/ha,
57 pines/ha, and 47 oaks/ha conserving present
oak clumps.
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Landscape Planning Contrast Between Initial
Planned Management Areas Vs. Landscape-planned
Management Areas
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Terminology is Confusing

• Related terms
• Historical range of variability
• Natural variability
• Natural range of variation
• What do these terms have in common?

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Natural Variability

• The ecological conditions and their variability
  over space and time relatively unaffected by
  peoplewithin a period of time and geographical
  area appropriate to an expressed goal.
• Related terms historical range of variability,
  reference variability.
• -- Landres et al. 1999.

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HRV in areal extent of open old ponderosa pine
forests in the Idaho Batholith
Morgan et al. 1998 Unpublished data
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Premises
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Premises for NV in management

• Disturbance structures ecosystems
• Variability is important
• Anthropogenic change decreases viability
• Fewer subsidies to systems within bounds
• Past is clue to the future
• Coarse filter
• Reference
• Context and guidance

Landres et al. 1999
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HRV and Desired Future Conditions
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• Bowl, ball and plate demonstration video
  available on web site

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Extreme Events and Desired Future Conditions

• Management variability
• Maintaining variability of disturbances within
  tolerable limits

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Identify social and economic concerns
Quantify existing conditions
HRV is only part of the decision-making process
Implement required actions
Determine desired future conditions
Develop Ecosystem Diversity Matrix
Delineate landscape
Coarse Filter Adequate Ecological Representation
Monitor, evaluate, and adjust
Describe Historical Disturbance Regimes
Check with species assessments
Haufler et al. 1996
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Sources of data
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Sources of Data

• Natural archives
• Tree rings (fire scars, climate, defoliators)
• Pollen, macrofossils, and charcoal from bog and
  lake sediments and pack rat middens
• Soil phytoliths
• Human archives
• Old maps, repeat photographs, journals, long-term
  and early data
• Models and expert opinions

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The Sources, Time Frames, and Spatial Resolution
of Available Data Vary Greatly
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Dated Fire Scars in Tree Rings

• The fire scars in this partial cross-section of
  ponderosa pine tree have been dated. The fire
  interval between fires can be determined by
  counting the annual rings between scars. In fire
  history studies, this is done for many trees

Photograph by T.W. Swetnam
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Charcoal and Pollen in Lake Sediments and Bogs
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Comparing Historical and Current Aerial
Photographs

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Simulation Models
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Limitations/challenges

• Extrapolating from points to landscapes
• HRV varies with scale
• Integrating space AND time
• Long time series for large areas
• Models of landscape change
• Understanding complex systems
• Changing climate

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Utility
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Natural variability is useful for...

• Evaluating and assessing change
• Establishing goals for ecological restoration
• Determining desired future conditions
• Setting priorities for action
• Understanding and illustrating change

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NV is less useful when

• Focus is on an individual species
• Historical patterns and processes are socially
  unacceptable
• Risk and uncertainty are high
• Biophysical conditions have changed greatly

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Utility depends on...

• Social and ecological context
• Issues
• Knowledge and understanding