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Disturbance. Why it Matters in (Landscape) Ecology and Resource (Ecosystem) Management. Definition (Pickett and White 1985).

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Why it Matters in (Landscape) Ecology and Resource (Ecosystem) Management

definition pickett and white 1985
Definition (Pickett and White 1985)
  • “A relatively discrete event in time that disrupts ecosystem, community or population structure and changes resources, substrate availability, or the physical environment.”
ecological importance of disturbance
Ecological Importance of Disturbance
  • “Resets clock”
  • Mixes ages, composition, structure at multiple spatio-temporal scales
  • Provides diverse habitat and PATHCES – important to biodiversity
  • Ecosystems are dynamic – growth, death, replacement. Disturbance is a major change factor
synergy in disturbance
Synergy in Disturbance
  • Not often studied; a very complex set of variables
  • Interactions between disturbance types (and chronic situations) recognized as important to landscape dynamics
  • Frequently mentioned when obvious (e.g. drought effects on fire)
types of disturbance
Types of Disturbance
  • Many different types, operating at many spatio-temporal scales
  • Different types produce divers results (over space and time)
  • Interactions can occur across scales
  • Type of disturbance present in an ecosystem often a function of components, structure of ecosystem as well as physical (climate, topography, etc.) factors.
studying disturbance
Studying Disturbance
  • Disturbance History, Behavior, Ecology
  • The record erasure problem
  • The reconstruction problem
  • The retrospective problem (natural experiments)
  • The replication problem
import of disturbance studies
Import of Disturbance Studies
  • Range of Natural Variation (Ecosystem Management)
  • Description of Important Ecosystem Component
  • Conservation Planning
    • Size of reserve
    • Management of disturbance within/without reserve(s)
    • Understanding of disturbance “behavior” within context of management
the disturbance regime
The Disturbance Regime
  • Method to describe disturbances in ecosystems
  • Several variables:
    • Distribution Area/Size
    • Frequency Magnitude (Intensity or Severity)
    • Rotation
    • Return Interval Synergism
    • Most common descriptors used are frequency (MRI or Rotation), severity and size
  • Major disturbance process in many forests
  • Important in grasslands also
  • Used by humans for millennia
  • Being introduced into tropical forest.
some controls of fire
Some Controls of Fire
  • Fuel Moisture content
  • Fuel Continuity
  • Ignition and heat spread
  • Fire triangle: Fuel, heat, Oxygen
  • Fire behavior triangle: Weather, topography, fuel




fire size
Fire Size
  • Mapped by air photo or satellite imagery
  • Normally the area affected by the fire (severity or intensity not considered)
  • Severity a/o intensity may be mapped within the fire polygon (e.g. scorch height, percent crown scorch, mortality)
  • Big Fires: Yacolt 1902 (239,000 acres, 38 people), Tillamook 1933 (311,000 acres), Coast Range 1849 (1million+? Acres), Biscuit 2002 (499,965 acres)

Tillamook Burn(s)

  • 1933; 311,000 acres (1259 km2)
  • 1939; 190,000 acres (769 km2)
  • 1945; 180,000 acres (730 km2)
  • 1951; 32700 acres (130 km2)

-Biscuit Fire 2002

-500,000 acres (2000 km2)

-Reflective of Present Fire

Management Issues:


Fight or Leave

Exurban Forest


Donato 2002 Science Paper:

Salvage Logging reduces

seedling regen and increases

future fire risk

OSU Dean, USFS and

Timber Industry letter

Issues of Academic Freedom



fire intensity
Fire Intensity
  • Fuel a major factor in intensity (size, shape, arrangement, moisture, continuity)
  • Patchiness of fuel adds to patchiness of fire (intensity)
  • I = 3 (10 FL)2 (in kw/m)
  • Surface, understory, crown fires (<1m, 1 – 3 m, >3m FL)
  • Crown fires release enormous amounts of E and can move very quickly
fire severity
Fire Severity
  • Difficult to measure
  • Frequently Ordinal (L, M, H)
  • When quantified often a percent of crown burned/dead
  • Reconstruction very difficult, not standardized in fire history studies
fire frequency
Fire Frequency
  • MFRI (Mean Fire Return Interval) – the average time between fire events.
    • MFRI = # intervals/total years fire intervals
    • MFRI needs at least 3 fires (2 intervals) to be calculated (although this would be a poor estimate of MFRI)
    • Useful in high frequency regimes (PIPO, etc.)
    • Area or Point calculation?
  • NFR (Natural Fire Rotation) – time needed to burn an area equal to the study area
    • NFR (yrs) = Total time period/% area burned in period
    • (Normally) Multiple fires, can have repeat in certain areas
    • Must define study area (extent)

MFRI: Fires in study area in 1555, 1849, 1871, 1882, 1891, and 1944

Intervals are 1849-1555=294; 1871-1849=22; 1882-1871=11; 1891-1882=9; 1944-1891=53



MFRI=78 years

Multiple Sites or individual site? Record Erasure? Variation?

NFR: Study Area total is 3500 km2 Study Time Frame from 1555 to 1998.

1555 Fire 949km2;1848 Fire 1876 km2; 1871 Fire 647km2; 1882 Fire 441 km2;

1891 Fire 498 km2; 1944 Fire 121 km2 .

949+1876+647+441+498+121=4532 km2 burned in (1998-1555=) 443 years

4532/3500=129% of study area burned in 443 years

NFR= 443/1.29 = 343.4

frequency and severity relationship
Frequency and Severity Relationship
  • Typically, the more frequent, the less severe and vice versa
  • Common assumption for other disturbance processes
  • More frequent fires remove fuels, etc. that cause high severity burns
  • Not completely proven
responses to influences of fire rowe 1983
Responses to/Influences of Fire (Rowe 1983)
  • Invaders: pioneers, short-lived. Fireweed
  • Evaders: long-lived propagules stored in soil. Serotinous cones of lodgepole pine. “Help” fire?
  • Avoiders: no fire adaptations; late-successional or “no-fire” environs. Hemlock, Sitka Spruce.
  • Resisters: Survive low- mid (higher?) intensity fires. Many fire adaptations. W. Larch, PSME, PIPO
  • Endurers: Sprout from root-crown. Oaks, Aspen, Madrone.
human controls of fire
Human Controls of Fire
  • Long history of different approaches to fire
  • Change in landscape patterns of patches and related characteristics has changed fire regime
  • Ex-urban development has made fire management far more complex
  • Major debate has always been around three-fold choices:
    • Control all fires aggressively
    • Prescribed burns and other early controls
    • Let burn
  • High intensity flow of water in river/stream systems
  • Affects bed structure and composition, sediment deposition, inputs to streams.
  • Flood Hydrograph reflects intensity
  • Peak curve affected by humans, especially roads in PNW forests (Jones and Grant 1996).
  • R.O.S.E.s important in PNW floods (Pineapple Express)

Discharge Amount

  • Time Lag
  • Peak Discharge
  • Zone of Flood Risk
  • Normal Discharge
  • Recession Limb
  • Rising Limb

Hours from start of storm

Storm Rainfall event

landslide mass wasting
Landslide/Mass Wasting
  • Important input to streams
  • Synergy with rainfall, soil moisture content
  • Slope angle important (angle of repose)
  • Intensity a measure of volume of scar (inputs)
  • Colluvial deposits eventually make up stream bed (round alluvial) material
  • CWD also delivered
  • Roads affecting rate, amount of inputs
jokulhlaups yokel lowps
Jokulhlaups (Yokel-lowps)
  • Glacial outburst flood
  • Dam from glacier lobe fails, releasing lake behind dam catastrophically.
  • Quick or slow melting; lifting or bursting of glacier
  • Can cause mudflows across sandurs
  • Jokulhlaups of Columbia Basin were huge; came from Missoula Lake (about 40 events?) during Wisconsin era of Glaciation (ended about 10,000 years ago). Largest estimated at 2130 km3 of water
  • Important formation process for much of the landscape of Columbia Basin
  • Had effects in Willamette Valley also
snow avalanche
Snow Avalanche
  • Mass of snow flowing downslope
  • Enormous energies due to speed, mass
  • Controlled by many factors related to snowpack, especially stable slabs on unstable layers (not-bonded)
  • Triggered when stress applied to snowpack
  • Can focus in gulleys or cover large areas
  • Related patterns of vegetation, other ecological factors (e.g. grizzlies)
lahars debris flows
Lahars/Debris Flows
  • Hot or cold mixture of water, rock, mud that flows down a slope of a mountain, generally due to volcano activity
  • Triggers: landslides (rain, earthquake, eruptions), glacial melting/serac failure, eruption.
  • Almost always on volcanoes
  • Magma chamber underneath active volcano moves upwards, released violently. Explosive eruption of lava due to build-up of gases. Viscosity of magma another important factor: Thick  explosive (build-up). Thin  less explosive.
  • Lava, pyroclastic materials, ash, gases
  • Intensity can compare with hydrogen bombs
  • Cinder cone (boom), Shield, Composite,

Cinder cone



  • Trees uprooted during excessive wind events
  • Soil moisture content, topography, soil depth important considerations
  • Synergistic with fire, disease (supplies dead material, weakened trees susceptible to other disturbance)
  • Generally smaller areas than fires, other events
  • Provides small gaps for succession
  • Root-mound topography
  • Can be Isotropic (trees fall in one direction)
pest and disease
Pest and Disease
  • Many different diseases affect ecosystems
  • Often synergistic with other disturbance (weakened/stressed/dead organisms)
  • Insects, fungi, bacteria, viruses
  • Spread (dispersal) related to distribution of “subjects” and related behaviors, ability to move.
invasive species
Invasive Species
  • Introduced species that frequently have enormous impacts on natives
  • Lack predators, other controls
  • Out-compete or prey on existing species
  • Can change nutrient cycles, food web, other important ecological systems
  • Especially destructive on islands or similarly isolated ecosystems.
  • Can also alter disturbance regimes (e.g. cheatgrass)
managing disturbance or disturbing management
Managing disturbance (or disturbing management)
  • On-going experiment
  • Frequent failures (Los Alamos fire)
  • Overall attempt to re-introduce disturbance into ecosystems and try to restore RONV
  • TNC, other conservation groups, feds lead in this area.
  • Jones, J.A. and Grant, G.E. 1996. Long-term stormflow responses to clearcutting and roads in small and large basins, western Cascades, Oregon. Water Resources Research. 32: 959-974.
  • Pickett, S. T. A., and P. S. White. 1985. The ecology of natural disturbance and patch dynamics. Academic Press, Orlando, Florida, USA.
  • Rowe, J.S. 1983. Concepts of fire effects on plant individuals and species. In Wein, R.W. and D.A. Maclean (eds.), The role of fire in northern circumpolar ecosystems: pp. 135-54. New York: Wiley and Sons.