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DISTURBANCE
I.
What is disturbance? Disturbance has several definitions
A.
Demographic definition-event that results in the death of one or more dominant individuals
B.
Community definition-event that results in the alteration of composition or structure of vegetation
C.
Ecosystem definition-event that results in the release of a previously unused resource.
D.
All disturbances are characterized by their disturbance regime, which includes
1.
Intensity
2.
Frequency or return interval
3.
Predictability
4.
Spatial extent
5.
Seasonal timing
6.
Synergism with other disturbance agents
E.
Common disturbance agents
1.
Fire
a.
Releases nutrients stored in biomass
b.
Species killed depends on the intensity of fire
(1)
Canopy fire
(a)
High temp; widespread mortality, even of mature trees
(b)
Western U.S.
(2)
Surface fire
(a)
Lower temp; shrubs, seedlings and saplings killed, mature trees
survive.
(b)
Southeastern U.S.
2.
Flooding
a.
Annual rhythm to flood cycle
(1)
Bankfull discharge (flooding) once or twice a year
(2)
Sediments deposited on floodplain fertilize soil
(3)
Flooding erodes portions of the channel and destroys some riparian
vegetation
(4)
However, some riparian species are dependent upon floods and changes
in flood regime for establishment. Low energy floods followed by a long
dry interval needed for reestablishment of cypress.
3.
Treefall and windthrow
a.
Structure of vegetation is altered
(1)
Canopy is opened; light is available in understory
(2)
Center of gravity of tree is higher as tree grows
b.
Occurs on several spatial scales
(1)
Individual treefall
(a)
Pit and mound topography
(2)
Hurricane windthrow
(a)
80% of forest on Puerto Rico has been disturbed by hurricanes in
the past 100 years
4.
Disease and pest infestation
a.
Disturbance effects specific species
b.
Strongly alters composition and structure over large area
c.
Example: Chestnut blight
(1)
By 1950 in North America had spread to 23 states
(2)
Maples, oaks, and beeches have replaced oak in the canopy.
5.
Geomorphic disturbance
a.
Mass movements under the influence of gravity and water
b.
Overwash disturbance on barrier islands and sandy coastlines
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II.
How did the ideas of disturbance develop?
Early roots of disturbance patch dynamics were in the criticisms of the facilitation model of
A.
Clements. Critics held that climax is rare. Disturbance is frequent; not enough time for climax to
develop. Chance disturbance events constantly interrupt and redirect any orderly sequence of
succession. Many communities cannot propagate themselves without disturbance, particularly
fire.
B.
There existed a need for formalization of disturbance concepts and ideas. There was no one
unifying conceptual or theoretical framework even though it was widely recognized that
disturbance was ubiquitous in nature.
C.
The concept of gap-phase dynamics (Watt)
1.
Gap phase dynamics incorporated disturbance as a normal component of vegetation
dynamics
a.
Gaps refer to a patch in a forest created by the death of a canopy tree. These
gaps become localized sites of regeneration and subsequent growth.
b.
Any one point in a forest goes through a more-or-less predictable cycle from
open gap, to young tree, to mature tree, and then when the tree dies the cycle
repeats. His insight was, if one were to look at a large area of forest, various
gap-scale elements would be undergoing similar cycles of gap dynamics, but
independently.
c.
Averaged over a large area, there would be some elements in all stages of
succession represented (somewhere) at any given time.
d.
This facilitated that idea of a dynamic equilibrium. Cyclical replacement in a
particular spot, averaged over the whole forest, would maintain the same basic
type of forest.
III.
Disturbance patch dynamics
A.
Formal ideas about disturbance were made widely known with the publication of “The Ecology
of Natural Disturbance and Patch Dynamics” by S.T.A. Pickett and P.S. White in 1985. Some
general tenets of disturbance patch dynamics:
1.
Disturbances are common in nature. Disturbances generate patchiness, or spatial
heterogeneity.
2.
The extent to which a disturbance has been historically prevalent for an area, the
degree to which it is predictable from the impacted organism(s) point of view,
determines species responses.
a.
If a disturbance occurs that has been historically prevalent, species are more
likely to have evolved responses to the disturbance for which they are adapted.
b.
If disturbance is outside what is normally experienced, then the species response
will be less predictable, and more dependent upon the ecological interactions
among the species.
A corollary of this is that the abundance and distribution of species reflects the
c.
sorting effects of the disturbance regime.
3.
Some parts of the landscape are more vulnerable to one type of landscape than
another. Different habitats can moderate or intensify the magnitude of a disturbance
event.
4.
Disturbances are often synergetic, with the occurrence of one linked to another
5.
Disturbance and diversity are intertwined
a.
Disturbance is an opportunity. Disturbance prevents competitive exclusion. It
creates the environmental heterogeneity needed to allow other organisms to
establish.
IV.
Disturbance patch dynamics has influenced many areas of ecology and resource management in four
major ways:
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A.
B.
C.
D.
Disturbance is not always a disaster.
1.
The idea that disturbance is a disaster for nature is misleading. Disturbance is the key
to coexistence and renewal in many communities. Recognition of the importance of
disturbance led to a paradigm shift in resource management. Disturbance, particularly
forest fires, were no longer seen as ‘bad”, but instead necessary.
2.
Inhibition model of succession for frequently disturbed landscapes. No species
replacement—disturbance triggers change in species composition instead of species
sorting. One community dominates between disturbances.
Resource extraction began to be modeled after natural disturbances
1.
Patch dynamics used as a model for timber harvesting. By mimicking the gap-phase
patterns of disturbance, forest is kept in a steady state. Harvesting is rotated over an
area to allow successional recovery.
Contributed to a more non-equilibrium view of nature and succession
1.
Disturbance regimes change, such that the idea of a plant community in balance with its
environment is sometimes simplistic.
2.
There are multiple equilibria that develop in response to the natural, historically prevalent
disturbance regime and to infrequent, historically unprecedented disturbance events.
The vegetation dynamics model of Tom Vale (1982) captures this non-equilibrium view
Because disturbance is common, it is unrealistic to try to manage land with the intent of
keeping it the same.
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FIRE ECOLOGY AND POLICY
1.
Most terrestrial ecosystems in North America are influenced by fire to some extent. Be able to identify
locations for these ecosystems on a map, state the predominant forest tree species, the type of fire (where
given), and the fire return interval (how many years transpire on average between fires)
a.
Fire-dependent ecosystems of North America
(http://www.nifc.gov/preved/comm_guide/wildfire/fire_6.html)
i.
Midwest tallgrass prairie
ii.
Southwestern California Chaparral
iii.
Ponderosa Pine in the Southwest and Intermountain West
iv.
Lodgepole Pine Communities of the Rocky Mountains
v.
Longleaf Pine Communities of the SE US.
vi.
Jack Pine Communities of the Great Lakes Region and Canada
vii.
Alaska's Boreal Forest and Tundra
2.
Types of wildfires
a.
Canopy/crown
i.
Dominant vegetation removed
ii.
High temperature fire
iii.
Widespread mortality
iv.
Large spatial extent
v.
Strongly lined to fire weather
vi.
Example: Yellowstone Fires
b.
Ground
i.
Burns organic soils
ii.
Creates new seedbed
iii.
No evident flame front
iv.
Slow moving
v.
Example: Okeefenokee ground fires—subsurface organic matter burns
c.
Surface
i.
Burn through understory
ii.
Grasses regenerate because growth tissue is underground
iii.
Seedling and saplings killed; adult trees spared
iv.
Lower temperatures than canopy/crown fires
v.
Example: Longleaf pine forests
(1)
2-5 year fire frequency (summer)
3.
Longleaf pine is adapted to fire and requires it to perpetuate itself in the landscape.
a.
Historically, fires were promoted by inhabitants
i.
Native American Indian practice of burning increased cover of longleaf. Some scholars believe
that these burning practices allowed bison to range must further east in the past, as far as
Georgia and the Florida panhandle. Early “cracker” settlers also practiced a form of ecosystem
management whereby forests were burned frequently to clear undesirable undergrowth and
insects, and to promote game. Later inhabitants suppressed fires in the interest of protecting
forest resources and property.
b.
Morphological and physiological adaptations
i.
Thick bark
ii.
Cone serotiny: cones require fire to open up and drop seeds
iii.
Punctuated growth
(1)
Grass stage (10-25 years).
(a)
Tree resembles a grass, low and close to the ground, thus protecting it from fire.
Meristematic (growth) tissue protected by fringe of needles
(b)
Most of the photosynthate made by the longleaf is used to build up its roots
(2)
Rapid height growth stage
(a)
Sudden, rapid growth within next 3-4 years extends growth tissues above fire
front. Tree becomes tall and slender.
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iv.
c.
d.
e.
Abundant resin production
(1)
When species grow in monotypic stands as a result of fire (as with longleaf pine)
diseases can spread rapidly and result in high mortality. As an adaptive consequence,
longleaf produces abundant resin (sap), that contains chemicals to thwart fungal and
insect attack. Longleaf pine, for example, is less susceptible to mortality from the action
of the southern pine beetle. Sap blocks the insect from reaching sensitive growth tissue
underneath the bark. Red cockaded woodpecker makes nesting cavities in longleaf that
secrete sap which deters tree-climbing predators.
These adaptations allow the longleaf to thrive in a high fire environment. Pines are able to replace
themselves in the canopy without an intervening successional stage. Resistance to frequent fire allows
some trees to reach ages of 450 years.
With fire suppression, longleaf can be lost from the landscape. Hardwoods like turkey oak and laurel
oak, and other pines less adapted to fire (slash and loblolly) replace longleaf pine. Flammable
wiregrass understory may be lost as well.
Difficult to return to initial conditions via restoration. Growing season prescribed fires (March thru June)
are needed, along with wiregrass seed collection and reseeding, longleaf plantings, and often manual
removal of hardwoods.
4.
Effects of fires
a.
Can reduce cover of tree species other than pines.
i.
Some hardwoods (like oaks) that are also fire-dependent. Fire opens the canopy and allows
high light conditions to develop that are needed by species that are shade intolerant when
seedlings.
b.
Alters nutrient cycling
i.
Burns litter where there is slow decomposition, thereby releasing nutrients.
ii.
Fire also releases nutrients trapped in tree biomass. Nutrients returned to the soil and new
growth initiated
iii.
Fire may leads to loss of some nutrients, particularly nitrogen, as it is volatilized and lost to the
atmosphere
c.
Species regeneration
i.
Fire provides a mineralized (devoid of organic matter) soil some species prefer for
regeneration.
ii.
Fire opens canopy and promotes sunnier conditions in the understory to enhance regeneration
of shade intolerant species
iii.
Enhances dispersal through cone serotiny in pines
iv.
Stimulates vegetative (clonal) regeneration in some species. New growth may initiate from
underground tissues of trees not destroyed by fire.
d.
Creates spatial (environmental) heterogeneity
i.
Allows coexistence of early and late successional species
ii.
Reduces spread of plant pathogens
e.
Alters stand age structure
i.
Uneven-aged stand structure: lots of young or old trees indicating that a large stand-clearing
fire has not occurred recently
ii.
Even-aged stand structure: trees all have the same age, indicating that establishment took
place after a large fire
f.
Maintains community boundaries (prairie: forest).
5.
How fire suppression influence vegetation cover
a.
Stem density increases----there are more stems (trunks) of trees
b.
Structure of fuel load becomes more vertical----an abundance of stems prevents some trees from
falling all the way to the ground. They end up leaning on each other after they die from shading each
other out.. As a result small surface fires may become crown fires—flames climb up these dead
trunks, which then become known as “fire ladders”.
c.
Fuel load increases
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d.
6.
Scale of fire disturbance increases: more fuel increases the likelihood that a fire may burn hotter and
spread to a larger area.
A brief and generalized history of fire in North America
a.
Before human settlement, high lightning flash frequencies enhanced naturally occurring forest fires.
Vegetation and fire have evolved for millenia.
i.
Timing of lightning ignition with rainfall determines frequency of fires. In the southeastern US,
thunderstorms are often accompanied by high rainfalls. Rain is less plentiful in the Southwest,
hence lightning strikes are more effective as an ignition source.
b.
It has been hypothesized that the extinction of megafauna during the Pleistocene, as a result of the
hunting skills of hominids, may have led to decreased grazing by herbivores, in turn creating larger
fuel buildup for fires.
c.
Burning by native Americans common
i.
Removal of predators/noxious insects and clearing of the land to make it safe
ii.
Fire-driven hunting
iii.
Farming—clearing land for agriculture with fire
d.
European explorers arrived to gaze upon a fire-influenced landscape that was open and parklike.
However, settling of continent corresponded with a lowering of fire frequencies. While logging and
deforestation was common in some locations, in others, the lack of fire from Native Americans led to
incursions of shrubs and trees into land that was once kept open by fire.
e.
While some of the practices of the native Americans were continued by European settlers, they were
never as ubiquitous on the landscape. Fires by the early part of the 20th century were by far seen as
destructive, destroying important forest resources, and a period if active suppression was initiated.
f.
By 1960's fire policies began to account for the fact that forests need to burn and suppression policies
were relaxed. ‘Prescribed’ natural fires were adopted as a management tool.
g.
However, this policy has become more complex after several decades of prescribed burns and letburn policies. Fuel buildup after years of suppression, combined with a renewed emphasis on
prescribed burning, has lead to the escape of controlled burns into populated areas near national
forests. Recent wildfires have burned large areas of national forests and adjoining communities in the
Western US and even in parts of the humid eastern states. More people live at the urban-wildland
interface, bringing the potential for natural fires that become uncontrollable, or for prescribed fires that
escape into developed areas. Recent policy seeks to thin forests more aggressively so as to reduce
fuels back to a level where controlled burns may be safer and less likely to lead to loss of timber and
property.
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