Download Disturbance and succession

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Disturbance and succession
Sources of natural disturbance
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Continental drift…… millions of years
Climate change………hundreds to thousands yrs
Volcanic……………..decades to hundreds yrs
Disease epidemics…..decades
Fire………………… annual to centuries
Freezes…………….. annual to decadal
Storm………………. monthly to decadal
Fire
• Huge wildfires are
very destructive
– Crown fires
– Soil organics
burn
– C ignites at 500
oC these
temperatures can
penetrate 1m
into soil.
– Fire present for
hours.
– Plant rootstocks
killed
– Severe erosion
follows
– Large area
burned, reduces
source for
recolonization.
Destructive fires
Most fires are not so severe
• Light-moderate fires
– Surface fires
– High surface temperature,
but at 2cm depth <50 oC.
Fire passes in 2-5 minutes.
– Many taller plants survive
though scorched.
– Animals either flee or
survive in burrows.
– Fire uneven, often leaves
unburned/lightly burned
patches
– Roots remain, soil held.
Everglades fire
burning above the water
Physical effects of fire
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Scorching of leaves reduces productivity
Reduction of live biomass
Reduction of surface organics
Ashing and oxidation of soil chemicals
– More soluble…more likely to be leached (solution).
– Loose ash easily washed away (suspension).
– Volatilization …loss of chemicals in smoke…70%
of nitrogen.
Fire can shape
flora and hence
fauna
• Fire sensitive species are
killed outright by fire.
• Introduction of fire can
change dominant species.
• If fire is occasional leads
to secondary succession.
• If fire is frequent get a
pyrophillic community
adapted to fire.
Adaptations for
fire tolerance
• Corky bark that
insulates from heat
• Root resprouting
Epicormic buds
• Epicormic buds…..buds
beneath the bark that
sprout following fire.
• Epicormic sprouting is a
sign of general stress on
a plant.
Australian “grass trees” flower as
a result of exposure to smoke
• Synchronizes flowering, so
that insects pollinate
flowers.
• Burned area offers suitable
habitat for seed to become
established.
• Note grass trees themselves
may or may not have
burned (these ones did).
Natural fire regimes altered by:
• Roads acting as fire
breaks.
• Fire suppression
• Prescriptive burns
What is the relationship between
the variables here?
Do the same
observations apply
to this reef?
Intermediate disturbance
Hypothesis (Connell 1978)
• At very low disturbance
competitive exclusion
limits diversity.
• At very high
disturbance harsh
conditions limit
diversity.
• Highest diversity at
moderate disturbance
regimes
Fire is also bad if too frequent
• S. Africa: Land on right burned too often and
rare Proteas (shrubs) are missing
Hubbard Brook
• Long term ecological
research (LTER) site.
• Experimental
deforestation,
succession
suppression and burns
of entire stream
catchments.
• Effects measured in
stream water.
Net primary productivity and
succession
• NPP = GPPrespiration
• NPP maximal in
immature stages
of succession.
• Mature phase
has senescent
trees
Hubbard Brook
fire results
Nitrate
• Nitrate is an important
nutrient.
• Nitrate and sulfate are
strong acid anions, and
their leaching acidifies
the streams, but will
Sulfate
leave the soil more
basic.
NO3 and SO4 content of streamwater
following fires in their catchment
Fire increases nutrient leaching
• Nutrient flow from a
burned area is elevated
for 2-6 yrs post fire.
• Oxidation of chemicals
1-2 yr after
increases solubility,
decreases acidity
(increase of 2 pH units)
3-7 yr after
• Sulfate concentrations in
ELA streams: mean Nutrient
monthly SO4
Pre-fire
load
concentrations in the
Northwest stream before
and after the 1980 fire.
Seasonal
variability
• Nitrate stored
in biomass
and released
in the spring
melt.
• Loss of
biomass post
fire.
Hubbard brook
clearcut (no fire)
• Slower release of
nitrate than following
fire.
• Why? Can you explain
these patterns?
• Watershed 2 is
clearcut
• Watershed 6 is control
Fire summary
• Disturbance depends on intensity of fire.
• Frequent low intensity fires will shape
composition of community to include fire
tolerant species.
• All effects of fire on the watershed may not
be immediately apparent.
Not just chemistry also basic
biological questions
• Has the fire promoted reproduction?
• Has the fire promoted growth?
• Has the fire increased or decreased the
presence of insects, reptiles or mammals?
• Has the fire stressed the plants, or were they
totally fire adapted?
• Are the impacts on wetlands that burn as
severe as on uplands (or more so?)?
Soil temperature in a tropical
forest
• Note how large
clearing radically
increases soil
temperature.
• Dry conditions lead to
oxidation of surface
litter and release of
chemicals. Leaching a
threat.
Primary Succession
Colonization of new areas
Accreting shoreline (e.g. spit,
buildingColonization
beach, sandbar) of new
New volcanos
New coral atolls
Artificial reef
areas
Colonization of a new area
• Follows succession from pioneers to
competitors…..but all have to disperse
there.
• Distance from source is important…can
larvae survive long enough to be
transported there? Can seeds be blown
there? Can mammals swim there? Can birds
fly there?
Two ways to study succession
• Follow one location from disturbance to
maturity, ex. Krakatau, Mt St Helens.
• Select similar habitats at diffent times since
similar disturbance, ex. Glacier Bay,
building riverbank
Succession in the intertidal
• Macroalgal
succession over 30
months on
experimental
concrete blocks.
Ulva
Sea lettuce
Primary succession in Glacier
Bay, Alaska
• Steadily retreating
glacier since 1850s.
• New land surface
revealed…primary 1912
succession.
• Oldest succession
where ice first
retreated.
1850
Glacier Bay
Succession
• Retreating glaciers
expose new land
surface of till.
• Rate of retreat ca. 65
km in 200 years
• Succession follows
broadly predicatable
path
Nutrient changes at Glacier Bay
• The initial soil is nutrient poor.
• Alder is an N-fixer, spruce and
hemlock are not.
• “forest floor” reflects N in leaf
and wood litter.
• Why is there a peak in forest
floor N at the transition to
spruce-hemlock.
• Why does soil N decline in the
spruce-hemlock zone?
Simulation showing nitrogen
inputs during 2ndry succession
Importance of alder
(ALRU) as a
nitrogen fixer and
Ceanothus (CEVE),
early in succession.
Lichens, and wood
decomposition
important (lesser)
sources late in
succession.
Nitrogen sources
Biotic and abiotic influences
• Nitrogen likely to be
limiting nutrient early
in succession….why?
r-K strategies
r
K
Unstable environment, density
independent
Stable environment, density
dependent interactions
small size of organis m
energy used to make each individu al
is low
large size of organism
energy used to make each individu al
is high
many offspring are produced
early maturity
few offspring are produced
late maturity, oft en after a prolonged
period of parental care
short lif e expectancy
long lif e expectancy
each individu al reproduces only once individu als can reproduce more than
once in their lifetim e
type III survivorship pattern
type I or II survivor ship pattern
in which most of the ind ividuals die
in which most indiv iduals live to
within a short time but a few live
near the maximum l ife span
much long er