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Disturbance and succession

Sources of natural disturbance

• 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.

Destructive fires– Fire present for hours.

– Plant rootstocks killed

– Severe erosion follows

– Large area burned, reduces source for recolonization.

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

• 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 = GPP-respiration

• NPP maximal in immature stages of succession.

• Mature phase has senescent trees

Hubbard Brook fire results

• Nitrate is an important nutrient.

• Nitrate and sulfate are strong acid anions, and their leaching acidifies the streams, but will leave the soil more basic.

Sulfate

Nitrate

NO3 and SO4 content of streamwaterfollowing 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 increases solubility, decreases acidity (increase of 2 pH units)

• Sulfate concentrations in ELA streams: mean monthly SO4 concentrations in the Northwest stream before and after the 1980 fire.

Pre-fire

1-2 yr after

3-7 yr after

Nutrient load

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 areasAccreting shoreline (e.g. spit, building beach, sandbar) New volcanosNew coral atollsArtificial reef

Colonization of new 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.

UlvaSea lettuce

Primary succession in Glacier Bay, Alaska

• Steadily retreating glacier since 1850s.

• New land surface revealed…primary succession.

• Oldest succession where ice first retreated.

1850

1912

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 rUnstable environment, densityindependent

KStable environment, densitydependent interactions

small size of organism large size of organismenergy used to make each individualis low

energy used to make each individualis high

many offspring are produced few offspring are producedearly maturity late maturity, often after a prolonged

period of parental careshort life expectancy long life expectancyeach individual reproduces only once individuals can reproduce more than

once in their lifetimetype III survivorship patternin which most of the individuals diewithin a short time but a few livemuch longer

type I or II survivorship patternin which most individuals live tonear the maximum life span