Seed Dispersal by Fire

 

Introduction-seed dispersal by fire

Animals are able to flee any fires that destroy their habitat.

deer-with-a-wildfire-in-the-background

In contrast plants and trees have no such means of escape.

forest fire ablaze at nightnoaa.gov/features/04_resources/fire2.html

In regions prone to fire many plants and trees have evolved some remarkable adaptions which ensure the survival of their species.

One such adaption includes the amazing ability to harness the heat and smoke of fires to help disperse their seeds.

  • So how are plants and tree species able to do this?

Releasing seeds from the canopy- Lodgepole pine trees

The Lodgepole pine tree (Pinus contorta)….

Lodgepole pine tree Pinus contorta Lassen Volcanic National Park, Tehama County, California.Rudi Riet CC-Share Alike 2.0

….stores large numbers of seeds in its cones. After the cones mature they stay on the tree’s branches for anything between 10 and 20 years.

Lodgepole Pine with cones against backdrop of Lincoln PeakWalter Siegmund CC by SA-3.0

A thick resin binds the seeds inside the cones. The resin will only melt in response to high temperatures caused by forest fires.

Cross section of lodgewood pine cone showing seeds bonded together by resinwebpages.uidaho.edu

Once the resin has melted during a fire the cones open up, releasing the seeds trapped inside.

burnt cone opened by heat of fire on branch of Lodgepole pinephoto credit: Jim Peaco, US Parks Service

The seeds remain unharmed even when the cones are badly burnt.

Burnt lodgecone opened on ground releasing seedsImage credit: nps.gov

The intensity of the fire also kills adult trees. Once the land has been cleared of adult trees the seeds can germinate and young saplings have enough space and light to grow.

Lodgepole saplings of the same age grow after fire has killed parent treesphoto credit: Jim Peaco, US Parks Service

Cones can only open up and release seeds if the fire is a ‘proper’ one generating high temperatures….

Forest fire sweeps through Lodgepole trees

…. and not a smoldering one of the low temperature variety.

Smoldering fire will not release seeds from Lodgepole pine conesImage credit: nps.gov

Releasing seeds from the canopy- different species of Banksia

In the savannas of Australia wildfires are commonly triggered in the summer months by lightning strikes or sparks caused by falling rocks.

savanna fire in northern AustraliaGrant Stone at news.cornell.edu

Wildfires can also be man made. Over the past 60,000 years indigenous Aborigines have become well practiced in the managed burning of landscapes with the aim of regenerating growth or flushing out wildlife hiding in the forests.

Aborigines use fire to hunt kangaroos

The 170 species of Banksia, endemic to Australia, are very reliant on fire as a mechanism for dispersing seeds.

The Banksia prionotes, endemic to southwest Australia, can reach heights of up to 10 meters.

Banksia prionotes north of Kalbarri National Park Cas liber CC-S.A.3.0

Banksia prionotes has a distinctive inflorescence (or ‘flower spike’) …

Banksia prionotes inflorescence at Reabold Hill in Bold Park Floreat Western Australia Gnangarra Creative Commons CC-BY 2.5

…made up of thousands of bright orange flowers.The inflorescence (or ‘flower spike’)….

Banksia_prionotes close up Gnangarra Creative Commons CC-BY 2.5

….matures into an infructescence. An ‘infructescence’ is the fruiting stage of a ‘flower spike’.

Banksia prionotes young folliclesCas liber CC-S.A.3.0

Dried fruits called follicles grow on the outside of the ‘flower spike’. Only a few flowers on each ‘flower spike’ ever produce dried fruit.

Banksia attenuata follicleCC-BY-SA (all versions)

Two conditions need to be met before the follicles open up….

Banksia prionotes infructescence mature cone after seed releaseHesperian CC-BY-SA-3.0 and GFDL

…..to release the seeds.

Banksia prionotes seed released from follicle after firesHesperian CC-BY-SA-3.0 and GFDL

The first condition is that the parent plant needs to be ‘cooked’ at temperatures of between 265 °C (510 °F) and  330 °C (625 °F).

Banksia burning bush at Eneabba noth of Perthaustraliangeographic.com.au/news/2011/

For the second condition to be met the follicles need to be moistened with water.

Once a follicle becomes moistened the ‘seed separator’, which separates the two seeds found inside each follicle….

Diagram of inside of follicle of banksia species showing seed separator separating the seeds

….curls up.

Banksia Marginata Seed separatorCC by SA 3.0 JJ Harrison89@facebook.com

When the seed separator become dry it straightens out.

Banksia prionotes seed_separatorHesperian CC-BY-SA-3.0

Through successive wet-dry cycles the seed separator levers the seeds out of the follicle until the seeds fall to the ground.

lever showing analogy with seed separators of banksia prionotes levering seeds out of follicle through successive wet and dry cycles

Seed release and germination of the Banksia prionotes

Banksia seedlings post fire

….occurs not in response to fire, but in response to the onset of rains following a fire.

 ground after bushfire during rainshowing possibilities for germination of banksiatransform.fairfaxregional.com.au

The parent tree may not survive the heat generated by a fire. This image shows a B prionotes sapling growing under a dead parent tree.

Banksia prionotes seedlings after fire Burma road nature reserveCas Liber CC-BY-SA-3.0 and GFDL

Contrast B. prionotes with B. hookeriana. Banksia hookeriana will only ever release seeds…

Banksia hookeriana

…after an intense bushfire in which temperatures reach anything between 340 °C (645 °F) and 500 °C (930 °F).

banksia pod opened following fire

If bushfires take place too often, B hookeriana plants are killed before they reach fruiting age and before they have developed a substantial store of seeds.

If bushfires do not take place often enough B hookeriana plants die of natural causes between fires.

Banksia hookeriana is a strongly serotinous. It only releases seeds in response to an environmental change (ie a bush fire) and not spontaneously at seed maturation.

Dispersing seeds through ‘heat shock’

Acacia suaveolens (Sweet Wattle) is a species of plant of the Fabaceae family. It is endemic in south-east Australia.

Acacia_suaveolensnorthsydney.nsw.gov.au

Acacia suaveolens seeds grow inside pods…

Acacia-suaveolens-Sweet-Wattle seed podswestgatepark.org

… and when the seeds reach maturity the pods open up releasing the seeds which fall to the ground.

Acacia suaveolens pods open to release seedsflickr.com/photos/128520717@N02/16391788990/

Seeds lie where they fall, accumulating in the soil under the parent plant.  A ‘seed bank’ of dormant seeds increases in number year on year.

This image shows a large number of Acacia seeds which has been collected from just a single handful of soil.

large number of acacia seeds retrieved from a single soil smapleacademic.sun.ac.za/cib/

Some seeds are transported to ants’ nests where the fleshy outer elaiosome is used to provide food for the lavae.

After ant lavae have eaten the fleshy elaiosome the seeds are removed from the nest to be dispersed and then abandoned by the ants.

ant dragging an Acacia europhylla seed by the elaiosome fsd2010.org/miscellaneous/frugivores_of_the_month/17.htm

The seeds remain fertile despite the ant lavae eating all the elaiosome.

Any seeds lying in the soil cannot yet germinate . The testa, the tough external skin of the seed, prevents the seed sucking up water and germinating.

Cross section of Acacia ramulosa seeds showing testa, the tough external skin

It is only after bushfires generate extreme heat and high temperatures that the testa cracks, allowing the seed to suck up water and germinate.

The proportion of dormant seeds whose testae crack open under conditions of extreme heat varies according to the amount of heat transferred to the soil during the fire and the depth of seed burial.

Research has shown that exposing the seeds of Acacia suaveolens to soil temperatures of between 60°C and 80°C is ideal to crack open the testae and allow the seeds to germinate. Exposing the seeds to temperatures greater than 80°C leads to increasing ‘seed death’.

After a bushfire the seedlings can grow in places where there are low levels of competition for light and space and a high availability of nutrients.

This image shows a large number of young Acacia seedlings growing after a bushfire has destroyed all other vegetation.

Mass recruitment of acacia seeds after a fire academic.sun.ac.za/cib/news/20121022

Dispersing seeds through ‘smoke shock’

Some seeds need to ‘inhale’ smoke before they germinate.

Whispering bells (Emmenanthe penduliflora) is a grassland wildflower whose natural habitat is the….

Emmenanthe penduliflora in Palm Canyon, just south of Palm Springs, CaliforniaCreative Commons Attribution-Share Alike 3.0

…’chaparral’ of the western United States.

chaparral canyon fire near Rancho Penasquitas

The Emmenanthe penduliflora has a ‘bank’ of seeds stored in the soil which only germinate in response to ‘smoke shock’. They will only germinate in response to chemical signals emitted by smoke and charred wood.

This image shows the seed coat of a healthy Emmenanthe penduliflora which has been greatly magnified under an electron microscope.

Emmenanthe penduliflora seed coat magnified before exposure to smokejxb.oxfordjournals.org/content/49/325/1317.full.pdf

After exposure to smoke for just three minutes cuts appear in the seed coat; these cuts allow smoke to enter inside.

A chemical reaction now takes place which starts the process of germination.

Emmenanthe penduliflora after exposure to smoke showing scarificationjxb.oxfordjournals.org/content/49/325/1317.full.pdf

Seed disersal of species that benefit indirectly from fires

Calluna vulgaris

A European heathland species Calluna vulgaris, often called Scotch heather,…

Flowering-heather-on-heathland Calluna vulgaris

…. accumulates substantial seed banks in the soil underneath the plants. Their seeds germinate in response to exposure to light and by heat shock after a fire.

This species relies on fires to sweep through heathland…

burning heathland pebblebedheaths.org.ukpebblebedheaths.org.uk/management/swailing.ashx

….clearing the land of all vegetation. Such conditions are ideal for light to trigger seed germination.

This image shows young Calluna vulgaris growing on land cleared of vegetation in the year following a fire.

Calluna vulgaris on Braderup Heath growing one year after burn

Epilobium angustifolium (‘Fireweed’)

Fireweeds have long distance wind dispersed seeds…

Eloise Butler Wildflower Gardenfriendsofthewildflowergarden.org/pages/plants/fireweed.html

…which frequently colonise distant sites stripped of all vegetation and tree cover…

Epilobium angustifolium fireweed in alaska growing following a forest fireustaxpayerswill.wordpress.com/2015/09/#jp-carousel-6861

…..following forest fires.

forest fireustaxpayerswill.wordpress.com/2015/09/#jp-carousel-6861

The Yukon Flats of Alaska once contained forests of black spruce trees before the region was ravaged by forest fires.

fireweed in Yukon flats once colonised by spruce treesphys.org/news/2013-07-flammable-boreal-forests-north-america.html

Indirect dispersal of seeds -flowering after forest fires

The South African Fire lily (Cyrtanthus ventricosus) only ever flowers after a fire.

Cyrtanthus ventricosus in soilispotnature.org/node/646244

It appears within 12 days after a fire has burnt itself out.

field of Cyrtanthus ventricosus amongst bouldersColinpatersonjones.co.za

It bright red flowers are visible against the charred landscape….

Cyrtanthus ventricosus flowering66squarefeet.blogspot.co.uk

… making it very attractive to plant pollinators.

The butterfly Aeropetes tulbaghia, visiting the Fire Lily, Cyrtanthus ventricosus, very soon after a fire on the Cape Peninsula, South Africa.Copyright : Colin Patterson Jones

Within 15 weeks of flowering the Fire lily releases shiny, black winged seeds which disperse on the wind. With all the lily bulb in vegetative state waiting underground for a firesurrounding ground vegetation cleared by fire, the seeds have a better chance of landing in a suitable place to germinate.

If there are no bushfires the lily bulbs remain underground in a vegetative state- sometimes for 15 years or more.

Links and further reading

A wealth of information about species that live in the fire prone chaparral of the western United States can be found at waynesword.palomar.edu

Information about Banksia species endemic to Australia at Oznativeplants.com

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