Animals are able to flee any fires that destroy their habitat.
In contrast plants and trees have no such means of escape.
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.
The Lodgepole pine tree (Pinus contorta)….
….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.
A thick resin binds the seeds inside the cones. The resin will only melt in response to high temperatures caused by forest fires.
Once the resin has melted during a fire the cones open up, releasing the seeds trapped inside.
The seeds remain unharmed even when the cones are badly burnt.
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.
Cones can only open up and release seeds if the fire is a ‘proper’ one generating high temperatures….
…. and not a smoldering one of the low temperature variety.
In the savannas of Australia wildfires are commonly triggered in the summer months by lightning strikes or sparks caused by falling rocks.
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.
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 has a distinctive inflorescence (or ‘flower spike’) …
…made up of thousands of bright orange flowers.The inflorescence (or ‘flower spike’)….
….matures into an infructescence. An ‘infructescence’ is the fruiting stage of a ‘flower spike’.
Dried fruits called follicles grow on the outside of the ‘flower spike’. Only a few flowers on each ‘flower spike’ ever produce dried fruit.
Two conditions need to be met before the follicles open up….
…..to release the seeds.
Hesperian 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).
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….
When the seed separator become dry it straightens out.
Through successive wet-dry cycles the seed separator levers the seeds out of the follicle until the seeds fall to the ground.
Seed release and germination of the Banksia prionotes…
….occurs not in response to fire, but in response to the onset of rains following a fire.
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.
Cas Liber CC-BY-SA-3.0 and GFDL
Contrast B. prionotes with B. hookeriana. Banksia hookeriana will only ever release seeds…
…after an intense bushfire in which temperatures reach anything between 340 °C (645 °F) and 500 °C (930 °F).
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.
Acacia suaveolens (Sweet Wattle) is a species of plant of the Fabaceae family. It is endemic in south-east Australia.
Acacia suaveolens seeds grow inside pods…
… and when the seeds reach maturity the pods open up releasing the seeds which fall to the ground.
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.
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.
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.
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.
Some seeds need to ‘inhale’ smoke before they germinate.
Whispering bells (Emmenanthe penduliflora) is a grassland wildflower whose natural habitat is the….
…’chaparral’ of the western United States.
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.
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.
A European heathland species Calluna vulgaris, often called Scotch heather,…
…. 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…
….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.
Epilobium angustifolium (‘Fireweed’)
Fireweeds have long distance wind dispersed seeds…
…which frequently colonise distant sites stripped of all vegetation and tree cover…
…..following forest fires.
The Yukon Flats of Alaska once contained forests of black spruce trees before the region was ravaged by forest fires.
The South African Fire lily (Cyrtanthus ventricosus) only ever flowers after a fire.
It appears within 12 days after a fire has burnt itself out.
It bright red flowers are visible against the charred landscape….
… making it very attractive to plant pollinators.
Within 15 weeks of flowering the Fire lily releases shiny, black winged seeds which disperse on the wind. With all the surrounding 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.
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