Fire for Plants
Is fire really evil? Maybe it’s just a natural part of the environment.
You hear a crackling in the distance. The air turns warmer than it already was. You turn your head just in time to see smoke dribbling out from the bushes.
The disturbance reaches a dry, fallen shrub on the ground. Without warning, it bursts into flames. Before long, it has spread to small plants and patches of leaves on the ground. The bottoms of the tree-trunks turn black, and their fruits get well cooked on one side if not the other.
Birds flee shrieking from the trees; smaller animals burrow into the ground while larger ones run away from the scene. One of them is you.
It’s such common knowledge that there’s no need to spell it out: Fire burns. Fire destroys. Fire is dangerous. Fire has a bad image.
And now, I’ll try to explain to you why fire is good for plants.
What is fire? At a chemical level, it’s the combustion of oxygen and a fuel like wood. Molecules from the fuel join with oxygen in the air, leaving behind an oxidised product such as charcoal.
The combustion of fire is divided into six states, a bit like the launch of a rocket. Light a flame somewhere, and let the countdown begin…
Preignition. The wood is getting hot and start to release smoke. It’s losing water.
Ignition. Now the wood is releasing heat by itself! It doesn’t need any external energy to keep burning.
Flaming. As you may have guessed with the name, flames are appearing thanks to the release of inflammable gas. In this phase the heat is going upward and can reach 1600 degrees Celsius.
Smouldering. Release of particles: smoke, smoke, smoke.
Glowing. The most efficient phase of the combustion, there is no smoke anymore. This phase has a temperature of 600 degrees Celsius and can last a very long time. It’s the best time to cook and grill your steak.
Extinction. The fire is ended by a lack of oxygen or fuel. It doesn’t release energy anymore.
That’s the basic sequence of a burning fire. But how the fire behaves in in that sequence depends on what you’re trying to burn. The woody and green bits of plants are made of more-or-less hard molecules, which can influence how your fire turns out.
Firstly, there’s the density of wood to think off. If the wood’s more dense, there’s that much more molecules to go through, so you’ll end up with a dim, slow-moving fire that stealthily makes its way forward. Different kinds of wood also have different heat-transport capacities, which means heat can spread quicker in some wood than in others.
The ash content, moisture, and flammable volatiles also have to be taken into account. Perhaps there’s already some ash in the wood, providing a barrier that the fire has to struggle through. Or maybe there’s water, which has the same effect.
As for “flammable volatiles”, that’s random other things hiding in the wood that’ll suddenly leap out at you in flames when the fire gets to them.
To get all this into some sort of order, there are certain properties that people try to measure. There’s the “flammability” of a material — which tells how likely it is to catch fire. It’s a bit hard to define, but most people describe it as the time between when it starts heating and when it catches fire.
Petrol is highly flammable. It catches fire the instant you apply heat to it.
There are other things to measure too. “Sustainability” is how long the fire can continue to burn without adding extra heat from outside, like coals that keep a low steady flame without any extra heat.
“Consumability” is the mass of fuel burnt by the fire, while “combustibility” is the speed at which that burning happens.
It’s hard to get these values exact, though, because there’s an experimental bias. Measurements often focus on the flaming part of combustion, ignoring what goes on before and after. Moreover an experiment on a particular fuel cannot define the flammability of a species or a plant community.
Fires are not new to the Earth. They were there long before anything started living on it.
Natural fires can be started by spontaneous combustion when something just catches fire on its own, or by sparks from a falling rock, a meteorite impact (yes, sometimes!), volcanic activity, or the most important: lighting.
According to one estimate, 10% of global biomass is burnt by lightning-induced fires. Every year, lightning starts up five thousand fires in the US national forests alone.
Since humans entered the game, they changed the dynamics of fire in the landscape. Humans burn trees to free space for cultivation, or for creating their pastures and buildings. (Humans don’t live in the pasture, but their cows and goats do).
Today, a fire can start because of conflict of interest or crazy pyromaniacs.
One usually thinks of fires spreading across the land. But did you know they can also spread through the sky?
A forest fire emits so many gases and ashes that it creates a cloud above itself. From this cloud full of energy, lighting is created and sent to the ground where it starts new fires.
Lighting can hit areas far from the cloud, dispersing the fire across the forest.
As you may know, different types of clouds go by different names. “Pyrocumulus” is the term for clouds created from the water and smoke emitted by a fire. Under the cloud, wind can be very violent and generate a firestorm.
But it can also create rainy conditions, which will hasten the “extinction” phase that puts a fire out.
In the landscape, the spatial distribution and quantity of fuel influences the way fire spreads. The orientation and organization of the vegetation changes the way it decides to move.
A patchy grassland doesn’t give the same fire as an eucalyptus forest. Grasslands have way more available biomass, with the thin grass all tightly packed together, allowing the fire to spread easily. The forest’s separate patches need the wind to transport hot dust from one patch to another, making new fires that much harder to start.
Too much compacted straw, however, may also have difficulty burning, as less oxygen will be available to use use up.
The fuel load on an area is influenced by the how fast the vegetation is growing, and how fast dead plants get decomposed back into the soil.
After Chernobyl nuclear accident, a lot of tiny bacteria got disrupted. Decomposition slowed down, creating an accumulation of biomass. If fires had started on this dry accumulated biomass, the consequence would be catastrophic: the fire would burn radioactive component, and disperse it all across Europe through its ashes.
Wind and topography also change the movements of fire. Wind often helps it to disperse further and quicker. Meanwhile, a hilly topography won’t move the fire the same way as a valley or a flat meadow.
What decides the impact of the fire on the vegetation is not only the intensity of the fire but also how long its action lasts. It’s possible to jump over a fire but way more complicated to sit on embers.
The regional rain precipitation, and the humidity of fuel, could slow or extinguish the fire. Wet fuel don’t burn, so the fire needs to evaporate all the water before it can start the ignition.
Of course, that’s assuming the fire even starts in the first place.
Dry and hot weather is the ideal weather for ignition. Usually, wind helps to get rid of air humidity, while the sun warms the place in preparation. If the drought is prolonged, then fuel ignition is highly possible and there is danger of fire.
But what kind of fire? Fire comes in so many forms, but people have draw up three broad categories.
Surface fires are the most familiar ones. They burn the surface dead litter and the low vegetation. They last as long as there is fuel on the surface to continue burning. Most start by the ignition of a dry surface material — like a spark from a falling rock or a carelessly discarded cigarette.
Crown fires, on the other hand, burn right up through the treetops, moving from crown to crown. They are often dependent on surface fire, and always start from it.
But the greatest fire of all is probably the groundfire. Most of the time, groundfires are started up by a fire at the surface, but they really take off from there. They burn the subsurface vegetation such as peat or duff, and keep on burning under the surface through a slow, glowing combustion.
They are slow to burn, difficult to control as we can’t see them, and can last a very very long time: I’m talking hundreds of years!
Extreme fire event are extreme in size. Fortunately, they are also extreme in rarity. They usually occur when many small fires merge together and become one single colossal giant. For example, the Big Blow-Up in the US in 1910 burnt more than a million hectares. In Russia, the Great Siberian Fire burnt about a hundred-million hectares of the Russian forest.
These large fires are relatively rare, but they’re responsible for 97% of the burnt area. The larger these fires are, the higher the probability that they skip some bits, leaving unburnt islands inside the fire perimeter. These unburnt areas can represent 5% of the area in a twenty-thousand-hectare fire class.
Climate change is increasing the temperature and so changing the plant distribution on Earth. According to a model (published in 2008 by Giglio and colleagues), current dry and hot places such as Sahara desert or Australia will start getting less fires because of their loss in vegetation. Meanwhile, current cold areas like Greenland will have more vegetation and therefore will be more susceptible to fire.
How does fire impact the ecosystem? Again, it really depends on a lot of things like soil type, vegetation, and the local climate.
During a fire, mobile animal can hide underground or run out far from the fire. But what about plants? They can’t just flee away, so they’ve had to come up with other strategies.
Some plants are able to regrow quickly after a fire, while others simply make strong defenses for themselves. Cork, for example, is an external protection made by the oak Quercus suber, to protect itself against burning temperature. Some plant put all their important organs such as reserve or meristems underground, in what are called geoxyles, to avoid high temperature.
Fire may also trigger the reproduction of a plant by dispersing its seed or provoking its flowers to sprout in a flourish of serotiny — or perhaps a last-ditch attempt to keep the family going.
Fire is part of the Earth system and was always present. But now, human activities threaten the equilibrium which have shaped plant communities for millions of years. Global warming also changes the fire pattern on the Earth.
Let’s hope we survive.
It’s possible to study the history of fire by finding charcoal or ash in the sediments. Traces of previous fire can also be found on the growth strips of trees.
One million years ago, humans changed fire dynamics in the landscape. They altered the number of fires that came up, and their timings. They also altered the land use, land cover and therefore the quantity and quality of available fuel.
Despite all that, it’s still very hard to differentiate natural fires from human-created ones. In New Zealand, where humans arrived less than a thousand years ago, it’s clearly visible that human activity increased the fire activity — but which fires were caused by humans and which we naturally occurring? Nobody knows.
And maybe that’s the point. Fire is a natural part of the environment, as it has always been. So — humans or no humans, what’s the difference?
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