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Why are some wildfires more severe than others?

Wildfire Operations & Mitigation Expert – and Associate Forester at Frontera – Conor Corbett dives into the factors contributing to wildfire severity, and why the BC landscape is so susceptible to them.


Happy Holidays from Frontera


Growing Wildfire Significance in BC

Hi all! Conor here again. At Frontera, we are often asked why we are seeing such a large increase in wildfires in recent years. Here in BC, we are seeing a huge increase in wildfire frequency, intensity, severity and damage. The three most significant wildfire seasons on record (2023, 2018 and 2017) have all occurred in the last 7 years. But to really understand why these fire seasons have been so severe, we first need to understand what influences wildfires in the first place.



The Fire Triangle – Understanding How Wildfires Start


All fires – not just wildfires – are chemical reactions that require heat, fuel, and oxygen. We call this the fire triangle. When we burn gasoline in a car's engine, the fuel is gasoline. Wildfires are different in that the fuel is vegetation: trees, shrubs, grass, logs, even the organic material in the soil. In a car, the initial fire reaction starts with heat from a spark plug, which creates a spark to start the chemical reaction. Most wildfires get their "spark" from lightning, but can also start from human negligence through discarded cigarette butts, unattended campfires, or arson.


All wildfires require heat, oxygen, and fuel; but this does not explain why some wildfires are so large, intense, and destructive. Most people are surprised to hear that a vast majority of wildfires in BC are very small, low intensity, and easily extinguished. Only a small handfull of wildfires become the large conflagrations we hear about in the news. This is because the behaviour of a wildfire can vary widely, and depends on a few additional factors: fuel composition, weather, and topography.


Topography


Topography is the simplest to understand: steep slopes can support more intense wildfire behaviour, particularly when south facing. Heat rises, so a wildfire naturally wants to travel uphill. South facing slopes receive more sunlight and heat, creating a drier environment which can give rise to a hotter, more volatile wildfire.


Weather


Weather is also relatively simple. Hot, dry, and windy weather supports more extreme wildfire behaviour. Heat, particularly long heat waes, dries out the forest, making the vegetation (fuel) easier to ignite and allowing fire to spread more quickly. Wind exacerbates wildfire, not only by causing the wildfire to spread, but also by increasing oxygen to the fire.


Fuel Composition


The fuel component of wildfire behaviour is the most complex. The flammability of vegetation in a forest depends on its composition and arrangement. Different types of vegetation are more flammable.


Conifer trees with needles and grass have foliage with a high surface area, which means they dry out very rapidly. Dead vegetation cannot hold moisture very well, so it is also quite flammable. Deciduous trees have leaves, which retain much more moisture, and thus are less flammable.


Organic material in the soil can also struggle to hold moisture, and in periods of drought even dirt can become flammable! An additional component is how this vegetation is arranged. Forests with high continuity, both vertically and horizontally, allow fire to spread rapidly and become very intense. When a fire is able to travel into the canopy of a conifer forest, the intensity dramatically increases in what is called a crown fire.



Wildfires in BC


Let's bring it all together into a BC context. In BC, we have tons of forest, and most of it is coniferous. These forests are very continuous, with lots of vegetation that can support wildfire. We also live in a very mountainous place, with lots of slopes that can increase intensity of wildfire.

Finally, much of BC experiences long dry periods in the summer with high temperatures and little rainfall. It is no surprise that we see lots of large, intense wildfires! In our next post, we'll explore why these extreme wildfires are becoming more and more common.



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Frontera Forest Solutions is a forestry consultancy located in North Vancouver, British Columbia. With clients all around Western Canada, Frontera specializes in helping communities develop wildfire resiliency though innovative forest management an urban forestry, grant writing and training and curriculum development.


To explore how we can work with your community, reach out here.


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