An expert explains the science of wildfires

Dr. Ellen Whitman is a forest fire research scientist and works for Natural Resources Canada in the Canadian Forest Service.

Whitman works on fire ecology and fire remote sensing, and most of her research is focused on northern Canada, including in the NWT and the Yukon. Whitman grew up in the Annapolis Valley and did her master’s on fire research at Dalhousie University. 

The Halifax Examiner spoke with Whitman on Wednesday about wildfires, how they behave, and what Nova Scotia can do to reduce the risk of wildfires.

This is our conversation:

Halifax Examiner (HE): How do wildfires behave and does the spread depend on where the wildfire starts?

Dr. Ellen Whitman (EW): The fire behaviour we get in an ignition does really depend quite a lot on various things together, but we always talk about fuel, topography, and weather. Those are the three things that are most important to how that fire spreads or how that fire behaves, which is the technical term we use.

Often what we’re seeing in the most severe fire behaviour you can get, if you think about a crown fire and that’s where the fire reaches the tops of the trees and it’s moving as a front as opposed to along the surface, that typically happens in conifer-dominated stands or new leaf trees. As we get into more mixed wood stands, for example, both conifers and broad leaves [trees] or broad leaf only, we see less extreme fire behaviour.

However, the weather and topography also matter. So, for example, if you think about a slope, fires spread faster up slopes because that flame leans forward and preheats the fuels that are ahead of it. On the other hand, if you have wind that is going downhill, but the fire is uphill, you have a slower fire spread, which is called a backing fire as it’s trying to move downhill, which is difficult for a fire or a fire to spread against the wind.

Weather matters a lot. Just because a fire happens in a conifer area, if it’s quite wet or the fuels are not available to burn or if you have a very short burn, say one day and then it rains, all of these things make that fire behaviour less severe and can also reduce the likelihood of that fire spreading more severely. It’s the interaction of all three of those things together that determines fire behaviour. 

One fuel I didn’t mention is grasses. Tall, dry, standing grasses often allow for very rapid fire spread. It’s what we call “flashy fuel.” It ignites very, very quickly and then it’s out again very quickly after that.

HE: What we know about the fires in Nova Scotia is that they were started by human activity. Can you give me examples of human activities that are more likely to start a fire?

EW: Human activity, we keep it very broad as a category in terms of what causes fires. So, that can be something like arson or intentional fires that are started by a person. But it can also be something like a spark from a railway. Both of those are human-caused fires, but the actions behind them are quite different.

When we think of members of the public and things that lead to unintended fires, we are typically talking about things like campfires not being put out properly and getting away after being left alone for hours. Cigarettes and matches, they do still matter. Putting something lit on the ground, you may not see it start a fire but if you walk away it can smoulder in the ground for a little bit and take off.

There’s obviously equipment that can start fires as well. When we talk about using four-wheelers or quads in the woods, it might be surprising to people, but if you’re moving through thick, peaty mud, and that gets onto an exhaust pipe and heats up, it can fall off and start a fire. Or operating a chainsaw that goes against a rock causes a spark. There’s a broad diversity of human-caused fires and I don’t think we have a great understanding of which are the most problematic. But in all cases, it’s generally unintentional, but people need to be super careful.

HE: The fires in Nova Scotia started in a very dry environment. Can you tell me how that fire might react in dry conditions once it ignites?

EW: Drought and dry fuels are really important to what happens with the fire. Often what we see with the fires, whether it’s lightning or human caused, is sometimes you can start a fire and if it’s super windy and super dry, it will get away from you very quickly and you can see that happen right before your eyes. Often that’s the result of wind and I think there were quite severe winds involved in the fires in Nova Scotia. 

Drought, on the other hand, creates this additional complicating factor. The fuels are very dry. For one thing, they’re quite susceptible or they’re open to burn, so it’s hard to suppress that fire once it gets away. And then also, you can have these situations where someone has absolutely no idea that their activity caused the fire because if there’s something very dry, you can have things burn for quite a long time undetected at the surface and then hours later that fire can take off once it encounters the right winds and the right fuel. When you have a long-term drought, like Nova Scotia had this spring, the fuels make ignition much more likely and then the fuels themselves that are burning once the fire is active make the spread more volatile as well.

HE: So, that’s something to think about over the summer months since we’re just into June and we’ve already had these fires.

EW: Often in places like Nova Scotia where we have a lot of broad-leaf trees, we have this initial high-risk period in the spring before the trees are fully green, so they don’t have those nice, big wet leaves on them. That period is generally quite high-risk for human-caused fires. And then later in the year, when we get into more of a drought situation in other parts of Canada, that’s when we tend to experience more lightning-caused fires. Although, those green leaves should still persist through a drought, so later in the summer hopefully if there were to be another dry spell, the risk may be somewhat lower. But yes, another dry spell in the summer could absolutely allow more fires to happen again.

HE: What are the differences between the fires in Nova Scotia and say the fires happening in Ontario and Quebec right now?

EW: A major difference there is the forest type, for one thing. You do have a patchier landscape in Nova Scotia in terms of the availability of those continuous conifer fuels. Those fires burned in conifer-dominated areas and created some of the biggest fires Nova Scotia ever experienced. However, the fires that are happening in other parts of eastern Canada right now are in the boreal forests. It’s a much more conifer-dominated landscape. There’s lots of continuous fuel available. Also, the population density up there, even though the total population there is much larger, in northern Ontario and northern Quebec, the density of people is quite low. You have lots of undisturbed natural boreal forests that can let fires spread quite far because there’s a lot of continuous fuel, it’s not broken by things like roads, parking lots, people’s lawns, which are quite low flammability. 

The other key thing to highlight there is, at least in the case of Quebec, it seems to be most of those fires ignited within a short window after a big lightning event. Generally in Canada, lighting-caused fires are about half of our ignitions, but they make up the majority of area burned because they are ignited in places where there are no people nearby to notice them, so they get detected less quickly. They wait around for a few days before they take off, smouldering in the dust or the peat. That makes them hard to detect as well.

HE: Can you speak to the development issue and where we build homes and how to take the fire risk into consideration?

EW: I think it’s a challenge anywhere in Canada in that it’s a forested landscape and often fire scientists are guilty of just calling the forest fuel. Many people in Canada love to live in beautiful, natural landscapes. We’re really intermingled and intermixed in the forests and what we tend to call the wildland-urban interface. Generally speaking, the wildfire risk is much lower in more densely packed urban areas. So, like the peninsula of Halifax. There are not a lot of continuous forests whereas in other areas like in the suburbs or rural areas, we tend to be living more intermingled. That’s a challenge and not everyone wants to live in a dense urban development and that’s not something we can necessarily force onto people. 

However, when you’re living in a wildland-urban interface area, there are mitigating actions we can take around urban planning. So, making sure people have multiple routes of egress or adequate options for firefighting in an evacuation.

And also fuel mitigation on their own properties. Nova Scotia has a lot of private land, so that means the people who own that land and live there have the responsibility and also deal with people with the authority to treat the fuel on their land. If the homeowner is concerned about the fire risk and living on a property surrounded by conifer trees, you can think about not having branches that are touching your roof.

That’s the kind of thing we’d recommend when people look into a program called FireSmart, which we have in Canada that provides guidance on how to manage fuels around private properties in rural areas or urban areas, if you’re concerned, and try to mitigate that risk. That can be things like what we call home hardening, such as having, for example, a metal roof, which doesn’t allow embers to land on it and ignite. Or building with materials that are not flammable. And also, other things like making sure stacks of wood are piled separately from the house. Or making sure you have a large area around your house. We often say something like having 300 metres as a gap between your property and the trees. But all of that is described in FireSmart.

HE: What does Nova Scotia need to think about now as a province and as property owners about reducing the risk of wildfires and the damage they can cause?

EW: Certainly people in severe fire weather conditions need to be careful about their own actions and don’t be the person who’s unintentionally responsible for setting a fire. Thinking carefully about putting out fires, campfires when you use them. Making sure if you’re operating equipment in hot, dry, windy weather you’re taking precautions or delaying that activity. 

Nova Scotia does have a lot of rural communities and wildland-urban interface communities and that is a challenge going forward, but there’s a lot of opportunities to adapt and create resilient communities. So, thinking about things like urban design or personal mitigation risks.

I think that there are challenges with climate change. Atlantic Canada benefits in many ways from having this mixed wood Acadian forests with a broadleaf [tree] component, which historically reduced risk. And also just being closer to the ocean. There’s generally a higher humidity on the coast. When we get into these situations where the weather is very warm, perhaps there’s been an extended drought, and particularly if you really notice it’s hot, the relative humidity is low, and it’s very windy, that’s when we get into these dangerous fire situations. With climate change, that could become more frequent, even if there’s more precipitation because warm temperatures evaporate moisture very effectively. Being prepared for that and planning ahead and identifying those dangerous situations when they occur.

Click here to visit our Nova Scotia wildfires resource page.