Forest fires are used to be beneficial. How did they get to hell?
When the first people arrived in North America, they harnessed the benefits of small, periodic fires by setting up their own fires to make the ecosystem more productive. But with the Europeans, and then the spread of housing and industry across Western countries, came the concept of putting out fires: to protect lives and property, forest fires must be extinguished as quickly as possible. In the dry forests of the American West, without much microbial activity for recycling plants, this resulted in a dangerous buildup of fuel.
Consider the area around the town of Cranbrook in southeastern British Columbia. Before the fires were put out, its forests were mostly Ponderosa pine and Douglas fir, with fewer than 50 trees per hectare. The region will experience relatively moderate wildfires every seven years on average. Any low-intensity fire burning through grass, shrubs, and wood scraps would spare most trees, while keeping their populations under control.
But as a result of the fires being put out, there are now 10,000 trees per hectare, 95 percent of which are Douglas fir. Without regular fires to regulate tree aggregations, the species took hold. “This is a classic example of how, if you take the fire out of the system, you really get a huge change in species and structure — basically, intensity in this case,” Gray says. With trees now clustered so densely, fires can easily spread between them and spread across the landscape. Worse, he says, with 200 times as many trees per hectare now, “it’s not likely to be a surface fire,” he continues, or a fire that mostly affects shrubs. “It will be a high intensity crown fire, and it will kill everything.” In the fire of the crown, flames spread between the treetops.
Thanks to this combination of thick fuel and a lack of natural fires, the landscape has lost its “herd immunity.” Now, wildfires can spread quickly because they have many new areas that they can “infect.” And both plants and animals are less prepared to face this kind of wildfire. “Fire burns much higher, and the species that live there probably aren’t adapted to that level of heat,” Gray says. “And if the fires are happening over a large area, it will become very difficult for them to re-invade the site.”
If the forest is wiped out, it creates problems that can last for years. Animals that survive will not have cover to hide from predators. The resulting burn is also ripe for colonization by invasive species, especially opportunistic weeds, whose seeds begin to fall from the surrounding areas. If established first, they will wipe out any native species that also try to return to the burn scar. “They really benefit from these conditions,” Gray says. “And they can really change the environment of the site by making it very simple and sort of homogeneous.”
So how do you know if wildfires are “good” or “bad” for the landscape? By counting trees via satellites, drones and planes. In a low-risk fire, less than 20 percent of trees die. For a high-risk fire, the percentage is over 80 percent. The level of destruction can vary slightly within a single fire: the edges may burn more than the inside, or vice versa. Size is a factor too. “If the patch is big enough, the forest has to invade again from the edges,” says Gray. “If the fire is on 50,000 hectares, it’s a long process of recreating a forest.”
Forest fire ecologists also analyze soil structure and chemistry to determine the intensity of the fire. The presence of reddish iron oxide, for example, indicates a severe wildfire. If the scientists found that the root structures and buried seeds survived just fine, this is an indication of a less severe fire.