Forest fires were helpful. How did they get so hell?

When the first people arrived in North America, they took advantage of the benefits of periodic small fires by establishing their own to make the ecosystem more productive. Along with the Europeans, and later spread to housing and industry in western states, the concept of fire removal arose: To protect lives and property, fires must be extinguished as soon as possible. In the dry forests of western America, without much microbial activity in recycling vegetation, this has led to a dangerous accumulation of fuel.

Consider the area around the town of Cranbrook in southeastern British Columbia. Before the fire was extinguished, its forests were ponderosa pine and Douglas fir, with less than 50 trees per acre. The region would suffer a fairly light fire on average every seven years. Low-intensity fires in grass, shrubs and wood waste would save most trees, keeping the population under control.

As a result of the fires, there are currently 10,000 trees per hectare, 95% of which are Douglas firs. Without regular fire to regulate tree populations, the species took over. “That’s a classic example of how if you remove fire from the system, you’ll actually get a significant change in species and structure — basically density in this case,” says Gray. He says it’s even worse, with 200 times the number of trees per acre, “it’s likely to be a surface fire,” he continues, or one that mostly affects the bush. the flames spread among the treetops.

Thanks to this combination of dense fuels and a lack of natural firewalls, the landscape has lost that “herd immunity”. Now, fires can spread quickly, as they have several new sites that can “contaminate” them. And plants and animals are not so prepared against this massive fire. “The fire burns hotter and the species that live there are probably not adapted to that level of heat,” Gray says. “And if fires occur in a wide area, it makes it quite difficult for them to invade a site.”

If a forest is eradicated, it creates problems that can last for years. Surviving animals will not be covered hide from predators. As a result, burns tend to colonize invasive species, especially opportunistic grasses, as seeds enter the surrounding area. If they settle first, they will muscle all the native species that are trying to re-enter the burn scar. “They really take advantage of those conditions,” Gray says. “And they can really change the ecology of a site by making it pretty simple, homogenized.”

So how do you know if a fire has been “good” or “bad” for a landscape? Counting trees by satellites, drones and aircraft. In low-intensity fires, less than 20% of trees will die. High-intensity fires account for more than 80 percent. Within a single fire the degree of destruction can vary considerably: the edges can burn over the interior or vice versa. Size is also a factor. “If the patch is big enough, it should basically invade the forest from the edges,” Gray says. “If there’s a 50,000-acre fire, it’s a long process to restore the forest.”

Fire ecologists also study the structure and chemistry of the soil to find out how the fire went. The presence of reddish iron oxide, for example, indicates that the fire was very hot. If scientists find that the structure of the roots and the buried seeds have survived well, this is not an indicator of the fire that is so severe.