EACH SUMMER, forests burn. Australia, the Amazon, America’s western states and the Siberian Arctic are particularly susceptible to wildfires. Last year California saw the second largest fire in its history blaze through more than 950,000 acres, an area about the size of Rhode Island. This year the biggest ever fire in New Mexico charred more than 300,000 acres. Wildfires can be devastating for humans, but the scorched landscapes they leave behind are a crucial part of a woodland’s life cycle. What happens after a healthy blaze—and what changes when it becomes uncontrollably large?

EACH SUMMER, forests burn. Australia, the Amazon, America’s western states and the Siberian Arctic are particularly susceptible to wildfires. Last year California saw the second largest fire in its history blaze through more than 950,000 acres, an area about the size of Rhode Island. This year the biggest ever fire in New Mexico charred more than 300,000 acres. Wildfires can be devastating for humans, but the scorched landscapes they leave behind are a crucial part of a woodland’s life cycle. What happens after a healthy blaze—and what changes when it becomes uncontrollably large?

Wildfires are a natural way for forests to increase fertility. Over time, debris collects on the forest floor, forming a nutrient-rich layer. In normal times, it cannot be properly broken down and absorbed by plants. But when burned, the nutrients are released into the soil. Large trees in mature forests are best equipped to live through fires. Giant sequoias, which grow in California, develop a thick fire-resistant bark as they mature, protecting the heart of the tree. Some plant species in burn zones have even evolved to work with the flames. Serotinous seeds—such as those of the lodgepole pine, which grows in the American west—are “fire-activated”, meaning that a crop of saplings appears shortly after a blaze.

All this changes, however, when wildfires become uncontrollable. Larger and more destructive fires are happening more often. Rising temperatures caused by climate change mean more water evaporates from the ground in burn zones; snow drought in America’s mountains means less moisture makes its way to forests. That aridity turns trees into tinder just waiting for a spark (and in nearly 85% of American fires, this spark comes from humans). And in places over-zealous fire management has prevented many natural fires from burning, allowing a build-up of dead plants that is eventually capable of fuelling a much larger and less controllable blaze.

Huge wildfires upset the balance between destruction and regeneration. Some can completely wipe out a forest’s ecosystem. Even adapted plant species struggle in such extreme conditions. America’s National Park Service estimates that wildfires will kill as many as 19% of the world’s large sequoias within the next five years, despite their fire-resistant bark. Biodiversity in the Amazon is declining because mass deforestation leaves it more vulnerable to catastrophic fires.

Uncontrollable fires are also more likely to spread from forests to towns. In America, many fire-prone communities receive state and federal funding to rebuild damaged property. But there are few regulations to ensure that the new structures are any more resilient to fires than the old ones. Many homes are built in risky places. According to the US Forest Service, nearly two in every five homes in the lower 48 states are on tracts of land that the agency considers most at risk from wildfires. As more Americans move west, that share is growing.

Firefighters must think carefully about how best to manage the growing risk of wildfires. When they bring blazes under control too quickly, they store up trouble. But the fear of wildfires may encourage decision-makers to do just that. As intimidating as blazes can be, the health of forests depends on them.

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