Så förändrar megabränderna världen: "Omformar hela ekosystem"

De så kallade megabränderna, som får vanliga skogsbränder att blekna i jämförelse, har blivit allt vanligare på jorden. Bränderna har omedelbara ekologiska effekter – de dödar växter och djur som kanske hade överlevt mindre omfattande bränder. Men de får också mer långsiktiga konsekvenser. De jättelika bränderna kan utrota hela arter, förändra landskap och omforma hela ekosystem, skriver The New York Times. Bränderna har blivit så pass vanliga att forskare nyligen har döpt tiden vi lever i till pyrocen, eldens tidsålder. In our Pyrocene age, enormous wildfires aren’t merely damaging ecosystems but transforming them. By Emily Anthes October 15, 2023 On Aug. 15, a small wildfire was detected in the hills above West Kelowna, in British Columbia. The landscape was parched and the wind was fierce, and over the next few days the modest blaze exploded into a raging conflagration. It raced down into the valley and toward Okanagan Lake. Wind blew red-hot embers across the water, sparking new fires around the city of Kelowna. “I didn’t sleep much the night that the West Kelowna fires crossed the lake,” said Karen Hodges, who lives in Kelowna. “I could see the fires from my window. And so I was thinking about people I know in the valley and where their houses were.” Hodges, a conservation ecologist at the University of British Columbia Okanagan, also found herself worried about wildlife. She had been studying some Western screech owls that had been nesting in the heart of the fast-moving inferno. “That speed of fire would be difficult for animals to evacuate in front of,” she said. Had the owls escaped in time? And after Canada’s worst wildfire season on record, what would be left for the survivors? Fire is a natural phenomenon; some species actually benefit from its effects and even those that don’t can be remarkably resilient in the face of flames. But as fires intensify, they are beginning to outstrip nature’s ability to bounce back. “Not all fires have the same impact,” said Morgan Tingley, an ecologist at UCLA. “These megafires are not good for ecosystems.” Megafires, which dwarf typical wildfires in size, have an immediate ecological toll, killing individual plants and animals that might have survived more contained blazes. In the longer term, changing fire patterns could drive some species out of existence, transform landscapes and utterly remake ecosystems. This incendiary age, which some scientists have called the Pyrocene, could lead to “a wholesale conversion of what habitats are where on the planet,” Hodges said. “Right now, everybody is talking about fires and smoke and who dies, because of the immediacy of this fire year. But really, truly, the long-term consequences are much more severe and sustained.” Fire has been a planetary phenomenon for hundreds of millions of years, and plants and animals that evolved in fire-prone regions have adapted to periodic conflagrations. Some trees have roots that can re-sprout even if the trunk burns, while the mere smell of smoke will rouse some animals from torpor, a form of light hibernation. But in many regions and ecosystems, fires are becoming larger and more severe. In the United States, wildfires burn far more land today than they did three decades ago, especially in Western states. Globally, the risk of catastrophic fires could increase by more than 50% by the end of the century, the United Nations reported. Climate change is partly to blame, scientists said, but so are other factors, such as the expansion of highly flammable invasive grasses, which helped the deadly fires in Maui spread so quickly. More than a century of fire suppression has also left some forests thick with trees, giving flames more fuel. “When fires burn, they burn with so much intensity,” said Chris French, a deputy chief of the National Forest System in the United States. Even fire-savvy organisms may find themselves outmatched. In northern Australia, frilled lizards can survive low-severity fires by hiding in the tree canopy. But during severe fires, when flames leap higher, lizards that employ this strategy may perish. Fires are also spreading into ecosystems where flames are an unfamiliar threat. The megafires that erupted in Australia in 2019 and 2020 scorched the country’s rainforests, which contained many plants that cannot regenerate after burning. The animals in those ecosystems might be “fire naive,” said Dale Nimmo, an ecologist at Charles Sturt University in Australia. “They may not have been under any natural selection to detect the subtle cues of fire in the air, or through sound. And so they may not recognize the threat as it approaches.” The Algerian sand racer, a Mediterranean lizard, lives in a variety of habitats, only some of which experience frequent fires. In a 2021 study, researchers found that lizards collected from fire-prone sites reacted quickly to the smell of smoke, flicking their tongues and running around their terrariums. “In places where fire is not a common threat, lizards did nothing,” said Lola Álvarez-Ruiz, a biologist at the Desertification Research Center in Spain, who conducted the study. Fires that consume more fuel may also produce more smoke per unit of area burned, threatening animals far from the flames. “All air-breathing animals are going to be impacted by smoke exposure, because the chemicals in smoke are toxic,” said Olivia Sanderfoot, an ecologist at UCLA. Smoke inhalation can do more than cause respiratory problems. For months after severe peatland fires produced record air pollution in Indonesia in 2015, Bornean orangutans vocalized less frequently and their voices became harsher. The orangutans also moved less and ate more than they had before the smoke descended, but they still burned stored body fat, suggesting that their bodies were working overtime. That could be a sign of inflammation or stress, said Wendy Erb, an ecologist at the Cornell Lab of Ornithology, who conducted the research. The long-term consequences are unclear, but fires have become common on Borneo, which is the only home for the critically endangered apes. “We’re talking about all of the remaining living orangutans essentially being exposed to the smoke on a regular basis,” Erb said. Animals that survive the inferno must then find food, water and shelter on hot, dry, denuded landscapes where the risk of predation is high. (Surrounded by weakened prey, some predators thrive after fires.) Fortunately, fires tend to burn unevenly, ravaging some stands of trees while grazing or sparing others. These unburned islands can be a lifeline for fire-sensitive species such as caribou, which eat highly flammable lichen, as well as thin-barked fir trees. But some of today’s fires are leaving fewer of these oases. “You could walk half a mile, and you wouldn’t see a single living tree,” said Andrew Stillman, an ecologist at the Cornell Lab of Ornithology. “Increasingly, these fires seem to create habitat conditions that are outside of the norms that these species are adapted to.” That may be true even for fire-loving animals, such as the black-backed woodpecker. The birds nest in scorched trees and feed on the beetle larvae that colonize the charred trunks. But they prefer patches of burned trees that are near stands of leafy, living ones, which protect their fledglings from being picked off by predators, Stillman and Tingley found. After the enormous Rim fire in California in 2013, scientists searched for the woodpeckers at nearly 500 sites across the expansive burn scar. They found just six birds. “Even though it had created all this great burned habitat, it wasn’t the right kind of burned habitat,” Tingley said. Fewer clusters of living trees can also reduce regrowth. “In many places, we’re not getting regeneration because the seed source is lost,” said French, of the National Forest System. “It honestly looks like someone went in and just set off a bomb.” Scorched, vegetation-less soil, which does not absorb rain well, can also hamper regeneration. Flash flooding after fires can wash ash and sediment into rivers and streams, polluting the water, killing fish and reshaping waterways. After the Rodeo-Chediski fire in Arizona in 2002, repeated flooding washed away fertile soils that had taken more than 8,000 years to develop. “That has cascading impacts on the kind of plants that can grow,” said Jonathan Long, an ecologist at the U.S. Forest Service, who conducted the research. In the Northwest Territories of Canada, repeated fires have utterly transformed some forests. At one site, towering jack pines have given way to grasses and a smattering of “scrubby, stunted” aspens, which have light seeds that can be carried on the wind, said Ellen Whitman, a forest fire research scientist at Natural Resources Canada. “It is a very different place,” she added. Change is not necessarily bad. Fires can prompt overdue regeneration in places where flames have been artificially suppressed, and forests are not inherently superior to other ecosystems. Old-growth grasslands, which are biodiversity hot spots, are also under threat; in some places, grasslands have turned into forests, partly because of fire suppression. “So maybe in some ways, a bit of a balance is being restored,” Whitman said. But it could take a long time for new grasslands to build up biodiversity, and landscape transformations have ripple effects. In the Amazon, forest plots subjected to frequent fires began to resemble savannas; at these sites, ants and butterflies that favored forests declined, while species preferring open habitats moved in, scientists found. In North America, the loss of large, old-growth trees could reduce the ranks of forest specialists, such as martens and fishers, members of the weasel family that den inside tree hollows. Although the idea remains speculative, changes in fire activity could ultimately produce ecological communities that are more homogeneous, dominated by “generalist” species, such as coyotes and deer mice, which are flexible in their diets and habitats, scientists said. Today, increased fire activity could push more than 1,000 threatened plant and animal species closer to extinction, scientists calculated. And many plants and animals are already facing multiple stressors. In Canada, Western screech owls are threatened by land clearing and the expansion of invasive barred owls. “Then you throw fire on top of that as an additional thing that kills some of them, stresses others and changes habitat out from under them — you know, suddenly you’ve got too much to handle,” Hodges said. The West Kelowna fire burned some of the owls’ nest trees, she said, and the outlook is grim for a young, GPS-tagged bird that one of her students was tracking. “Its last known location was right in the middle of the fire that blew up so quickly,” Hodges said. “And we haven’t detected the signal since.” © 2023 The New York Times Company. Read the original article at The New York Times.