Flames from the Valley Fire covered a hillside along Highway 29 in Lower Lake, Calif. on Sept. 13, 2015. Credit Noah Berger/Reuters
Governments’ interference in the natural cycle of fires, along with climate change, has created more brush on forest floors and hotter, drier seasons.
By JOHN SCHWARTZ NY Times Sept 22 2015
NEAR COCHITI CANYON, NEW MEXICO — The hills here are beautiful, a rolling, green landscape of grasses and shrubs under a late-summer sky. But it is starkly different from what was here before: vast forests of ponderosa pine. ….. The result is bad news for forests here, in the West and around the world. A planet with fewer trees is less able to fight climate change, because trees absorb carbon dioxide from the atmosphere as part of photosynthesis. “The future in a lot of places,” Dr. Allen said, “is looking shrubbier.”
The fire season of 2015 in the American West is shaping up as one of the worst in the nation’s history, with more than eight million acres burned nationwide — more than five million acres in Alaska alone. Fierce wildfires this month have destroyed hundreds of homes in California. The Forest Service struggles under an increasingly costly mission: According to a report released last month, firefighting takes up more than 50 percent of its annual budget, up from 16 percent a decade ago. In 10 years, it could consume three quarters of its budget. Climate change has lengthened fire seasons, which are, on average, 78 days longer than they were in 1970, and the six worst fire seasons since 1960 have come since 2000.
The consequences of a century of forest policies to suppress fires are now combining with the hotter and drier seasons to create tinderbox conditions, producing high-severity fires that kill trees and are increasingly hard to bring under control. Dr. Allen and many other researchers have studied how to manage forests so fires are not as destructive. And in the case of the most destructive fires, they are studying what happens to those landscapes in the years after a blaze.
In an increasing number of cases, said Malcolm P. North, a research scientist with the United States Forest Service Pacific Southwest research station in Davis, Calif., “after the satellite trucks leave and everyone goes home, you have a charred condition on the landscape that does not have a historical precedent.” Fire, over eons, has been an essential and cyclical part of forest life; tree-ring records show fires occurring every five or 15 years for ponderosa pine in the Southwest, said Thomas W. Swetnam, a professor emeritus at the University of Arizona and an expert in tree-ring analysis. The fires tended to burn with low intensity, clearing underbrush, grasses and seedlings, leaving an uncluttered forest floor and helping some species of pine spread their seeds.
More than 100 years ago, that pattern of frequent surface fires was disrupted, first by widespread grazing by sheep and cattle, which cleared much of the grass and other undergrowth, and then by a government policy to suppress forest fires wherever possible. Fewer fires caused the forests to grow more densely, and for grasses and dead trees to accumulate on the forest floor. The hotter, longer droughts associated with climate change make the trees and ground cover drier; the result is a greater tendency for fires to “ladder up” to the canopy of leaves or needles above.
Dr. North, the California researcher, said that as much as 80 percent of California’s forests were in the kind of conditions that were likely to lead to the more destructive, tree-killing fires. Without mature trees near the fire-ravaged areas to spread their seeds, brush and grass are likely to grow in place of the conifers. That means forest recovery can be slow, or worse, said Donald A. Falk, a fire expert at the University of Arizona. “That’s a recovery process that could take centuries — and given where climate is going, it might never recover,” he said. Not everyone agrees with this gloomy assessment. William L. Baker, a professor emeritus at the University of Wyoming, has used historical land-survey data to argue in papers that large, severe wildfires are a natural phenomenon and are not necessarily worse than before. Over time, he has argued, the forests can grow back.
But Dr. Swetnam and others disagree with Mr. Baker’s conclusions, arguing that the land-survey records are not as reliable an approach to understanding fire history than the trees’ own rings, among other data sources.
Droughts are certainly not new, nor are large fires or even intense fires, Dr. Swetnam acknowledged. But the greater number of intense and large fires, and the repeated “burns on top of burns” like the ones that cleared the landscape around Cochiti canyon, are part of a pattern of worsening conditions exacerbated by the hotter droughts. He has studied sections of many trees from the former forest near the canyon, which provided 300 years of fire history before the 2011 Las Conchas blaze, which ultimately burned 150,000 acres. “Obviously, the forest had survived many, many, surface fires,” he said. “But this fire — this fire — killed all of the trees in this area. ”
He added, “I’m often pressed to say which is the most important factor in the changing nature of forest fires. I’m more inclined now to point to climate change than I was 10 years ago.” A sweeping recent paper from Mr. Allen and colleagues suggests that wildfires are only part of the damage that climate change is wreaking on the world’s forests. The hotter droughts that are associated with climate change are causing stress for trees, and are likely to grow worse over time, leading to increased tree mortality. By the middle of this century, “If the climate models are at all accurate, what in 2011 were really extraordinary conditions will be typical conditions for June” in the Southwest, he said.
Because dry and wet periods are cyclical, he said, year-by-year variation could lead to brief periods of even more extreme drought along the way. El Niño could bring wetter and cooler weather to this region, reducing the number and severity of fires for a while. But the long-term trends are anything but good. And what is happening in the West is a harbinger for much of the rest of the planet.
The trees in too-dense forests are already competing for water that the historically more sparse stands of trees might have found adequate; as drought increases, the stress will kill many trees outright and weaken others to the point that they become more vulnerable to predators like aggressive bark beetles.
Because of hotter drought, he said, “the future broad-scale vulnerability of forests globally is being widely underestimated, including the vulnerability of forests in wetter regions,” he said. With three trillion trees in the world, of course, there is still a great deal of carbon being stored. But that could change over time. And because forests provide an important natural mechanism for fighting climate change, the stress of hotter drought could become part of a feedback loop of increased warming. Landscapes like the scrubland in New Mexico absorb far less carbon than the forests they are replacing, said Mr. Falk of the University of Arizona. With forest fires and the decay of dead trees, which also puts the carbon they were storing back into the atmosphere, “These forest become carbon sources, where in the past, we’ve relied on them to become carbon sinks.”
A study published in July suggests this has occurred in California, where forests have shifted from net carbon absorbers to carbon emitters, according to Patrick Gonzalez, a forest ecologist with the national park service and the author of the report. An author for the Intergovernmental Panel on Climate Change, Dr. Gonzalez said that wildfires were “tipping the balance” between storage and emission. Even droughts that do not lead to tree death impair the ability of forests to absorb carbon dioxide. A study published in July from William R.L. Anderegg of Princeton University and colleagues has found that forests slow their growth for four years after a drought, and absorb less carbon during that period. Researchers in the past believed that forests recovered quickly, but Dr. Anderegg and his colleagues found that severe drought greatly curtailed the absorption of carbon over a broad range of tree species. “It’s absolutely clear that drought has these manifold and very severe impacts on forests,” Dr. Anderegg said. “Forests are more vulnerable than we thought. Drought has lasting impacts, even when the rain comes back and the soil becomes wet again.” So far, he said, the world’s forests appear to be a net absorber of carbon, but there are “some worrying signs that we could be starting to change that.” He added, “We could be in more danger than we thought, sooner than we thought.” Fire experts have long called for action to meet the challenge of forest fires in a changing climate.
In a paper published last week in the journal Science, Dr. North and colleagues argued for ending the national policy of fighting every fire, and for making more concerted effort to thin forests so more fires might only scorch trees without destroying them. The authors called the traditional policy of trying to suppress every fire “dangerous, expensive, and ill advised.” Thinning involves techniques like selectively cutting down trees in forests and burning sections of forest, carefully and under weather conditions that make fire easier to control. The alternative, Dr. Swetnam said, is more of the severe and repetitive blazes. “If you don’t come in and thin these trees with chain saws or fire,” he said, “you’ve got a bomb.” Tightly packed trees spread fire to other trees faster, noted Paul F. Hessburg, a research ecologist with the United States Fire Service at the Pacific Northwest Research Station. Thinning and prescribed burns, he said, “can break that contagiousness up.” Local citizens complain about smoke from controlled burns, he said, but some kind of fire is inevitable. “I think society is going to have to decide how it wants its fire and how it wants its smoke — whether in large-fire-sized doses, or in smaller-fire-sized doses,” Dr. Hessburg added. “We have it in us to decide what kind of society we want to live in.”
The problem with controlled burning, Mr. Allen said, is that the century of fire suppression has created forests that can readily burn out of control. “We were trapped in this Catch-22,” he said. But such thinning, he said, is nonetheless essential. Restoring forests that have burned is also expensive, Mr. Allen said, and not always effective. After the Los Conchas fire in 2011, the federal government announced a program to plant 5,200 acres of trees – a large number, though still just a fraction of the 150,000 acres of land that were burned. The seedlings, planted during a severe drought, also did not fare well; most died.
And even if the forest service could replant millions of acres of forest land, would those new trees be able adapt to the changing climate of the next 100 years? Those problems have led to high-level discussions about topics like climate adaptation and planting species that might be better suited to emerging environmental conditions — a process that goes by such names as “assisted colonization” and “assisted migration.”
Meg Krawchuk, an assistant professor of geography at Simon Fraser University, noted that such initiatives would be expensive, and contentious as well. Theoretically, she said, it might be attractive to shift species to areas where they might be expected to do well. “Given enough time and limited tinkering from humans,” she said, “one would think an appropriate response to changes in climate and to fire would be proposed by mother nature.”
As the fires in California are showing, conflagrations that threaten people and communities receive the most attention. Better land management can reduce the risk to life and property, but people will always be drawn to the lush beauty of forests, just as they are drawn to risky coastal areas and locations threatened by river flooding. “This is the human condition,” said Dr. Swetnam, standing on his land in the Jemez mountains of New Mexico. While the trees on his hillside property have been thinned, many neighbors have not taken that step — and he knows a major fire would easily race across his handful of acres. It is, however, home, and human beings have long preferred to overlook some obvious risks of natural — and increasingly human-created — disasters “In so many places, we live under the volcano,” he said. “We have a great capacity to set aside the risks we are taking.”