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The Last March of the Ents

Climate change could soon strip the Southwest of its forests.

southwest forests climate change

By John Briley | Photograph by Dave Cox

When hurtling through the woods, a maxim of tree skiing and mountain biking is to focus not on the trees, but on the spaces between them. But those spaces may soon become so vast—in the U.S. Southwest, and perhaps far beyond—as to render the advice moot.

Nearly three-quarters of the coniferous forests in the Southwest will be dead by 2050, and nearly 100 percent will be gone by 2100 due to climate change. This bleak forecast comes courtesy of a study, titled Multi-scale predictions of massive conifer mortality due to chronic temperature rise, published last December in the journal Nature Climate Change. The study, the latest in a mounting body of evidence prognosticating doom for pine trees in Arizona, New Mexico, and parts of California, Utah, Texas, and Colorado, also predicted death for more than half of the coniferous forests in the Northern Hemisphere by 2100.

If this seems inconceivably dark, it’s not: Climate change has already altered our planet and its ability to sustain the type of organisms that evolved to survive on the cooler and often wetter planet our parents and grandparents knew. Many of us choose to ignore what climate change has already wrought. It’s easy. Ocean acidification is hidden to all except scuba divers and snorkelers. Even for skiers, it’s hard to see our shorter winters. And unless you’re standing in a tract of beetle-killed forest, you might be hard-pressed to envision the Southwest without conifers.

A treeless moonscape certainly wasn’t on my mind as I ran the pine-scented trails of the Santa Catalina Mountains northeast of Tucson last March, nor while despoiling powder in the spruce and fir forests of Taos, or white-knuckling mountain bike trails in Colorado’s Elk Mountains in years past. To mountain folk the world over, forests are our sanctuaries. They’re where we go to escape industrialization, exert ourselves, and take in the grandeur of it all: snow on a hemlock; a cedar in sunlit relief. A piñon pine twisting skyward from its red-rock anchor is far more striking than any vigilantly refined bonsai.

Hoping for an optimistic loophole in the data, I call David Breshears—not to be confused with the filmmaker David Breashears, this Breshears is one of the Nature Climate Change study authors and a professor of natural resources at the University of Arizona, Tucson. He offers me no false assurances: “What is so powerful is that we’ve been looking at this since the mid 1990s, using a variety of models in different places, and we keep arriving at the same answer,” he says. “I have enormous confidence that we’re going to see much more forest mortality in the future.”

As temperatures rise from climate change, the increased stress means that trees will no longer be able to survive seasonal droughts of a few months or less, which are common in the Southwest and forecast to become still more frequent. Couple that with long extreme dry spells, such as one that peaked in the region in 2002, and drought and heat will take out more forest than they have historically.

This jibes with the 2014 National Climate Assessment, compiled by more than 300 experts and extensively peer-reviewed, which predicts that future droughts in the Southwest will be “substantially hotter…more frequent, intense, and longer lasting” than in the past. That report projects that—with continued growth in global emissions—the region’s annual average temperature will rise by up to 5.5 degrees Fahrenheit by 2070, and by up to 9.5 degrees Fahrenheit by 2099.

To better understand this damning forecast, I drive Highway 77 north from Tucson to Biosphere 2, the vivarium-turned-earth sciences research center, where forest ecologists conducted a prior landmark study on climate-driven tree mortality. There I meet Greg Barron-Gafford, Ph.D., an assistant professor of biogeography at the University of Arizona, and a colleague of Breshears. He explains the basis of the study: “After the drought of the early 2000s, we looked at aerial images to compare the tree die-off to a similar drought we had here in the 1950s,” said Barron-Gafford.

The images from the 1950s showed tree death confined to the lower-elevation edges of the forest—essentially, trees died where the pre-drought soil was already the driest. The 2000s drought, conversely, cut a swath up to elevations at which researchers would have expected wetter soils to protect trees. It was the worst piñon pine die-off they found in 103 years of regional forestry data.

With heat now identified as the main suspect in the increased die-off, Barron-Gafford, Breshears, and others brought 20 mature piñon pines into Biosphere 2 and set up one group at the ambient temperature—the mean highs and lows—for 2008. The second group of trees was subjected to average daily temperatures seven degrees Fahrenheit warmer than those 2008 means. The climate models show such an increase in temps will be the norm in the Southwest by the final decades of this century. “We then cut off their water and waited,” Barron-Gafford says.


Plants respond to drought by withholding water from their extremities, which is why a neglected houseplant droops. As drought persists, plants close tiny holes, called stomata, in their leaves (or needles). Stomata are a plant’s pores, allowing water from within to escape and cool the leaves much like sweat cools a human. But the stomata are also the sole entrance for the plant’s main food source, carbon dioxide, which is needed for photosynthesis. Closing the stomata prevents water loss, but also blocks CO2 from entering. Over time, this triggers a combo punch of overheating and a condition called carbon starvation. But that’s not the end of a drought-battling plant’s ailments. Trees drink from the soil through straw-like arteries, called xylem. When extreme thirst sets in, they slurp extra hard, and eventually the xylem collapse like a straw in a spent milkshake. Parched and starving, trees can’t produce the resin that protects them from pests, disease, and fire. Death follows.

The control group of piñons at Biosphere 2 with the temperatures set to 2008 levels survived roughly 25 weeks of drought, while those exposed to the hotter temperatures of our future died at around 18 weeks. It’s worth noting that all of the trees in the warmer group died before any of those in the other group. Based on the length of past Southwest droughts, the authors concluded that the predicted temperature rise would bring a fivefold increase in the frequency of tree-killing events. That initial work was published in April 2009 in the Proceedings of the National Academy of Sciences.

Fire wasn’t factored directly into the research discussed above, but according to the U.S. Forest Service, climate change has led to fire seasons nationwide that are on average 78 days longer than those in 1970. Fire is a natural part of the Southwestern forest ecosystem. But an increase of fires, droughts, and hot temperatures puts further strain on the trees. Piñon pine take 40 years or more to reach maturity. Moving forward, they likely won’t get that growth window. “A killing event every 20 years is not going to allow the woodland to get reestablished,” says Breshears.

Such a die-off in the Southwest and beyond would not only release more carbon back into the atmosphere by way of burning forests, but it would fundamentally alter the planet’s ability to capture future carbon emissions—further accelerating climate change. According to the National Climate Assessment, “In 2011, U.S. forest ecosystems and the associated wood products industry captured and stored roughly 16 percent of all carbon dioxide emitted by fossil fuel burning in the country.”

For the Nature Climate Change study published last December, a team of 19 coauthors led by Nathan G. McDowell, Ph.D., a forest ecologist at Los Alamos National Lab in New Mexico, studied piñon and juniper trees over five years in three large, controlled plots in New Mexico. They too simulated drought conditions, which killed about 80 percent of mature piñons and a quarter of junipers. They then plugged data into two climate models—a regional one, which predicted almost total mortality for the Southwest’s evergreen trees by the end of the century, and a global model, which forecast at least a 50-percent die-off of the Northern Hemisphere’s conifers by 2100. Even when the researchers modeled a conservative temperature rise of two degrees Celsius over pre-industrial levels, they found those dire effects would be delayed by only 10 years. 

For a less fatalistic take, I turned to Park Williams, a bioclimatologist at the Lamont Doherty-Earth Observatory of Columbia University, and also a coauthor of the Nature Climate Change research. Williams points out that the models used were simple and don’t account for a number of important factors, such as the region’s complex topography. One aspect the study doesn’t have a handle on is how trees will respond to increased CO2 in the air—a byproduct of climate change that in theory at least, should be good for plants. “We don’t really understand how trees are going to respond to increasing CO2 in the atmosphere,” he says, which, as noted, is the key fuel for photosynthesis. “Species with a consistent source of water may be OK.” Still, Williams concedes, “It’s really hard to find any positive influence of CO2 in the Southwest. It’s already warm and dry, and we don’t see places where forests respond well to drying.”

A forest, of course, is more than just a bunch of trees. It’s a safe house for ecosystems, habitat for creatures, the webbing that keeps soil from eroding, and a carbon sink that by its very existence can limit planetary warming. Those benefits, says Breshears, are too valuable to sacrifice without a fight. “People are starting to see what [climate change] can do, but it’s also on the slow end of what might eventually happen. So we need to act now,” he says. One way to do that: As a nation, we need to demand laws mandating cuts in industrial-scale emissions. “We will lose a lot [of forest] regardless, but we could preclude seeing the worst of this,” says Breshears.

In the meantime, ignore the spaces and take a long, hard look at those trees while they’re still standing.

From our High Summer 2016 issue. 

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