The Tree Coroners

To save the West’s forests, scientists must first learn how trees die.

  • Tree physiologist Nate McDowell, center, climatologist Park Williams, left, and ecologist Craig Allen, right, are studying how trees die to help predict how forests will fare in a hotter future.

    Michael Clark
  • The conifer forests in New Mexico's Jemez Mountains near Los Alamos National Laboratory still bear scars from the 2011 Las Conchas Fire. While the damaging side effects of warm temperatures, from drought to insect infestation to fires, have long been recognized as threats to forests, new research indicates that hotter temperatures alone will kill trees.

    Michael Clark
  • Nate McDowell, a tree physiologist at Los Alamos National Laboratory, pushes trees to the limits of moisture deprivation and heat in his outdoor laboratory in order to learn more about how trees die.

    Michael Clark
  • Researchers in Nate McDowell's research facility at Los Alamos check on a tree inside a chamber that allows them to keep the temperature 9 degrees Fahrenheit warmer than ambient air.

    Michael Clark
  • Trees in the facility are wired to monitor health.

    Michael Clark
  • Some trees in the facility are deprived of water by plastic troughs that divert rainfall.

    Michael Clark
  • Forests in New Mexico have experienced significant die-off in recent years due to wildfire, drought and beetle kill. USGS research ecologist Craig Allen says the damage is a preview of the impacts climate change could have on forests globally.

    Michael Clark
  • USGS research ecologist Craig Allen inspects a dead pine to gain information about how it died.

    Michael Clark
  • Cards attached to trees in the Los Alamos facility resemble toe tags.

    Michael Clark
  • During periods of drought, piñon trees like this one near Tres Piedras, New Mexico, close their pores to conserve energy and water, while junipers under extreme stress cut off circulation to some limbs. These tactics may not save New Mexico's piñon-juniper forests if the warming trend continues, putting old forests around the West – and the world – at risk.

    Michael Clark

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Williams also found a strong correlation between water stress and the forested acreage killed by beetles and wildfire in the past 30 years. "Even if we think of a couple degrees of warming as relatively minor," Williams says, "forests notice a couple of degrees, and they express it by dying."

Because the atmosphere's sponginess is so strongly dictated by temperature, climate models can help predict how fast it will climb. "I considered a scenario where we begin curbing emissions significantly yesterday," Williams says. "Even in that most optimistic scenario, we're looking at megadrought conditions by the 2070s." In other words, even if we began to aggressively control carbon pollution tomorrow, the heat guaranteed by past and ongoing emissions could still devastate Southwestern conifers.

"By 2050, it doesn't matter if it's wet or dry, it's just too damn hot out," McDowell explains. The sense of inevitability that accompanied Williams' conclusions changed how McDowell views his work. At first, he was intrigued by the novel scientific questions involved in tree mortality. "Now I feel like I have a moral obligation to speak up," he says. "We're not just going to lose a bunch of trees, we're going to lose most of them in the Southwest. By 2050, we could be looking at Albuquerque vegetation in Los Alamos," a landscape now surrounded by forests. "Albuquerque has grass and creosote bush."

Such radical changes are unlikely to be confined to the Southwest. A newer modeling effort that Williams and McDowell participated in estimates that the Pacific Northwest could lose 60 percent of its conifers to heat-induced water stress by 2100 – an especially sobering finding for McDowell, whose love of forests was lit at an early age by the old Doug firs on Puget Sound and the Olympic Peninsula. "Can you imagine the Olympic Peninsula without trees?" he asks.

Such a future is hard to imagine. Many Western forests still look healthy, with plump, verdant canopies. But even some of the healthiest-looking stands may already be stressed.

In mid-July, U.S. Geological Survey forest ecologist Nate Stephenson drove me to a long-term forest-monitoring plot in Sequoia National Park, a few hours north of Los Angeles. As we left the shrubby foothills, where one could break a sweat standing still at 9 a.m., the temperature dropped 20 degrees, shadows painted the pavement, and giant sequoia appeared – the titans of the Sierra Nevada. The plot itself was blanketed with ferns, and full of soaring sequoias and lichen-covered sugar pines.

Stephenson helped establish the network in 1982, measuring off the first plots with string. He is wildly passionate about the Sierra Nevada: In graduate school, he designed a thesis project that allowed him to hike 500 "glorious" miles a summer in Sequoia's backcountry. After he earned his Ph.D., he returned to Sequoia with no promise of permanent employment. He wasn't interested in going where the jobs were. Stephenson has now studied this place for 34 years. But it can still surprise him. When he expanded the plot network across different elevations in the early '90s to study how climate affects forests, he says, "It didn't occur to me that by the mid-2000s, we would already be able to detect an increase in tree mortality."

Around that time, Phil van Mantgem, a scientist who worked in Stephenson's shop, began analyzing growth and mortality in the plots. He expected dull results – birth and death rates usually reach equilibrium in old growth – but something peculiar appeared in his data: Background mortality rates – the rate at which trees die in a healthy forest – had doubled. "We thought we did something wrong," Stephenson says. "We tried to make it go away. We couldn't." The only possible cause they couldn't eliminate was the average temperature, which had risen almost 2 degrees F since the 1980s.

Stephenson and van Mantgem ran the same analysis for old-growth forests West-wide. They found the same pattern: At many high-, mid- and low-elevation plots, from California to Idaho, Arizona and Colorado –– even in Washington's Hoh Rainforest –– conifers were dying at double the rate they used to.

"Every year, you expect some people to die in your hometown," Stephenson analogizes. "If that death rate started to creep up slowly, it doesn't create a dead landscape all at once, but you would sit up and go, 'Oh my gosh, what's happening?' "

As in so many ecological stories, what's happening is complicated. "There is something tied to temperature that is probably responsible for what we're seeing," says van Mantgem. But exactly what that something is may vary from forest to forest. At mid-elevations in the Southern Sierra, where the sugar pines and sequoias live, the increase in mortality seems to be tied primarily to a temperature-induced increase in the atmosphere's demand for water – the same thing Park Williams expects to happen more in the Southwest. But at higher elevations and in wetter forests, like the Hoh, warmer temperatures may instead be favoring the fungi and insects that attack trees.

What the uptick in background mortality ultimately portends is also uncertain. But the forests' response to mild temperature increases, van Mantgem says, indicates their vulnerability. "(The results) might be telling us that they have chronic stress as things get warmer. Then if you get an acute stress, like a severe drought, it might be something that hits you over the head." That is, it might be something that takes out a centuries-old forest in a year, or two – or, in the case of a forest fire, overnight.

Jonathan Day
Jonathan Day says:
Dec 10, 2013 09:37 AM
An entire article about tree mortality and no mention of the extremely high tree densities that exist across the entire west, particularly in dryer ponderosa pine/juniper types. More trees are dying because there are vastly more trees on the western landscape now. To decrease tree mortality, increase individual tree vigor. Increase individual tree vigor by reducing competition for resources (mainly water). Reduce competition by reducing the number of trees on the landscape. The lack of frequent disturbance across most of these forest types is the real issue. Re-establish disturbance regimes and I believe we will have much more resilient forests. Hopefully these researchers are seeing the forest for trees and are thinking about these physiological plant responses in the context of larger landscape level processes.
Cally Carswell
Cally Carswell says:
Dec 10, 2013 10:36 AM
Thanks for your comment, Jonathan. Increasing resiliency through mechanical thinning and prescribed burns is addressed at the end of the story, and the problem of overgrown forests as it relates to fire is addressed in one of the sidebars. The scientists whose work is discussed in the story are definitely aware of the suite of factors that have made our forests vulnerable. The focus of this story was primarily on the vulnerability created by warming temperatures, which research is showing will be enough to zap many of our old trees even if public agencies had the resources and political will to undertake aggressive, landscape-scale thinning tomorrow. The scientists I spoke with believed that this kind of management could buy Western forests some time, but probably couldn't save them if we remain on our current warming trajectory. It should also be noted that some of the forests outside of the Southwest where upticks in background mortality have been documented are not considered artificially dense. Thanks again for reading.
David & Louise Stonington
David & Louise Stonington says:
Dec 12, 2013 09:31 PM
Thanks for the excellent research and report. We cannot continue to base our energy policies on fairy tales. It is time to put a carbon tax on oil, coal and natural gas to gradually reduce investment in these dangerous polluting fuels. With revenue from the tax returned to households, we would see lower energy costs for 60% of the population, and give others an incentive to use energy more efficiently. Citizens Climate Lobby is a non-partisan organization encouraging citizens to lobby Congress for legislation that lets clean energy strengthen our economy and stop the warming that is killing our forests.
Kirk Hohenberger
Kirk Hohenberger says:
Dec 20, 2013 10:30 AM
The true problem, is not putting a carbon tax on the burning of carbon fuels. But a tax on having kids. Population is the driving force behind all our problems of climate change, animal and plant Extinctions, Resource depletion, and overall unsustainability that seven billion people effect.Of coarse taxing people for every kid they have would not be popular, so I guess get ready for twelve billion people .
Kirk Hohenberger
Kirk Hohenberger says:
Dec 20, 2013 12:38 PM
How many people would be the number , living as we do now, burning fossil fuels, and driving cars, And Emitting CO2 into the atmosphere that the forests could and would be able to absorb ,and levels would remain neutral ,or constant, wouldn't that be the number of people on the planet that would be Sustainable? How many would that be?does anyone care?Its much easier and less Disconcerting to not think or worry about it.
Steve Laster
Steve Laster says:
Dec 20, 2013 02:48 PM
You're absolutely 100% correct!  Too many people (TMP) is the number one cause of all the planet's woes.  And it's this ecological principle known as carrying capacity that we don't seem to understand.  We focus on symptoms like food shortage, air pollution, poverty, overgrazing, energy shortage, I could fill this page with environmental problems that all stem from the same cause - TMFP.  
Put 1,000 cattle in a 1,000 acre BLM pasture, and in time it'll be a wasteland.  Put 300 fish in a 5 gallon tank and in no time all the oxygen is gone.  And here's one we see all the time.  Two pet horses in a big backyard, and the owner has to buy expensive hay because the land can't support the demand.   That's what exceeding carrying capacity is all about. It's right under our noses and no one seems to get it.  The earth is just a big pasture; a few more billion people added to an already taxed carrying capacity and the results are inevitable.  Maybe the pope has some good ideas.  Maybe the environmental media, like the one we're reading now, ought to focus on this and introduce the idea of carrying capacity to the masses.  We talk about incentives for good grazing practices, how about incentives for not having kids?!  
Kirk Hohenberger
Kirk Hohenberger says:
Dec 20, 2013 06:26 PM
 First you have to talk about it ,then people have to acknowledge it ,then we have to start doing something about it .right now I never see it in print or mentioned. How can something as wonderful and natural as having kids ultimately be a bad thing. How can mankind's incredible success ultimately lead to his demise? Ironic isn't it? David Attenborough wrote about 7 billion people becoming Earth's plague. All of us would think 7 billion large mammals were a plague if we weren't talking about people, and bias about ourselves.We Simply can't see it, when its us, and simply too many of us.