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

There are few better places than Frijoles Mesa to study the mortality of trees. This tongue of land lies partly within the grounds of Los Alamos National Laboratory in northern New Mexico's Jemez Mountains. To the west rises Cerro Grande, a mountain riddled with the charred skeletons of fir and pine trees. To the southwest are the lingering scars of another fire, one so intense that its heat alone killed trees that weren't consumed by the flames themselves.

The mesa itself is an exceptionally tough place to be a tree, even where the land has escaped conflagration. This summer, many ponderosas were so short of water that their weakened limbs snapped like pretzel sticks. The trees that sit behind a padlocked gate off State Road 4 were also struggling. This is tree physiologist Nate McDowell's outdoor laboratory. Here, he's enclosed piñon and juniper trees in transparent silos, cranked up the heat and deprived many of water – in order to watch them die.

McDowell spent his early career studying the towering conifers of his native Pacific Northwest and came to Los Alamos in 2003, eager to begin a U.S. Department of Energy job that would allow him to set his own research agenda. But looking out his office window at New Mexico's characteristic piñon-juniper woodlands, he had second thoughts. "This is not a forest," he scoffed. The stout, pear-shaped junipers – one of the most common species here – resembled ill-kept hedges more than trees, all arms and twisted torsos, barely showing any leg. "They were like a weed to me," he remembers.

Like weeds, junipers are durable. Those outside McDowell's window were still green, but the piñon around them were dead. During the deep drought of 2002 and 2003, piñon died throughout the Southwest in historic numbers. Had the Old Testament told stories of forest die-off, as it did of floods, the carnage around Los Alamos would have been called "biblical": More than 90 percent of the area's piñon succumbed. "What a bummer," McDowell sulked. "I'm a tree physiologist, and the trees are all dead. What am I gonna do?"

At first, the cause of the trees' demise seemed obvious. The punishing drought badly weakened them, and when beetles bored through their bark, the trees couldn't muster enough sap to pitch them out. Once inside, the beetles mated, multiplied, dug tiny tunnels and spread a fungus that cut off the flow of water and nutrients, killing the tree.

But Dave Breshears, a University of Arizona professor and arid lands ecologist who had studied the woodlands for years, suspected that the truth was more complicated. During the 1950s drought, tree death seemed less extensive, even though that drought was longer and drier than the more recent one. What was different about this drought was temperature: It was a degree or two hotter.

Breshears' observations inspired McDowell to take a second look at the struggling forest. It's common knowledge that trees die during and after a drought, McDowell says, but "nobody can predict where it will happen, when it will happen, what trees it will happen to. That means we don't understand it. That was exciting to me – there's a science question there."

Why do trees die? It's a deceptively simple question in urgent need of answers: Trees are dying at alarming rates not only in the Southwest but in Colorado, the Northern Rockies, Alaska and elsewhere. This summer in northern New Mexico, even junipers began to expire in droves.

It might seem surprising that, in 2013, we don't know how trees die. We understand tree growth so well that we can decipher its code – tree rings – and reconstruct droughts thousands of years in the past. So why is tree mortality such a mystery?

"There has been a long tradition in plant science where, if your plant died during your experiment, you were bummed out," McDowell explains. "It was like, 'Ugh, we've gotta start over.' The question was never, 'Why did it die?' " Besides, he adds, tree death didn't seem particularly pressing. "I think people inherently look at trees as these stable things in our lives, like mountains. We didn't know there was a problem."

Western forests are confronting new versions of familiar foes. In the 1990s, a series of warm winters and summers in south-central Alaska allowed bark beetle populations to explode and kill millions of old spruce trees. Beetles gained similar strength in the Rockies during mild winters in the late '90s and early 2000s, killing not only their usual victims but also entire hillsides of ancient whitebark pines, which live at altitudes once too frigid to support the insects.

Farther south, piñons were also attacked, but by a beetle that, unlike its fellows in the Rockies, typically preys only on the weak. Here, scientists believed the industrious insects were less the cause of death than the final straw: a strong shove to trees with one foot already dangling over the cliff.

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.