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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The piñons died during what Breshears dubbed a "global-change-type-drought." It's impossible to blame any particular weather event on climate change. Still, the drought was a glimpse of the future, when droughts are predicted to be hotter and drier. Breshears and his colleagues found that it took 15 months in extremely dry soils to kill the piñons around McDowell's office. The heat, they believed, had increased the overall death toll by siphoning more water from soil and plants, though they couldn't yet prove it.

Dramatic changes in Southwestern forests had been expected – eventually. Desert edges are already marginal tree habitat, and were predicted to become especially vulnerable to the future's hotter, more intense droughts. Still, the amount of dead wood around Los Alamos was startling. Piñons didn't die only at the ecological boundary between woodland and grassland, the dry end of their range where Breshears and others believed climate change impacts would first become visible. Instead, piñons died almost everywhere they grew.

No community can comfortably afford to lose its forests. Besides being nice places to hike and ski, forests provide food and shelter for birds and wildlife. Leaves scrub the air of pollutants humans saturate it with. And forests shelter winter snow, the source of most Westerners' water supply, filtering it to rivers and streams in spring.

More important from a global perspective is the fact that forests ingest an estimated quarter to a third of the carbon dioxide released by fossil fuels, effectively keeping the earth's burner turned down. When trees die, they not only stop absorbing CO2, but they also decompose, gradually releasing the carbon stockpiled in their wood. If enough forests collapse, the flame on the planetary heating element could turn from "low" to "high." Instead of slowing global warming, forests could start to make it worse.

Computer models either don't account for future tree death caused by climate change, or they do so simplistically. These shortcomings worry scientists, and with good reason: The most troubling thing it could mean is that the dramatic forecasts the models currently produce – the ones predicting not only a warmer climate, but also the fundamental transformation of life on earth – are understated.

Before scientists can more accurately predict our future climate, they have to complete a simpler task – at least, one that sounds simpler. They need to understand, in mechanistic detail, how trees meet their end.

After Nate McDowell spent a few years studying the inner lives of junipers, his attitude toward the trees softened. What junipers lack in majestic height and open, shady understories, they make up for in pluck and perseverance. McDowell, a spry, 41-year-old former endurance runner, began to appreciate these qualities. "They're just so tough," he says. "You have to respect someone who's tough."

Juniper doesn't cower in the face of drought. Even when extremely short on water, it doesn't close its stomata – the tiny pores on its needles that regulate the tree's basic bodily functions. Stomata allow trees to consume carbon dioxide and photosynthesize. They also let water escape, creating the tension that pulls water upward through the tree's circulatory system. If there's too little water in the soil, a tree's pipes can fill with air and break.

To prevent this, many trees close their stomata during droughts. Juniper, with its deep roots and sturdy build, doesn't. When extremely stressed, it begins severing the water supply to entire limbs – reducing the amount of water the whole tree needs to survive. This is why smooth, naked branches – the desert's version of driftwood – often protrude from living junipers otherwise covered in stringy bark and sharp needles.

Piñon is more cautious, slamming its stomata shut during drought. Perusing data Breshears and another colleague collected during the drought, McDowell had an epiphany: For a year, the piñons that died endured a level of water stress that should have kept their stomata shut. Photosynthesis is to trees what cooking is to people. It's how they eat. In trying to protect themselves from dying of thirst, he thought, maybe piñons had starved to death instead.

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.