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 Southwest has already experienced such sudden shocks. One of the most dramatic occurred just a few miles from McDowell's outdoor lab. There, in 2011, an aspen tree fell onto a power line, sparking a fire stoked by hot, dry weather and drought-seasoned fuels that burned 43,000 acres in its first 14 hours. The Las Conchas blaze raged through pine and fir canopies on the Jemez Mountains' eastern flank, killing entire stands. Some are unlikely to regenerate, ever, and are already being replaced by oak and locust shrubs. The worst-hit and driest areas have yet to sprout much of anything at all.

"That is the land manager's worst nightmare," Stephenson says of the Las Conchas Fire. "The biological potential has been lost, there's going to be soil loss, erosion, the trees' seed source has been killed off. That was not an easy transition."

If the warming trend continues – as it surely will without heroic intervention – Stephenson hopes land managers can slow the pace of change and influence its outcome for key forests in the Sierra. The Giant Forest in Sequoia National Park, for instance, contains the most massive tree on the planet – known the world over as the General Sherman. "It's a place of high social value," Stephenson says. To help protect the Giant Forest from sudden death in an insect outbreak or a big wildfire, managers can thin trees and set small, controlled fires to reduce competition and increase the resilience of individual trees. If research begins to show that certain species can't survive the future climate, Stephenson says, managers may decide to let those trees go, assisting their migration to more hospitable terrain and perhaps planting new species in their place.

So far, though mortality among many of the area's tree species is increasing, the giant sequoias seem unchanged. There's too little information to draw strong conclusions about the whole population, but preliminary studies suggest that, in their prime habitat, the trees are actually thriving, benefiting perhaps from an extended growing season. Coast redwoods also appear to be growing vigorously, perhaps basking in extra sunlight as coastal fog declines.

Of course, this trend could change. These two iconic giants slurp more water than any other trees on earth, and future changes in water supply could hurt. Desperate measures to save them – like installing sprinkler systems – are already being discussed. "Mortality of big trees is a one-way street," says Stephenson. "You can't replace them once they're gone."

Models are useful in planning for the future, but we needn't wait for them to be perfected in order to start grappling with the effects of climate change on forests. The mechanisms and trends scientists like Nate McDowell, Park Williams and Nate Stephenson are uncovering are already in motion – and gaining momentum. The future is all around us, plain to see.

That's especially true in the Jemez Mountains. USGS research ecologist Craig Allen has spent his career in this landscape, never growing bored. Allen is a whip-smart man with boyish, straight-cut bangs whom McDowell calls "one of the godfathers of tree mortality." Like Stephenson, a colleague and friend, he's studied the same place for 30-plus years.

Change itself does not surprise him. But some of the changes he's seeing now are painful. "Because trees live longer than we do, we tend to view them as timeless," Allen says. "It's unsettling when these landscapes flip overnight." Asked about his favorite tree species, Allen deflects the "unfair question," but acknowledges that he loves old trees the most. The longer a tree lives, the more visible its history becomes – in gnarled bark, fire scars, and in the case of conifers, flattened tops. "You can feel this sense of endurance," Allen remarks. "In human terms, we would call it wisdom."

Allen has spent a lot of time thinking – and publishing papers – about the global significance of the rapid changes in the Jemez. "I don't want to overstate the lessons of the Southwest for the rest of the world," he says. "But it's a preview of what could happen." Drought won't kill all of the world's trees; some forests may get wetter and grow better. Still, the expected increase in global temperature is so extreme that it could easily convince most trees that they've moved to a new planet, and outweigh the potential upsides of climate change for plants, such as more carbon dioxide to consume.

Forests' widespread vulnerability is already evident. A hot, dry spell in Europe in the mid-2000s wiped out oak, fir, spruce, beech and pine. Drought has picked off aspen, jack pine, and black and white spruce in Western Canada's boreal forest – at both high and low elevations. A once-in-a-century drought in 2005, followed by another five years later, killed vast numbers of trees in the Amazon rainforest.

"One thing we are going to lose, and it might be in most places later this century, is old trees," Allen says. "Even if the system can still grow abundant vegetation, the historically dominant old trees are dominant because they're tuned to that historic climate window – which is already not the climate that we're in."

As grim a prospect as that is, it is also an opportunity to make the forests we still have more resilient – and to start doing so now. It is possible, even in the Southwest. The region goes through natural wet and dry cycles, and within the next 10 years, says Allen, the current dry spell is likely to let up. That will ease the pressure on trees from wildfires, beetles and the weather itself, and allow land managers to more safely thin forests with low-intensity fires. Combined with landscape-scale mechanical thinning, these measures could soften the blow of the next drought, and reduce the risk of future catastrophic fires or insect outbreaks. If managers want to plant trees – perhaps more drought-hardy species from other places – the wetter cycle will give seedlings an opportunity to establish themselves.

Most of the old trees we love may still perish, but there are better and worse ways for that to happen. When landscapes change incrementally, they are more likely to maintain species diversity, soil health, and basic functions like erosion control. Past droughts that killed trees in what seemed like apocalyptic fashion, Allen says, in fact caused a gradual reshuffling of the landscape. The 1950s drought, for instance, killed most ponderosa at the dry end of the tree's range, but not all. It didn't leave a treeless landscape, prone to the kind of erosion and soil loss that follows severe wildfire. "It didn't leave a desert," Allen says. Piñon and juniper replaced the dead ponderosa. Going forward, change of this sort may represent a best-case scenario. The question is: How fast can new trees colonize landscapes as the old trees die?

If his daughter or his twin boys have children, Allen wonders, what will they want to know about the forests he knew? He is brainstorming a new project: documenting the size, age, diversity and three-dimensional structure of trees in old forests, archiving tree-ring samples and the histories they hold, and recording the sounds of birds and wind blowing through the canopy. He also hopes to document what ancient forests mean to people by involving artists, poets, ethnographers and the elders of cultures for whom forests are important. Such a project, he is starting to think, might be just as important as scientific research. Someday, even if only through a virtual experience, his grandchildren could still walk through the Jemez Mountains' ponderosa or the Olympic Peninsula's rainforest, and hear the whisper of the breeze through the treetops.

Jonathan Day
Jonathan Day Subscriber
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 Subscriber
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
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 Subscriber
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 Subscriber
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
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 Subscriber
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.