Once the leaves can’t thoroughly photosynthesize, the tree will stop sending its delicious and nutritious sap to those branches. The branches then die and fall off, leaving behind the perfect peeper shape behind.
Learning How to Kill Trees
William R.L. Anderegg A dead grove of trembling aspen in Colorado’s Grand Mesa National Forest.
Scientists trying to understand the future of forests on a warming planet have a strange problem: They do not know how to kill trees.
I don’t mean the trees in their backyards. I would bet that the average climate scientist, especially one who studies forests, is better with a chain saw than I would be.
What I mean is that researchers don’t have a particularly good idea of how trees die in the wild, and as a consequence, they have a hard time constructing computer analyses that can accurately predict the future of forests. These programs are fine at running equations that describe the growth of trees and other plants as a function of climate in a particular area. But when it comes to understanding what causes large scale die-backs, the analyses are rudimentary at best.
I encountered this problem in my long article a few months ago on the future of the world’s forests, in which I talked about forests’ displaying disturbing signs of stress, probably as a result of global warming. This is not a problem climate scientists saw coming, which illustrates the importance of getting a better handle on the life and death of trees. So it comes as good news to learn that one of the researchers I quoted in that article, William R.L. Anderegg of Stanford University, has offered substantial new insight into how trees die.
His paper was written with five co-authors and released Monday by a journal, Proceedings of the National Academy of Sciences. In it, Mr. Anderegg and his colleagues focused on the phenomenon known as “sudden aspen decline” that has killed broad swaths of aspen forest in Colorado and other states in recent years. At least 15 percent of Colorado’s aspen forests have died, and Mr. Anderegg, a native of that state, was as shocked as other Coloradans.
“I played as a kid in some of those aspen groves,” he said in an interview. As his research project for a doctorate at Stanford, he resolved to learn what he could about the situation.
Scientists already knew that sudden aspen decline was a delayed response to a severe drought and heat wave in Colorado in the first half of the decade, but they did not know at a systemic level how the trees were dying. In fact, the question has almost never been studied in any forest during a massive die-back.
Mr. Anderegg and his group spent long hours cutting limbs from dying trees to investigate their physiology, imposing a drought on nursery saplings and wild trees, and taking other steps to pin down exactly what was happening. As they did so, they were weighing the two main hypotheses about how trees die.
One of these posits that in a drought or other stress situation, trees close the pores that allow carbon dioxide to enter their leaves and water to evaporate. That would conserve water, but it would also slam the brakes on photosynthesis, and the tree might essentially starve to death for lack of the carbohydrates it usually creates in its leaves to build and maintain its tissues.
The second idea is that in a drought, trees undergo a kind of hydraulic failure, with the fine tubes that take water from a tree’s roots to its leaves filling up with tiny air bubbles. The idea is that this damages the tubes and can lead to problems for the tree even after rainfall resumes.
It turns out that Mr. Anderegg’s findings strongly support the second idea: that trees die of thirst, not hunger. He found plenty of carbohydrates in the roots of dying aspens, but also clear evidence that their water distribution systems had been damaged and could not recover even after the drought ended in 2004. In my interview with him, Mr. Anderegg compared trees to people: we die of thirst a lot more easily than we starve to death.
James Worrall, a scientist with the United States Forest Service who has also studied sudden aspen decline, described Mr. Anderegg’s paper (he was not involved in the work) as significant and the data as convincing. But he pointed out that much of the field work occurred in 2010, late in the aspen die-back, and the results might or might not be characteristic of what happened earlier.
He also said that insects had probably played a role in killing some trees weakened by drought, and the insect populations may have built up enough to kill even trees that had not suffered much in the drought. “Although certainly not the last word on how aspen stands die, this work is an important contribution that will certainly spur further research,” Dr. Worrall said.
Both Dr. Worrall and Mr. Anderegg pointed out that the two ideas about tree death were not mutually exclusive, so while hydraulic failure appears to be the answer in at least some episodes of sudden aspen decline, that does not mean it will be right in every forest die-back. People can die in many different ways — why not trees? So a lot of work needs to be done on other forest die-back situations to establish whether Mr. Anderegg’s results represent a universal pattern or just one among several possibilities.
Unfortunately, scientists may get plenty of opportunities for such research, given the rate at which forests seem to be dying. Mr. Anderegg is finishing his doctoral work at Stanford, and my guess is that he will wind up in one of the laboratories struggling to do a better job of forecasting the future of the world’s forests.
The Aspen’s Contribution to Ancient Cultures
Headache? BO? VD? Aspen’s got you covered.
Indigenous peoples of North America had a number of uses for the handy aspen, many of them being medicinal and still in practice today. The Blackfeet Tribe would brew fresh aspen buds into a sweet smelling tea, which can then be used as droplets for the eyes when they’re inflamed, or for long term use to ward off blindness, per Galileo . It can also be used as perfume!
Utes used aspens to retell significant events by carving scenes into the bark. As for their medicinal uses of the bark itself, it can act similarly to aspirin due to its related compounds to treat pain and inflammation, per Colorado Encyclopedia .
The aspen also helped the Nuche and other original peoples with a swath of health conditions ranging from heart complications, venereal diseases, to common colds and tummy aches.
Across the pond, Gaelic and Celts had a number of jobs for the aspen, including shields, oars and paddles, splints for busted bones, and flame retardant below the floorboards of homes, per Trees for Life .
Uses of the Aspen
Beyond an enjoyable, elongated, leaf peeping stare.
Other than making alpine landscapes as splendid as tree huggingly possible, you can use aspen for a number of purposes beyond its commercialized products like toothpicks, matches, chopsticks and saunas, per Colorado Encyclopedia .
Sunscreen
Aspens are covered in a powdery film that forms from the bark shedding old cells, per University of Colorado Boulder . It’s safe to use on skin as sun protection if you’re in a pinch.
Fire Starter
Aspen wood tends to burn way too quickly to keep a fire going. But because its fibers aren’t as dense, the bark is porous, and the internal tissue is gaseous, they burn quickly and easily.
Leavening
The chalky residue on aspen bark is chock full of yeast, enough to actually make bread with! Per Survival University .
A nutritious snack
The internal layer of aspens is where the tree contains its own food source, aka cambium . You can peel off the outer bark and eat the stored sugars inside, which are green and stringy like spaghetti. While bitter, it’s sweet enough to stomach as Indigenous tribes would use it in cakes and syrups, per Colorado Encyclopedia .
How to Grow a Quaking Aspen
Just add water! And fire! And avalanches! And…
Like the grand redwoods of California, quaking aspens can only thrive when fire gussies up the soil with sweet, sweet ash. Ironically, they’re also very easily killed by fires, per Colorado State . Rest assured the relationship between aspens and wildfire is quite healthy, as an entire stand can reestablish itself within 20 years.
Aspens like disturbed areas in general. They often go for openings created by avalanches, mudslides, and other situations that stuff nutrients right back into the dirt, per Nature . So essentially, if you knock an aspen down, it will get back up again, ain’t nothing—not even a natural disaster— gonna keep it down. Once established, aspens often live up to 150 years old, per Colorado State .
Unlike the redwoods, aspens need between 6,500 and 11,500 feet of elevation to sprout. Every once in a while the tree can grow up to a whopping 80 feet high, but typically maxes out around 35 to 50 feet. Aspens are also quite thirsty and needy for sunshine, making it pretty competitive with its neighbors. A t least the next door conifers are typically sturdy and stubborn.
Although aspens have quite the checklist for where they choose to settle down, you’ll find them surprisingly all the way down to Mexico and over in China, but only at high altitudes. These factors have made it the only native, widespread tree in Colorado, per Colorado State Forest Service .
