Vanishing Views

Will dwindling aspen stands fade fall colors forever?

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I sit alone among the clones, which loom overhead — quivering weirdly. But I am not scared. Truth be told — I do so love these clones.

Especially when the aspen turn all gold and trembly in the fall.

photo

Tom Brossart photo

The slopes of the San Francisco peaks and the top of the Mogollon Rim are among the few places that remain in Arizona to see the brilliant fall colors produced by aspen groves.

What does scare me is the dramatic decline in aspen groves across the Southwest — especially in Arizona where some studies suggest aspen have declined by 95 percent.

One of the defining trees of fall in the high country, aspen mostly reproduce by growing clones from a network of roots that can cover a whole mountainside — making them by some measures the largest single organism on the planet.

An above-ground aspen tree grows quickly and fades away in a century or so, but the true plant continues to grow beneath the surface, sending out new, leafy, white-trunked shoots for thousands of years. If you consider that great mass of roots the plant, then aspen may be among the longest-lived living things on the planet. One study in Utah put the age of a grove of clones at 80,000 years. By contrast, redwoods can live for 4,000 years and bristlecone pines for 5,000 years.

So the precipitous decline in aspen groves offers both insight into complex ecosystems — and the strangled cry of a canary in a deep shaft filling with gases.

A slew of recent research both documents the trend and raises fascinating questions.

Aspen have declined by 50 percent in Colorado, 60 percent in Utah and 95 percent in Arizona, according to a recently published study by researchers from the Forest Service’s Rocky Mountain Research Station. Study plots in many areas have shown a 50 percent decline in recent decades, with most of the stands giving way to thick forests of pines.

No one knows how to fully account for the great die-off.

Most experts blame a century of fire suppression and grazing by both cattle and elk. Aspen are among the first trees to come back after a fire clears away the thick, dark pines that would otherwise shade out the sun-loving aspen. After a fire, the flood of sunlight prompts the sprouting of a new generation of fast-growing clones from the buried roots, protected from the effects of the fire.

In a system not altered by logging, grazing and fire suppression, the aspen sprout quickly after fire or disease opens a clearing in the slower-growing pines. The aspen luxuriate in the sunlight for a century or so before the relentless pines close in. In a system where fires burn through every five or 10 years, the aspen always had clearings to exploit.

Unfortunately, the same qualities that help aspen adapt to regular fires now work against the survival of these ancient groves of clones. The Rocky Mountain Research Station researchers say an aspen tree can reproduce by releasing seeds — but that works only once every 100 to 200 years. That means if an aspen grove dies out, it can’t fill in easily from other neighboring colonies or trees — like ponderosa pine do routinely with their reliance on pine cones to sprout seedlings.

Aspen experts blame a variety of other factors as well. For instance, deer and elk love to browse on aspen seedlings. So some researchers think the explosion of the elk populations in the Southwest in the past 50 years may have also pushed the aspen toward extinction.

That theory took a hit recently when researchers in Yellowstone found to their surprise that the growing number of wolves in the park hadn’t helped out the dwindling aspen.

The U.S. Geological Survey researchers concluded that the decline of aspen in Yellowstone did coincide with the population explosion in the elk herds after the removal of the wolves in the 1890s. However, none of the dwindling aspen stands had staged a come back after wolves returned — reducing the number of elk by 40 percent.

Only fences that kept elk out altogether allowed some of the aspen stands to start producing sprouts from those buried roots that grew into new trees. The researchers concluded that rising elk numbers played the key role in the decline of the aspen, but the wolves haven’t eliminated enough elk yet to make a difference.

Another study implicated drought, rising temperatures and a host of plant pests and diseases unleashed by the trends. One careful study in southwestern Colorado published in Forest Ecology and Management documented the loss of 10 percent of the aspen in one area in less than two years. The study found that a plant canker and three types of beetles played a role in killing off the biggest trees at a dismaying rate. The aspen on south-facing slopes at lower elevations suffered the greatest loss, which implicated the effects of drought and rising temperatures.

Losing the aspen will also result in a big decline in wildlife.

For instance, one study showed that when pines crowd out aspen, it decreases water yields in streams and water available to other plants by at least 5 percent. In many areas, that’s like reducing annual rainfall by about 5 or 6 inches. Such changes can account for why streams go dry as the forest changes, just as most of the creeks flowing off the Mogollon Rim have flow now only right after a big rainstorm.

An aspen grove provides an island of diversity in a sea of pines. Aspen groves rank just behind streamside riparian areas in the diversity of plants, insects and animals they shelter. Converting a slope covered with aspen to pines or sage results in a 67 percent drop in undergrowth, according to the Forest Service study. Aspen stands harbor far more species of birds, bats, insects, snails, lichen and other creatures than the surrounding pine forests.

The alarming decline of the aspen throughout North America has prompted a rush of research, which has revealed unexpected complexities.

For instance, scientists still can’t fully explain the wonderfully hypnotic tremble of the quaking aspen’s leaves. The leaves have a distinctly tough, flexible, flattened petiole attachment to the stem. This allows the leaves to flutter and rustle in the slightest breeze. Scientists think that this lets sunlight fall even on the leaves far from the leading tips of the branches. This perhaps accounts for the aspen’s rapid growth rate, allowing the moving leaf surface to scoop up more carbon dioxide. Perhaps it prevents the high winds of the high country from stripping off the leaves. Alternatively, the trembling of the leaves might also play some role in extending the growing season in the frost-prone high altitudes where the tree thrives — and may also play a role in defending the tree against insects.

photo

Tom Brossart photo

Rising temperatures may be driving aspen out of lower elevations.

One recent study also found unexpectedly complex chemical defenses against insects in those lovely, tremulous aspen leaves. Most plants have a love-hate relationship with insects. On the one hand, they produce nectar and pollen to attract insect pollinators. On the other hand, they fill their leaves with chemicals to prevent insects from turning them into salad.

Researchers took a look at the details of this trade off in aspen, in a study published recently in the American Journal of Botany. The researchers figured the tree would offer up some leaves to lure insects, but load other leaves with chemicals to make sure the insects didn’t gobble them up.

Instead, they found that the tree loaded up the youngest, most tender leaves with both the insect attracting “nectaries” and the defensive “phenolic glycosides.” As the leaves grew older, the levels of both defensive chemicals declined, leaving the leaves increasingly vulnerable.

Another pair of studies demonstrated the complex response of the trees to rising levels of carbon dioxide in the atmosphere. Since 1960, the average amount of carbon dioxide in the atmosphere has risen about 26 percent, according to the National Atmospheric and Oceanic Administration.

One key study suggested increased carbon dioxide could dramatically increase aspen growth rates. The growth ring study of a grove of 1,000 aspen trees found that steadily rising carbon dioxide levels in the atmosphere in the past half century boosted growth rate of the trees by 50 percent. By contrast, neither oaks nor pines grew faster as a result of the rising carbon dioxide levels.

So why aren’t aspen thriving as the carbon in the atmosphere rises?

A partial answer came in another study involving a 38-acre experimental forest dominated by aspen and birch trees in Wisconsin. In that case, researchers have been pumping extra carbon dioxide into the tree canopies since 1997 to mimic levels of atmospheric carbon dioxide forecast for the second half of this century. Both the birch and aspen responded with 45 percent faster growth rates — at least initially.

However, after several years — the aspen began to falter while the birch continued to boom. What gives?

The answer could lie in the differences in the ability of the two species to scavenge nitrogen from the soil. The birch increased their uptake of nitrogen from the soil by 68 percent, the aspen by only 19 percent.

As a result, the researchers concluded that birch trees will overwhelm aspen trees by avoiding a nitrogen bottle neck.

So I sat on the hillside letting all that rustle about in my head, like the tremble of golden leaves.

I think that I would love aspen, even if they didn’t turn such a soul-stopping yellow in the fall. But this lurid defiance of winter puts me over the top. The flush of color results when the tree seals off those frail flutters as temperature drops. This prevents frost damage to the thin leaves from causing bacterial or fungal infections. Denied water from the roots, the fragile green chlorophyll breaks down quickly. This leaves a mix of other, tougher compounds in the plant behind, which turns the leaf yellow.

So I listen to the whispering of the leaves — an eager eavesdropper. I wonder how long this particular white-trunked world of filtered light and luminous green has existed — sprouting clones from its vast, hidden root until it covered the whole hillside. I note the lack of saplings — the dying giants and feel a flush of sorrow.

Did this now faltering grove take root before the first humans made their way to North America. Did mastadons and ground sloths nibble its first clones?

I cannot bear to think that they are dwindling, with their inexpressible tremble of beauty. I cannot bear to think that we have played a role in that great dying.

So instead, I will set all that aside and sit here and listen to the music of the quivering leaves, safe and happy among the clones.

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