As I did stand my watch upon the hill, I look'd towards Birnam, and anon, methought, the wood began to move . . . Within this three mile may you see it coming. I say, a moving grove. -- Macbeth, Act 5, Scene 5
“Trees aren't supposed to do that,” says University of Colorado ecologist
Tim Seastedt as we climb through 11,000 feet atop Niwot Ridge, high above
Boulder, Colorado. “Except in Macbeth, trees are supposed to stay pretty
much at home.”
But up here, Seastedt says, where the average annual temperature barely
nudges above freezing, where late winter air masses avalanche over the
continental divide at a hundred miles an hour — up here, where the harshest
alpine conditions in North America have been recorded — tree-like islands
of fir and spruce actually do creep across the tundra.
The movement happens as wind and cold kills living tissue on the exposed,
windward side of a tree island while new shoots and roots sprout in the
protected, downwind direction. The net movement downwind of about 2 to
4 centimeters a year may make a slug look like a thoroughbred by comparison,
but on a time scale of centuries, the movement adds up. If it were possible
to view a time-lapse movie spanning hundreds of years in just a few minutes,
the groves would be seen to move across the knolls and swales of the alpine
tundra like ships pushed by the wind across a storm-tossed sea.
The mobile tree islands are a form of krummholz, a word that means “twisted
wood” in German. Krummholz is found in the transition zone, or “ecotone,”
between the forest and treeless tundra in alpine and arctic areas throughout
the Northern Hemisphere. Stunted and shaped by wind, ice and cold, it can
take many forms. At the edge of the continuous forest, and even higher
on wind-swept slopes, the tops of stunted trees may extend in winter above
the snowpack, where they are thrashed and ice blasted. Bark on the windward
side is abraded, forcing growth to occur only downwind. The result: a flaglike
appearance.
But “flagging” is by no means the most severe krummholz deformity caused
by wind and cold. At the highest extent of tree growth, a tree may grow
only inches high, rivaling even the most assiduous pruning of a bansai
expert. But it may also send out ground-hugging branches to great distances,
sometimes forming a sprawling cushion as wide as 50 feet in diameter. And
in the Rockies, as well as other mountainous areas, stunted trees often
huddle together for protection in isolated, wind-sculpted groves — the
tree islands that stand all alone amidst the wide open meadows of the treeless
tundra.
The close clumping of vegetation in krummholz tree islands protects
more than the trees themselves. For prey such as voles that frequent tundra
meadows, the islands provide quick cover from predators. For birds, such
as the white-capped sparrow, they offer cozy spots for nesting in an environment
that otherwise provides scant shelter. For wintering animals, such as gophers,
tree islands give effective shelter from howling winds.
And for another species — us — krummholz provides other kinds of benefits.
The tenacious vegetation helps stabilize snow and fragmented rock on steep
mountain slopes, reducing the risk of avalanches and landslides that would
otherwise take a heavy toll in lives and property. And Krummholz may be
important for scientists monitoring global warming’s effects on vegetation
in far northern regions. Here, studies already have shown a transition
from krumholz to normal forest coinciding with warming temperatures.
Global warming is one reason for Seastedt’s interest in the mobile tree
islands. He is using them on Niwot Ridge to see how a possible advance
of treeline into the tundra due to rising temperatures might trigger changes
in soil chemistry that ultimately could reinforce the climate change. In
the end, though, Seastedt’s interest in the tree islands lies mostly in
their ecological importance and the simple fact of their mobility.
“What’s absolutely fascinating to me is that these trees move across the landscape,” he says.
Up here on Niwot Ridge, a biosphere reserve on the eastern slope of
Colorado’s Front Range, the balmy conditions of this early July day belie
the harsh picture painted by climatological averages. At 10 a.m., the sky
is a crystalline blue. Only a few cotton-ball cumulus clouds hover over
the craggy continental divide a few miles west. Off to the east, in the
far distance, the hazy, high plains stretch to a dim horizon.
Seastedt leads the way up the ridge. Tall and lanky, he has short-cropped,
dirty blond hair and skin deeply tanned by long hours spent in the open.
Also in our group is Susan Sherrod, a recently minted ecologist who studied
under Seastedt. Sherrod’s dissertation research involved Niwot Ridge’s
never-seen but ubiquitous gophers. Their continual tunneling beneath the
rocky tundra puts to shame the gold miners who once labored within the
granite embrace of these mountains.
All around us is evidence of the gophers’ engineering efforts: ropy
mounds of earth strung across the landscape like cooked spaghetti tossed
across a floor. Seastedt explains that during winter, the animals avoid
the icy fury at the surface, digging tunnels through the snow and the ground
instead. But this raises a disposal problem: What to do with the excavated
dirt? The gophers’ answer: Use it to backfill their snow tunnels. In early
summer, when the snowpack melts, the backfilled dirt filling the former
tunnels settles onto the ground, preserving a record of the gophers’ wintry
travels.
Here and there we also see middens, stashes of seeds and dried vegetation
that gophers hide under the snow “for those bad days when it’s 20 below
and the wind is blowin’,” Seastedt jokes.
Gophers, Sherrod adds, are “keystone engineers.” Through their burrowing
and middening activities, they have dramatically altered the landscape,
“creating an environment that wouldn’t be here without those impacts,”
she says. Sedges and leafy forbs seem better adapted than grasses to live
with the disturbances created by the gophers. “If it weren’t for the gophers,
these meadows might well be grasslands,” Seastedt says.
And were it not for the krummholz trees, the gophers might not be here
either. The islands, Seastedt explains, act like snow fences, causing drifts
to accumulate. These drifts significantly increase the snow cover in an
environment that otherwise would be scoured almost clean by the wind. The
gophers depend on the snow cover for protection from the elements. “Krummholz
allows gophers to overwinter,” Seastedt says. “Without that snow cover,
they’d be toast.”
Condos, towns and other high-country outposts of civilization in North America might also be toast were it not for krummholz and other trees of timberline. According to Steve Arno, author of “Timberline: Mountain and Arctic Forest Frontiers,” timberline trees in Europe still have not recovered from grazing and wood cutting in Medieval times. As a result, snow, soil and rock on steep slopes are more apt to slide in avalanches and landslides, says Arno, a recently retired research forester with the U.S. Forest Service. “Timberline areas have been denuded with disastrous effects,” he says. In North America, though, timberline areas in general and krummholz in particular are more intact. “Their root systems are doing what isn't being done in Europe: stabilizing snow and fragmented rock,” Arno says.
As we head up Niwot Ridge, out of the subalpine zone and into the krummholz
ecotone, the spruce and fir are becoming increasingly stunted and wind-sculpted.
Many trees are flagged. Higher still, we emerge into the realm of the mobile
tree islands. Were it not for the jagged peaks all around us, I could swear
we were in a botanic gardens: The sedge- and forb-filled meadows between
the tree islands are laid with a veritable technicolor carpet of flowers.
Yellow buttercups, among the first flowers to emerge in spring, are scattered
everywhere. And interspersed here and there are purple, spikey flowers
in the genus Dodecatheon, which means “12 Gods” in Greek, according to
Sue Sherrod. As she tells it, the flowers got this name because their great
beauty convinced the ancients that a minyan of protective deities had to
be watching over them.
As I look out at a krummholz island, I see low trees grouped together in an aerodynamic shape. At the windward side, the trees are severely stunted, attaining no more than the bansai height of six inches or so. Moving back, the trees heighten and the island widens. At the very rear, the trees are about chest to head-high. In profile, the grove looks strangely like the front of a bullet train.
I walk over with Seastedt to take a closer look. At the front of the grove, facing toward the wind, there’s considerable die-back of vegetation. Here, under the full assault of the wind, snow is mostly scoured away before it can form much of a protective drift. So this part of the island is eroding away. Then Seastedt and I get down on our hands and knees and crawl as far as we can into the thicket of branches at the center of the tree island. He points to branches that have contacted the ground, sent out roots and successfully anchored themselves to the ground. This growth seems to be toward the lee of the island, downwind, away from the nasty assault of winter wind and cold. With erosion of the island on the windward side, and expansion at the rear due to new growth, it now seems abundantly clear how an island of trees like this can migrate downwind.
Recent research by Seastedt and his graduate students has revealed how the migrating tree islands alter the environment as they pass over tundra soils. And the results may have implications for global warming research.
Over the past few years, Seastedt and his students have sampled the soil in around 35 mobile tree islands, including sites that the islands have passed over. In the “wake” of a tree island, they have found, the soil becomes significantly depleted in the element carbon compared with undisturbed tundra. According to Seastedt, tundra soils contain among the highest concentrations of carbon per square meter of any biome. His research suggests that if global warming were to cause trees to encroach on the tundra, a substantial amount of that carbon could be lost; some of it could wind up in the atmosphere in the form of the greenhouse gas carbon dioxide. If this were to occur, it could be a positive feedback on the warming. Such a feedback would be relatively small if it occurred at all, he concedes. But scientists considering the complex interactions between soil, vegetation and atmosphere might want to take the carbon depletion phenomenon into account to get as nuanced a picture of global warming as possible, Seastedt says.
Continuing higher up the ridge, we finally emerge onto the treeless tundra. I ask Seastedt how long-lived the tree islands are. “Based on carbon dating of the oldest pieces of wood connected to a living tree, I’d say the patterns we see up here have been in place for 500 to 1,000 years,” he says. So it seems that the islands got established up here long ago, perhaps during a prior period of warming. Since then, the wind has been pushing them steadily downhill. At a rate of two to four centimeters per year, they should have moved about 20 meters, or 65 feet, in 500 years. Considering this, Seastedt jokes that maybe one day they’ll converge at the edge of the subalpine zone, piling up in a slow motion assault on the ramparts of the forest.
In Shakespeare's play, a messenger brings Macbeth news of a similar assault, the seeming approach of a grove of trees toward his castle on Dunsinane Hill. But Macbeth does not believe a forest can move: “That will never be. Who can impress the forest, bid the tree unfix his earth-bound root?”
Macbeth, of course, got it partly right: No grove of trees was advancing on his castle. It was, instead, an army of soldiers camouflaged with branches. But ultimately he was wrong about trees. They can, indeed, move.