It is the second day of March, and the title of this blog entry is Atolls, which, despite the photo above, might lead a reader to expect or at least hope for a literary and photographic cruise to someplace tropical, someplace in the South Pacific. That might be a welcome diversion to readers who are weary of winter.
Alas, this is not that trip. So right off the bat, let me offer an apology to my Flagstaff readers, who, during the snowiest winter in recent memory, are probably sick of shoveling the white stuff, and might enjoy a blog entry about, say, Tahiti. But although I would no doubt love Tahiti, I write about places I know. And this is not an essay about tropical atolls.
This essay is indeed about atolls—but not ones where you might wear a bikini or a speedo. So, if you are up for it on this second day of March, shed your imaginary bikini, and don your imaginary snow suit. Or stay in your flannels and curl up by a warm fireplace with some Bailey’s and coffee as I take you to an entirely different kind of atoll. A timber atoll high in the North Cascades.
This one is not formed by a coral reef; it is formed by a resilient, determined ring of a remarkable tree called Tsuga mertensia: the Mountain Hemlock.
Like their tropical counterparts, the atolls of which I speak are small islands surrounded by water—only the water, in this case, is frozen. And these alpine atolls also form through a process of gradual accretion, as life builds upon life.
Of course, they are not really atolls at all. It’s a metaphor, first employed by an alpine botanist in the 1930s. But it is an apt metaphor, one that captures the way these outcrops of trees lay claim to the high ground amidst billowing waves of snow. They look like islands in the summer, when the meadows around them are a sea of wildflowers. And they look even more like islands from October to June, when they are surrounded by snow.
How do these atolls form? And why, among all possible distribution patterns that a tree species might adopt, does the Mountain Hemlock adopt this one?
The answer comes into clarity at the nexus of several key facts about climate, topography, and biology. First of all, snow is very, very deep here. The growing season is short. There are hazards to be avoided. It can be tough, for any living thing, to go it alone in such a place. In the subalpine zone, a seedling is more likely to survive if it is in the lee of a larger tree. Hence, the presence of one tree makes life easier for the next one, and the next one.
But I’m getting ahead of myself. I shall return to the atolls. First things first: let me introduce you to the tree itself.
In fall and winter, atmospheric rivers from the Pacific take aim at the Northwest coast, giving the lowland forests a long, deep drink. These forests host behemoth trees like the Coast Redwood, Sitka Spruce, Western Hemlock, and Western Red Cedar. Douglas fir dominates the Puget Sound. Along the rivers, big leaf maples drop their soccer-ball-sized leaves. All of these are magnificent trees. In fact, in terms of both diversity and sheer majesty, I can’t think of any place with better trees.
But where the Pacific storms bump up against the steep slopes of the North Cascades, where the wind picks up speed as it sweeps upslope, where mist wraps around ridge lines, and where the dense clouds drop heavy wet snow—tons of it—a different tree takes over. A tree that was built for just this climate and just this topography. It is not as big as the lowland trees, but what it lacks in size it more than makes up for in character.
It is the quintessential tree of the west side of the mountains. It is the brooding tree that so often is shrouded in mist, the one with the dark and hoary silhouette up on the ridge. It is a dignified tree. In the hours before dawn, it is a bit spooky to hike through a forest of Mountain Hemlock. The word primeval comes to mind.
At lower elevations, it is a large tree that grows thickly and drapes the slopes; at high elevations, it is the tree that forms attractive ring-like clumps in a meadow. (Yes, atolls.) It is the snow-plastered tree that graces the postcard of your ski vacation to Mt. Baker. (What? You’ve never had a ski vacation to Mt. Baker? What are you waiting for? It’s as good as Tahiti.)
One of the most distinctive features of the tree is a droopy leader right at the tippy-top. It also has graceful branches that subdivide at the end into many small, supple twigs, creating a web-like bough that can hold big loads of snow until a skier or climber happens to be directly underneath them. This leader and branches give it a distinctly different texture than the stiff and prickly spruces that occur in the subalpine zone on the dry side of the Cascades.
The tree produces tons of small, narrow cones that are brown with papery scales when mature, and a beautiful luminescent shade of purple when immature. The needles are bluish-green, short, and flat, reminiscent of rosemary. They are arranged spirally around the twig, creating a ‘bottlebrush’ appearance which is accentuated in midsummer, when the growing tip erupts in bright new growth, like little stars at the end of each twig. The bark is grey and shallowly furrowed.
To my mind, Mountain Hemlock captures the spirit of the North Cascades as well as any tree.
It’s clear from its name that it is not a flatlander. And it’s a hemlock, which means that it is a west-coaster. It inhabits coastal ranges from Southeast Alaska to California, where it is found in only the wettest parts of the Sierra Nevada. Aside from a few pockets in Northern Idaho and the northwest corner of Montana, it is not a Rocky Mountain tree. It likes things damp, shall we say. And, even though it is superbly adapted to snow, it is not a tree that wants its roots to be in frozen soil.
Further south, in California, it is only a high altitude tree, but in the North Cascades it starts as low as 3000 feet and extends all the way to timberline, which is generally between 6000 and 7000 feet. Mountain Hemlock inhabits the middle kingdom, between the giant trees of the valley bottoms and the wilderness of rock and ice above. On these startlingly steep slopes, it forms extensive, dense stands with few other species in the mix.
If you are hiking up into the high country from one of the valley bottoms, it will take a long time. For several miles and for an elevation gain of four thousand feet, you will see a lot of tree trunks. At first, you’ll be in the lowland forest of the Western Hemlock, the Western Red Cedar, and the Douglas Fir. It’s a lush forest, filled with mossy nurse logs, and the dense understory of salmonberry, red huckleberry, thimbleberry, devil’s club, salal, and various other shrubs.
After you’ve gained about 1000 feet (depending on your trailhead elevation), you may notice a change in the trees; you’ve entered the Silver Fir zone. After gaining about another 1000 feet, you will be entering the domain of the Mountain Hemlock. You’ll be in this domain for a long time, and for a lot of switchbacks. From here until timberline, it will be the dominant tree.
At the lower end of its range where it blends with the Silver Fir, Mountain Hemlock is a large tree, though not as large as its sibling, the Western Hemlock. The largest known Mountain Hemlocks in Washington approach 200 feet and are about 6 feet in diameter. (This is big, but not nearly as big as the lowland and coastal behemoths of the aforementioned species, some of which can reach 300 feet and be nearly 20 feet in diameter.) A more typical large Mountain Hemlock might be 100 feet tall with a trunk a couple feet wide. Like the Western Hemlock, it is a tall, straight tree with a cylindrical form.
The forest is somber and dark. There isn’t as much of much of an understory as in the lower forest (although there are delicious huckleberries!). Brief and occasional glimpses of the snowy peaks above will tantalize you, through the heavy boughs of the hemlocks. Ever so gradually as you ascend, the hemlocks get smaller, but it’s only noticeable over miles and hours.
Now and then you may emerge from the forest into an avalanche path, a brief and chaotic strip of sudden sunlight, slide alder, downed trees and debris. Kind of a battle zone. Tongues of snow, set like concrete, will last well into late summer. As much as any tree, Mountain Hemlock bears the brunt of natural violence. Avalanches regularly rip through timber, leaving behind a tapestry of bright and dark green stripes. It’s common to see large trunks snapped in two, like toothpicks. But this destruction notwithstanding, Tsuga mertensia, more than any other tree, has made its peace with snow.
At the upper extent of its range, as it approaches timberline, the Mountain Hemlock can appear to be a whole different character. The higher you go, the smaller the trees become, both in terms of height and girth. Whereas they are tall and cylindrical at lower elevation, trees in the high country, burdened as they are with deeper loads of snow, can be beautifully misshapen. It is a creature defined by its relationship with snow.
Their crowns may be truncated or their boughs widely splayed. Their trunks may be J-rooted or bent. Without the benefit of a human pruner, these trees in the subalpine zone can become a piece of bonsai art. And like other trees that live at timberline, Mountain Hemlock can take on a gnarled, stunted form (known as krummholz) where it hunkers down in deference to fierce weather.
In the upper drainages of the Nooksack, the Baker, and the North Fork of the Skykomish, mountains of startling vertical relief form pockets that catch the prevailing southwesterly winds. These topographical catcher’s mitts bear the full force of the atmospheric rivers that hit the Cascade range. As a result, these watersheds are among the wettest places in North America.
Annual precipitation in these places can easily exceed 140 inches. Most of it comes in the winter. At the Mt. Baker ski area, annual snowfall averages around 600 inches and in many winters exceeds 800 inches. In the winter of 1998-99, the ski area recorded 1140 inches of snow—a world record. There are pockets of the North Cascades, particularly far up in the Baker River watershed, where the snowfall probably exceeds that of the ski area. These are areas where the Mountain Hemlock thrives.
In one quite distinctive way, Tsuga mertensia demonstrates how it has evolved to succeed in such a snowy place. A very high sugary sap content that imparts unusual flexibility in the trunk of the tree. This renders them extremely ‘bendy,’ so that they can be held down all the way to the ground by heavy snow loads, and then spring back up when the snow melts. BOING!!! Backcountry travelers are familiar with these ‘catapults,’ and are careful to walk around them rather than straddle them.
The Cascades are so effective at wringing moisture out of the wet towel of the atmosphere that the country on the other side of the mountains, the eastern slope, is spacious dryland forest of Ponderosa Pine. Not far beyond that, it is essentially a treeless grassland, thick with ticks and rattlesnakes (but not without its charms). But here on the western slope where the Mountain Hemlock lives, the situation is steep, wet, and green. Cross-country travel is diabolically difficult. Route-finding is too. The North Cascades are a stand-offish range, not offering their treasures easily. But the treasures are sublime.
By now you may be wondering, ‘What about the atolls?’ I did not forget.
I’ve been speaking in a somewhat inexact way about ‘zones’ of vegetation. The concept is familiar to anyone who has been down the Grand Canyon, or just driven a highway that goes from a low place to a high place. In each zone, there are not only characteristic trees, but shrubs, berries, flowers, fungi, birds, mammals, and all manner of living things. As elevation is gained, the species change, and so do the patterns of growth, of size, of density and spacial arrangement.
For many human travelers, the zone that is most sublime and most delightful is what is known as the subalpine. The word ‘subalpine’ simply means below the alpine, which is accurate enough but not very poetic. What it is: slopes of butter-yellow avalanche lilies, timbered knolls, waterfalls, boulder gardens threaded by chattering streams, and glorious cirques jeweled with lakes that mirror snowcapped peaks.
It’s where you camp the night before you attempt one of those high peaks, and wake up to a frosted world. Frozen or flowing, water infuses the landscape; it is in tiny pools, rivulets that gather into streams that tumble and plunge toward the rivers down below. It’s in dewdrops that bead the leaves of every flower and bush.
From high summer to the first snows of October, it’s a place of extravagant life. A lot of living is packed into a few short months. A whole lot of procreating. A lot of interspecies drama. A lot of color. The wildflowers waste no time, and the bumblebees, butterflies, and birds are right there with them. Tiny pollinated urns turn into plump berries on scarlet bushes, and hefty bears fill their bellies. Ptarmigan chicks scuttle about in the heather, and marmots hoot at intruders.
Many modest summits top out in the subalpine, offering views that rival those from any hard-gained alpine peak. Trapper Peak is one such place. Glorious undulating ridges offer miles of delightful scrambling, alternating between rocky outcrops, meadows, and dense thickets of scrappy, dwarfish trees. Sibley Ridge is one of my favorites.
While at a lower elevation Tsuga mertensia thickly carpets the slopes, in the subalpine it is sparser, more widely scattered. It lives in clusters that are most often situated on small rises or ridges. In other words—atolls. The trees do not populate bowls or depressions; they are staking out the high ground. Why? Because that is where the snowpack is thinnest, which means that is where the growing season is the longest.
It’s where young trees are safe from avalanches that sweep away or bury everything in their path, and it’s where tiny saplings feel the warmth of the June sun, long before the surrounding meadow is free from its heavy blanket of snow. And, after a period of years, it’s where organic matter builds up over the mineral soil, to nourish a new generation of trees.
Despite how these outcrops and promontories are exposed to the full force of winter gales, it is precisely here that young Mountain Hemlocks find a hospitable place to get started in life. Especially if they can grow close in the orbit of an old survivor. The typical pattern is that there are, within a small atoll, one or two trees that are significantly older and larger than the rest.
I like the way the process is described by Christopher J. Earle, on the Gymnosperm Database. Earle says, “Once a clump contains at least one tree tall enough to project above the winter snowpack, blackbody radiation emitted from the tree causes the snow to melt sooner and faster near the tree than in the open meadow. In this way the tree alters its environment to reduce snow accumulation and produce conditions conducive to both the growth of existing trees and the establishment of new trees.”
There is a correlation between snowpack and the establishment and expansion of atolls; when there is a longer growing season, trees stake out new ground. If there is a string of winters that are less snowy than average, these trailblazers will survive and grow tall enough to establish a safe harbor for subsequent seedlings. On the other hand, a string of heavy snow winters will inhibit the expansion of atolls and prevent new ones from getting started.
In an extended period of drier and warmer winters, atolls are likely to expand and new ones will become established. In time, the forest could grow and the meadow diminish. Sequential years of heavier-than-normal snow would inhibit the process.
Sometime in the distant past, one intrepid tree had to start things off. If the winged seed from a Tsuga mertensia cone was lucky enough to land on a good spot, it might make it. The first years would have been the hardest. Then there comes a time when this persistent pioneer tree becomes shelter for another. And so the atoll begins.
The way these Mountain Hemlock atolls are dispersed across the landscape is very satisfying to the eye. It is as aesthetic as any designed garden. One way to think of it is that this particular instance of beauty is shaped by hardship.
Or, to be more precise, it is shaped by Tsuga mertensia’s response to hardship. In our thinking about evolution and adaptation, we often recognize the importance of competition—especially in situations where life is particularly precarious. Do we appreciate, also, the importance of cooperation? Do we understand the ways in which one tree grows in the grace of another?