| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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A Closer Look Tundra Tussocks
Alaska’s rolling tundra can turn even mild-mannered hikers into cursing, limping stragglers. The North Slope’s pastoral-seeming meadows trip up unsuspecting ankles with mushy moss pits and shin-high mounds of grass known as tussocks. Being a stubborn sort, I try to cross the tundra by hopping from mound to mound. Then, just as I least expect it, a grassy pimple pitches me off into a muddy sinkhole. Here at Toolik, a research station three hours south of Prudhoe Bay, people compare a tussock tromp to slogging across a sea of basketballs. But although these basketball-sized bumps may be the bane of tundra hikers, they also shelter some of the world’s toughest plants and animals. Tundra dwellers put up with extreme conditions: frigid temperatures, a short 60-day growing season, and extremely low precipitation. And year-round frozen ground, called permafrost, lurks only a foot or so below the surface. Without the rotund tussocks, many of the tundra’s tough residents wouldn’t have a place to lay their heads—or set down their roots. I am spending the summer belly-down among the tussocks. Scientists suspect the tundra may be among the first to face the firing squad as global climate warms. So I will be helping Toolik researchers pull up patches of plants to test what would happen to these communities if some of its key inhabitants disappear. Tussocks squat on the tundra like living haystacks. A tundra plant known as cottongrass (Eriphorum vaginatum), which gets its name from the white puffballs on its grassy tips, makes up the spiky mound. A single plant sprouts tillers—packages of roots, stems, and leaves—midway up its stem. As each batch of tillers dies, the grass’ actively growing region pushes upward, leaving behind a thick scaffolding of dead tillers. Between permafrost and freezing temperatures, microorganisms that break down dead material to the south work much more slowly here. A single tussock may shoot out 300 to 600 living tillers at a time on top of this mounting mound. Tundra tenants consider tussocks prime real estate. Although an average tussock only grows about eight inches high, it creates a range of housing options for other plants. Species that favor dry conditions take up residence in the tops of tussocks, while moisture-loving mosses snuggle in between the mounds. Green, red, and bone-white mosses feel cool and spongy as I pull them up to examine them more closely. They come up in clumps, their slender, feathery tassels interlocked to form tangled mats. Mosses are lush oases of moisture here. Deciduous and evergreen shrubs—tiny forms of their southern relatives—take advantage of this small drink of water, often rooting themselves amid the moss. The dwarf birch, Betula nana, barely tops my ankles; it can’t rustle up enough nutrients to really stretch out. Lingonberry (Vaccinium vitis-idaea), a relative of the cranberry popular with bears and humans, bounces out scarlet berries in the fall. The leathery leaves of Labrador tea (Ledum palustre) have been used for many years by native Alaskans to make medicinal drinks for everything from cramps to tuberculosis. The ground stocks up a meager buffet of nutrients for these plants in the form of peat, a compacted mess of dead plant material. If things heat up, microorganisms will start working away, decomposing the peat and returning this stored carbon to the atmosphere. Lichens, on the other hand, don’t tap into the peat’s nutrient stash. These combinations of fungi and algae hang on to tussocks for support, then suck moisture and nutrients straight from the air. Caribou produce a special enzyme that helps them digest reindeer lichen as they munch their way southward from calving grounds in the high arctic to warmer winter lands. One smaller arctic mammal stays put in the winter. Arctic ground squirrels (Spermophilus parryii), or siksiks, get their native Inupiaq nickname from the clicking sound of their signature whistle. The furry rodents are so entrenched in the tundra experience that limericks have been scribbled in their honor along the green walls of the camp’s wooden johns. When not eyeing unattended sandwiches, siksiks dine on tundra grasses and plants. In winter, they drop into deep hibernation. A few years ago, Toolik researchers found that the torpid rodents can supercool their blood during their long winter’s nap, dropping it to below-freezing temperatures. Siksik colonies snooze the winter months away in burrows lined with lichen mattresses. Other local rodents actually make their homes inside the wiry tussock domes. Singing voles (Microtus miurus) and tundra voles (Microtus oeconomus) will eat their way into tussocks to form nests inside the masses of tillers, like hobbits digging homes inside hills. Sometimes a vole’s voracious mowing can even shear tussock tops right off, providing a cut-away view of the vole’s inner sanctum. Rambling rodents are a special treat for the grizzly bear. These ever-hungry giants claw into the ground in search of mobile meals, but spend most of their time munching on easier-to-catch berries and grasses. Grizzlies and another tundra loner, the gray wolf, are relatively scarce in the high Arctic, preferring the year-round prey found farther south. Spotting one of these in the distance could transform an already weary tundra explorer into a quaking mess. But the peskiest predator in the Arctic is actually much, much smaller. With at least two dozen mosquito species in Alaska, all available airspace seems to be choked with these insects on patrol for exposed skin, whether human or caribou. One creature’s pest is another’s dinner guest. Fish that live in the tundra’s shallow lakes feast on larval and adult mosquitoes. Mama mosquitoes drop their eggs in moist clumps of mosses, bogs, and standing water. These eggs stay dormant through the winter, lying in wait until the sun’s rays—and unwary victims—appear. Mosquitoes also supplement the diets of gulls and other shorebirds, which flock inland for the summer feast. Birds such as the long-tailed jaeger (Stercorarius longicaudus) would prefer to eat voles, but will snack on mosquitoes. Jaegers, loons, and arctic terns head back to the coast when winter comes. For the tussocks, however, there’s no escaping the onset of winter, though they do put up a strong front against snowdrifts. Spring visitors to Toolik have arrived to find flowers bursting up through the snow. Recent studies suggest that photosynthesis occurs underneath the white stuff as well. Ecologist Steve Oberbauer and his crew noticed that tundra plants emerged from the snow with a reddish-brown coloring, which forms a natural plant sunscreen. Wondering how the plants would know to lube up before they saw daylight, the scientists peered beneath the snowdrifts. They found that ice lenses, created by melting and refreezing snow, act as skylights for the buried tussocks. The process also traps carbon dioxide so the plants can get started photosynthesizing after the light returns but before the spring snows have melted. Now it is the middle of summer, and tussock plants are using the 24 hours of sunlight to stock up on supplies for the rest of the year. The long daylight hours have made me tired. Confused by constant sunshine, I have been reluctant to turn in at night. So I have found my own place among the tussocks: the grassy mounds and soft mosses make a perfect spot to curl up for a nap. Cameron Walker is a freelance science writer based in Northern California. |
