Several groves of quaking aspen trees enhance the landscape of the area Lewis called "the prarie of the knobs," from the number of knobs hillocks being irregularly scattered through it. Each grove consists of a large number of clones of a single parent tree. Because the earth sciences as we know them were still in their infancy, Lewis could not have recognized the knobs such as those shown in this photo as moraines or piles of glacial debris, which may have been deposited here as recently as 17,000 years ago. The groves pictured here surround the vestige of a glacial tarn or pond, which now is rimmed by crystalized salt leached from the surrounding soil.
Lewis and Clark notice aspen
More than a year into the expedition, having traveled hundreds of miles through flora and fauna new to them, Lewis recognized a tree native to New England in the middle of Montana. On July 16, 1805, at "tower rock"–the first "gate of the Rocky Mountains"–near Cascade, Lewis "passed a small stream on which I observed a considerable quantity of aspin [sic]," the party's first record of quaking aspen, Populus tremuloides. The generic name, Populus (po-pull-us) is Latin for Aspen. This is North America's most widely distributed tree, occurring from the Atlantic to the Pacific, from interior Alaska, down the Rocky Mountains to Mexico.
The captains noticed aspen again along the Dearborn River on July 18, 1805; near Prickly Pear and Spokane Creeks near Helena, on July 20, 1805; and on July 2, 1806, at Travelers' Rest near Lolo.
Aspen life strategy
Each seed in every long catkin of unisexual aspen flowers-without-petals is equipped with a tiny white filament that works like a parachute, enabling it to ride wind currents for great distances.
Aspen's broad distribution is not due to tolerance toward a range of climates. Just the opposite. It has a narrow range of environmental tolerance. But it produces trillions of wind-disseminated, short-lived seeds each year with the probability that one will land in a suitable environment, germinate, and develop into a sapling. This life strategy is referred to as "fugitive" species—many seeds scattered so one will land in a rare but suitable refuge. The requirements for successful germination and seedling establishment are so exacting that the probability of success is near zero. The secret is that when a rare seedling establishment occurs, the sapling develops into a grove that may persist for hundreds or even thousands of years by periodic rejuvenation.
Aspen seed requires a flat seedbed that remains moist throughout the summer, free from shade, and free from browsing by wildlife. The last time such seedbeds occurred on a wide scale was on protected silt beds left by retreating glaciers at the end of the ice age. Most of today's aspen groves are believed to have originated at that time, rejuvenating periodically over the ten millennia since the original seeds germinated. If age is calculated from seed germination, these groves are America's oldest living trees, even though the aboveground portions are generally younger than 100 years.
Aspen rejuvenates by producing two kinds of roots, lateral roots that may radiate out 100 yards or more from the parent tree just below the soil surface, and sinker roots that grow down several feet toward the water table. Lateral roots have about two buds per inch than can develop into sucker shoots. In the young sapling the buds do just that, bypassing critical seedling requirements by nursing by root connection to the parent tree.
Aspens regulate their own stand density and structure
The Benefits of Genetic Diversity
This photo, taken in October of 2004, illustrates evidences of genetic diversity among adjacent clones of aspen. Aspen groves self-regulate their stand density and structure. Thus the grove at right foreground already shows its bare branches, while the one beyond it is still clothed in pale green leaves. The leaves of the next grove left of the bare stand are bright orange, and ready to fall. The darker greenish-orange foliage of the grove at extreme right are at an intermediate phase. These adjacent clones are running toward dormancy on slightly different schedules related to diurnal durations.
Nature's plan requires them to anticipate the need for autumn dormancy before their buds freeze. They retract all the nutrients—especially nitrogen, potassium and phosphorus—from their leaves and put them in the twigs, then harden the bud-scales enough to make them frost-resistant. The precise day and time of the onset of dormancy is related to elevation and latitude, and that timing is crucial. Sudden radical changes in the weather can spell disaster or disappointment. If they grow dormant too late, they risk destroying the ability to retrieve the nutrients in the leaves and not hardening the buds. If they're not hardened in time, they freeze and die; if they harden too soon, they miss part of the growing season. The schedule observed by a given clone is genetically imprinted, along with its preferred elevation and latitude, in every one of its light, wind-blown seeds.
Leaves of the aspen produce a hormone called auxin that inhibits the development of lateral root buds. As the grove develops, enough hormone is produced and transferred to the roots that sucker buds become dormant. A natural enzyme degrades the hormone as it travels from leaves to root, so sometimes sucker shoots develop from roots that extend far from the parent, such as into a meadow. This strategy, plus shaded branch removal, keeps the canopy open, prevents overcrowding and waste of resources to support sucker growth that cannot survive in shade. Dormant buds rapidly develop into sucker shoots if the lateral root is severed from the parent tree by logging, girdling by fire or animals, or by insect defoliation.
Rejuvenation most commonly occurs after a fire girdles overstory trees. This releases thousands of buds to form new shoots and establish new groves. Each grove is composed of genetically identical trees. Adjacent groves can often be recognized by subtle differences such as bark color, leaf size, date of bud break in the spring or leaf color change in the autumn.
Funded in part by a grant from the Montana Cultural Trust.