MTPR

Quaking Aspen, Disturbance, And The Return Of Yellowstone's Wolves

Aug 18, 2019

Yellowstone National Park has lost much of its aspen forest cover — not just to the detriment of the aspen, but to all the other species that depend on them, including many birds, black bears, snowshoe hares, porcupines and beavers. I, too, miss the aspen, its knobby white trunk standing in stark relief against the dark-barked backdrop of spruce, fir and pine that forms most of my visual diet.

A few young aspen trees line the sidewalk in front of my apartment building. I’m a little surprised that they are here; quaking aspen isn’t typically used in landscaping. It’s an unruly plant, sending shoots up precisely where they’re not wanted. I once spent an entire afternoon removing little aspen sprouts from the middle of a wilderness trail. The area had burned two years previously, but not badly, so the mature aspen trees along the trail were sending up sprouts - also known as suckers - from the root system. This form of vegetative reproduction results in the growth of clones, extensive stands of quaking aspen with identical genes. Aspen clones can cover many acres. I swung my Pulaski that day many more times than I care to remember.
 
Old aspen stands must be maintained by disturbances in the forest, because quaking aspen is not shade-tolerant. After a few decades, conifers will shade out and kill the aspens, unless they are removed by fires or logging. Once the roots are exposed, the aspens will send up their suckers to thrive in the light. Regularly maintained by fire, aspen clones can survive for thousands of years, even though the individual trees may only live for 150 years or so in the Rocky Mountain West.

Quaking aspen is the most widely-distributed tree in North America, but in the Rocky Mountains, it’s declining. This is probably due to a combination of fire suppression and heavy browsing by deer and elk. Quaking aspen stands are preferred habitat for deer and elk because of their plant diversity, which is much greater than in conifer stands, and because the leaves, twigs and bark of the aspen themselves are highly nutritious.

Yellowstone National Park has lost much of its aspen forest cover* — not just to the detriment of the aspen, but to all the other species that depend on them, including many birds, black bears, snowshoe hares, porcupines and beavers. I, too, miss the aspens, because they’re one of the most distinctive trees in the West. Quaking aspen stands out from the crowd, its knobby white trunk standing in stark relief against the dark-barked backdrop of spruce, fir and pine that forms most of my visual diet. The faintest breeze sets the leaves to a fit of shivering, forest music that may also protect the tree against windstorms. This is because the petioles that connect the aspen’s leaves to its branches are flat and twist easily. This also allows the leaves to clump together in the breeze, reducing air drag upon the tree. In any case, to hear aspen music there need not even be any wind; it’s enough to strike the tree trunk with my walking stick.

In the fall, entire mountainsides flanked with aspen turn a resplendent gold. When people back East think of the Rockies, this is what they see: an ocean of yellow leaves at the edge of a broad meadow, shimmering in a cooling autumn breeze. It’s the classic calendar photo. It isn’t what I see out of my window. Looking out my window, I remember how those little aspens, not much taller than a man, had brightened my autumn evenings, ever-trembling.

* 2019 update on the aspen stands of Yellowstone National Park's northern range:
In Yellowstone Science - Volume 24, Issue 1, John Klaptosky asks: Are aspen benefitting from wolves? The answer is yes.

"Aspen appears to be benefitting from the reintroduction of wolves. Barring any unforeseen circumstances, these large predators are here to stay in Yellowstone; and their continued presence on the northern range should help maintain the elk herds at lower densities, providing a long-term benefit to aspen, unlike the decades that followed the discontinuation of culling activities and the ensuing increase of elk on the landscape ... This physiological process is translating into the emergence of widespread aspen stand recovery, in spite of continued levels of browsing across the northern range. In light of this recent development, it is premature to say aspen have recovered or will return to anything like historic levels both in range and size. Many existing stands of mature aspen trees on the northern range established and flourished at the tail-end of the Little Ice Age, a period where conditions were predominantly wetter and cooler. These environmental conditions do not exist on the northern range today, so aspen is expected to behave and adapt differently. Recent data are highly encouraging and suggest a positive trend forward for aspen. How this plays out on the future landscape remains to be seen."

"Field Notes" is produced by the Montana Natural History Center.

(Broadcast: "Field Notes," 8/11/19 & 8/16/19. Listen weekly on the radio, Sundays at 12:55 p.m., Tuesdays at 4:54 p.m., or Fridays at 4:54 p.m.,  or via podcast.)