Aspen: A Keystone Species
Part of the Young Naturalist Awards Curriculum Collection.
Restless leaves sift the ever-brilliant Colorado sun into undulating patterns, wavering across the pale trunks of an aspen grove. Wind rustles the branches as I step among the trees. Across the ground before me, overgrown with grasses and vegetation that seem determined to bind it to the earth, stretches a fallen aspen log. I have stepped and stumbled over it many times while wandering our corner of the forest on the west shoulder of Pikes Peak, but this time I stop and sit quietly down. Old branches, gray with weathering, still arch over fronds of false solomon's seal and golden banner, while at the opposite end of the trunk, white ruffles of shelf fungus sprout from scarred bark tissue. This is Ganoderma applanatum, a fungus that attacks and weakens the bases of older aspens, rendering them susceptible to windfall. I run my hands over the smooth bark, thinking of the intense spring winds that probably caused this tree's downfall.
I have lived most of my life in the Pikes Peak region of Colorado, among aspen and conifer forests. Four years ago, my family moved to ten acres of land adjourning the Florissant Fossil Beds National Monument. I began to keep a nature journal, documenting in words and drawings what I observed around me. As my eyes increasingly opened to the events of the natural world, I began to perceive the key place aspen holds in supporting and sustaining the biodiversity of our region. Aspen has always held a special place in my perception; but since beginning my nature journals, I've come to appreciate more fully its great array of ecological roles. Aspen's importance, to the montane ecosystem as a whole and to the myriad of creatures who turn the groves into microhabitats, is immeasurable. My interest lies not only with aspen's interaction with other species, but with its own biology. With aspen visible from my bedroom window, and with daily walks through their sheltering groves, my observations and sketches of them have taught me to revere the trees both for their interactions with their world and for their intrinsic beauty.
Quaking aspen (Populus tremuloides) is the most widely distributed tree species in North America. It stretches the entire breadth of Canada and covers a broad range of territory across the western U.S., from Alaska to New Mexico. Gazing at the downed aspen log and the numerous other standing trees in the grove, I realize that part of this success may be attributed to the tree's singular method of reproduction. Although catkin flowers appear in early spring and germination by seed does occur, most aspen groves are the result of natural cloning. A parent tree sends up new sprouts called suckers from its own root system once it is firmly established. Young trees formed by this process reap the benefits of a reliable store of nutrients and moisture and have a better chance of survival than a seedling dependent on its own roots for nourishment. The shelter of a grove protects young suckers from sun-scald damage as well. Aspen bark has a smooth outer membrane or periderm, thin and highly permeable. In many tree species, new bark cells grow beneath the old layer of dead cells which, as the trunk expands, splits apart to form furrows and wrinkles, as in firs or cottonwoods. In contrast, the cells of aspen periderm continually reproduce and expand to keep pace with the tree's increasing diameter. Old cells are sloughed off as a chalky residue, brushing off onto my fingers as a whitish powder.
Aspen root systems can remain dormant for centuries, awaiting the right conditions for regrowth. Dense conifer stands frequently shade out aspen, but the aspen roots remain healthy and alive underground until their evergreen neighbors are felled by fire or wind or man. Sunlight then awakens the aspen's roots and triggers the growth of vertical stems, called ramets. Aspen are often one of the first plants to revive and revegetate after a fire. Fossil leaves dated fifteen million years ago suggest that at one time two separate species of aspen flourished in slightly different habitats. The modern species is probably a hybrid of its two ancient ancestors.
From the moment it emerges from the ground, an aspen begins its interactions with other organisms. Insects are among the first to take advantage of the tree, feeding and burrowing in the leaves and stems. The foliage of the young ramets around me is already lacy from incessant chewing. The caterpillar of Weidemeyer's admiral butterfly (Basilarchia weidemeyerii) prefers aspen as a host plant, as do chrysomelid leaf beetles. Gall-moth larvae (Ectodema populella) bore into the petiole tissue just behind the leaf blade, forming rosy, pea-sized galls. The striking black and yellow larvae of the moth, Malacosoma disstria, share the leaf surfaces with minute eighth-inch galls made by spider mites. Leafhopper nymphs and inchworms cast transparent shadows through the leaves, and large silk-skein nests of tent caterpillars stretch around frameworks of several twigs. Sitting in the aspen grove I know so well, I watch ants shimmy up and down the branches, tending flocks of aphids feeding on plant tissue. Aphids secrete a honey-like substance from the tips of their abdomens, triggered by the tapping of ants eager for a drink. In return, the ants patrol the aphids' territory, providing protection from potential predators. Larvae of bark beetles bore tunnels inside the branches, feeding on inner bark tissue. Explosive beetle populations and excessive damage are kept under control by woodpeckers and flickers, probing with slender bills below the bark for an insect meal.
As an aspen grows to maturity, it encounters interactions on a new level. All North American ungulates occasionally browse young sucker growth, but aspen bark is a major winter food source for elk. When other vegetation is low or snow-buried, elk use their incisors to scrape off aspen bark. Damage to the tree is seldom very great, since elk tend to browse lightly, taking only a bite here and there throughout a grove. They rarely girdle a tree except in times of severe stress or food shortage. Although barking trees is primarily a winter activity, elk have also been known to feed on aspen in summer when other resources are abundant, suggesting that its value goes beyond nutrition. Native Americans traditionally used strips of bark to bandage cuts, sprains, and broken bones. Modern studies indicated that aspen bark contains vitamins or chemicals similar to those used in aspirin. Perhaps it is a physiological need, rather than hunger, that draws elk to aspen. Male elk, in particular, need constant nutritious forage in order to grow an impressive pair of antlers for use in combat, courtship, and establishing rank. Examining the fallen tree, I discover pairs of blackened tooth marks at a place that was once five feet off the ground - testimony to elk foraging many years ago. They are intermingled with wider abrasions resulting from deer and elk rubbing velvet antlers against the trunk to relieve itching and discomfort. Injuries to the periderm from antler-rubbing may be disastrous for an individual tree, but do not affect the grove as a whole.
A pair of shorn antlers lies buried in leaf-litter underneath the topmost boughs of the aspen log. There is something similar in the upward, branching gestures of branch and bone. Though discarded and weather-tarnished, each still retains some of its original grace, a fragment of former glory. Neither branch nor bone's service to the ecosystem ends here. Cast-off antlers provide an important mineral supply for small rodents, mainly voles, who gnaw on the antlers to replenish their stores of calcium. The voles themselves also feed on aspen bark during winter, nibbling at the base of trees from tunnels excavated between the snow and the ground. Vole feeding generally occurs in areas where the vole population is high, especially on grassy south-facing slopes and in riprian areas along streams. During the icy yet sun-drenched days of January and February, the aspen periderm absorbs enough heat to melt a small space in the snow around it, providing voles with room for feeding and movement.
Loss of bark to the voles results in dark scaly scar tissue around the base of the tree. I observe this in another nearby ramet covered in rough blackness for about a foot of its trunk length. I reach out and run my fingers along the grooves. The bark is crusty, almost scale-like to the touch. Snow depth of previous winters can be deduced from the vertical extent of vole damage. In this montane aspen grove I have never seen the snow level rise higher than three feet; but in wetter areas of the subalpine, it can reach four to five feet, with vole damage increasing accordingly.
Rabbits, hares, mice, and woodrats also occasionally resort to aspen as a food supply, feeding on the succulent growth of young suckers. Porcupines gnaw on aspen limbs, but their impact is rare, and results from unusually high porcupine populations and consequent food shortages. While I often encounter ponderosa pine trees with branches stripped of bark by porcupines, I have yet to see an aspen debarked by one. A nuttall's cottontail makes its home in a secluded corner of the grove and seems to enjoy nibbling the young aspen leaves and buds in spring.
Aspen leaf buds form early in winter, remaining dormant until the revitalizing sap flow of spring. They seem to be a preferred delicacy of many species of birds. One winter morning last year I glanced out the window and was rewarded with the sight of several juvenile pine grosbeaks flickering among the aspens and tearing open buds with their thick, sturdy bills. They stayed for half an hour until, as if signaled by a warning I could not perceive, all lifted off from the grove and scattered across the landscape. Their brief visit provided enough time for the birds to seriously impair the leaves-to-be, but the time for opening was still three months away, and the trees had time to produce a second crop of leaves to sigh and shimmer in the breath of a summer wind.
Meandering further down the aspen log, I reach a crumbling scar just below the first branches. From this old aspen "eye" (the result of self-pruning, which discards lower foliage that receives inadequate light), sprouts an unusual fungus. Crusty, charcoal-clack growth supports a firm, red ochre hoof shape. It is very common in our area; I have seen and marveled at it many times. Now at last I can give it a name: Fomes igniarius conk. The external growth that caused my many inquiring looks is only the outward signal of heartwood rot within. Conk fungus enters the aspen through abrasions in the bark, whether caused by animal impact or the tree's purposeful dropping of a branch. Examining the wood around the scar, I find horizontal strings of holes where a yellow-bellied sapsucker took advantage of the tree's weakened condition. Conk fungus is a sign to the sapsucker that it need only break through the outer bark layer to reach sap and the rotted, easily extracted wood within. Besides feeding on the sap, yellow-bellied sapsuckers excavate nesting cavities in diseased or dead aspens. On the fossil beds, I have come across sapsucker holes in dying ramets frequently, and every summer I listen to the birds' forceful drilling. When the aspen nesting holes are abandoned by their original occupants, other cavity-dwelling birds quickly fill the vacancy: mountain chickadees, red and white-breasted nuthatches, pygmy nuthatches, various warblers, warbling vireos, and violet-green swallows.
Tiny flammulated owls also nest in the aspens. Adults are only five or six inches tall and feed primarily on moths, which are at their greatest diversity and abundance in the rich forests of moist north-facing slopes. Consequently, flammulated owls are most likely found in mixed conifer and aspen forests where moths proliferate. Several years ago I was fortunate enough to accompany my father on a field seminar focused on banding and observing these small, secretive birds. We located a nest approximately twenty feet up in a huge aspen ramet and brought down two owlets. I can still recall my sense of joyful wonder as I stood cradling the frail creatures in my hands. Weightless as the clump of gray down feathers that they closely resembled, the owlets gripped my palms with reptilian feet, surveying their surroundings and handlers with deep, obsidian eyes. I thought of them often afterward, imagining the two growing to adulthood, taking a first, experimental flight, and eventually gliding silently back and forth to nests of their own. I have monitored the aspens close to home for any sign of owl inhabitants, and now the sound of the flammulated owl's tremulous, wavery call drifting through a summer evening carries a new poignancy.
I wander alongside a small stream not far from the aspen grove that has become my second home. A belted kingfisher plunges with a musical splash, and red-winged blackbirds dart in and out of the willow thickets and aspen stands surrounding the rivulet. I round a particularly large, leafy gooseberry bush and am suddenly confronted by a stand of aspen stumps, felled virtually overnight by beavers. A path of crushed vegetation points the direction in which the trees were dragged. I kneel down to study the aspen remnants. The beaver's carpentry work looks smooth from a distance but, touching the wood, I can feel uneven ridges where their teeth scalloped a stump. Continuing to walk until the beaver pond is in view, I see fresh logs woven into the structure of the beaver dam. Beavers tend to select trees within a hundred yard radius of their building grounds. Although most of their cutting is directed toward the construction and upkeep of a dam or lodge, beavers also fell aspens for food. Unlike the light browsing of elk and voles, an entire tree must be sacrificed to feed a beaver. Beavers may seem ruthless in their cutting of trees, surpassed only by man in altering their environment to fit their own needs. However, their demolition of the aspen grove serves a purpose similar to that of fire - aiding in the prevention of over-mature trees and the promotion of new growth.
Beyond the beaver dam in the valley, two tremendous ancient aspens spread their gnarled limbs in a high protective canopy over me and the other smaller members of the grove. Their trunks are studded with sets of claw marks ascending to the level of the first branches. A black bear once climbed these ramets to scent mark its territory, digging its claws into the periderm for a firm hold. The scratches form pentagonal arcs as wide as my hand splayed out on the trunk. They have healed into slightly raised black swellings, yet another addition to the trees' record of encounters with other species.
On the slope above the stream I find a clearing densely populated with young aspens amid the surrounding conifer forest. Older trees show lines of charred bark along their lower trunks, and burned aspen logs lie decaying in the wake of a fire that swept through several years ago. Human intervention prevented the fire from spreading far or afflicting many conifer trees, but the few thin barked aspens interspersed with fir and spruce were more vulnerable and suffered heavy damage. Now surveying the exuberant growth of the young ramets, I gain a deeper understanding of the aspen growth cycle. Fire is essential to the health and rejuvenation of a grove - burning away dead undergrowth, over-mature ramets, and conifer branches that shade out the sun. Fire provides a form of renovation, a return to the beginning. Without fire, an aspen grove becomes stagnant and over-mature; the shade of the tallest tree eliminates the possibility of any new growth. The roots grow dormant under the shade of evergreens. Yet with the removal of older growth, the resilient young suckers continue the life cycle.
Without the aspen, the biodiversity of the Montane Life Zone would be significantly decreased. Most species in a given ecosystem owe their existence to the myriad connections to other species, none overly abundant but together creating a functional whole, an interdependent community able to meet the needs of all its inhabitants. There are also organisms that play versatile roles, supporting and strengthening all of the others and providing the building blocks of a habitat. Aspen is one such keystone species. The threads of life are all interwoven; in this mountain forest, aspen makes possible many of the connections.
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