Nature’s Gold – The Aspen
Every morning I set off to the river and to my refuge, the Bosque, walking under dying cottonwoods whose bark is peeling away from dead tree limbs. Just in the past summer a few of these magnificent matriarchs have crashed to earth providing nutrients that will one day support some other kind of ‘tree of life’.
My dreams tell me that new as yet unknown species will begin life here, offering me comfort because as a dreamer I have learned that my dreaming body knows what I do not, no doubt because she is attached to the body of this precious earth.
When I pass by the coyote willows I think about how cottonwoods, aspens, and poplars are part of the Willow family. Recent genetic testing reveals that this diverse group also includes passiflora (my beloved passionflowers) and wild violets too! – I was astonished by this last piece of information until I realized that all require adequate water to thrive.
Coming from the Northeast where quaking aspen can be found throughout the state of Maine I was still surprised to learn that these trees stretch across the entire continent. Aspens hold the title of being the most widespread tree in North America. They can be found from the Midwest, across Canada, north into Alaska and across the West through to Arizona and into New Mexico.
One aspen tree is actually only a small part of a larger organism. A stand or group of aspen trees is considered a singular organism with its central life force hidden underground in an extensive root system that I think is analogous to the human brain (I am not alone in this speculation). Before a solitary aspen trunk appears above the surface, the root system may lie dormant for many years until conditions are just right. In a single stand, each tree is a genetic replicate of the other, hence the name – a “clone” of aspens used to describe a stand.
Older than the massive Sequoias or the Bristlecone Pines, when the Pando clone was discovered, scientists named it with a Latin word that means “I spread.” Pando is an aspen clone that originated from a single seed and increases by sending up new shoots from its underground expanding and complex root system.
Pando is believed to be the largest, most dense organism ever found on earth. It weighs nearly 13 million pounds (6,600 tons). The clone spreads over 106 acres, consisting of over 40,000 individual trees. The exact age of the clone and its root system is difficult to calculate, but it is estimated to have started at the end of the last ice age. It was first recognized by researchers in the 1970s and more recently proven by geneticists. Its massive size, weight, and prehistoric age have caused worldwide fame.
Located in central Utah (Fishlake National Forest), Pando is dressed in verdant green throughout the summer. Her fluttering leaves bring relief from summer’s intense basin heat. In autumn the oranges and yellows and sometimes crimson leaves rival the most spectacular New Mexican sunset.
However, there is deep cause for concern because Pando is showing signs of decline. The organism is not regenerating, invasive insects are present, as are diseases. Unfortunately throughout the west, diebacks of other aspen stands are becoming more common. No one mentions climate change or loss of habitat as an issue.
The prevailing logic is that even if the trees of any stand are wiped out, it is still difficult to extinguish an aspen’s root system permanently due to the rapid rate in which it reproduces, thus there is hope.
It’s hard to decide what is most memorable about aspen: the vibrant gold leaves in fall, the startling pearl white stands, or the magical sound of the “quaking” leaves.
Among swaths of dark green conifers, the deciduous aspen stands thrive in a variety of environments. Aspens quickly colonize recently burned or bare areas (with birches in areas that support the latter like they do in Maine). They prefer moist soil but can survive near springs in desert conditions. Like the rest of the willow family these trees must have access to water. Because the Southwest is under siege from increasing drought as a result of climate change, I wonder if lack of adequate water is the reason why these trees are not regenerating.
One of the most fascinating aspects of aspens is that they grow all winter, a fact I didn’t know, but should have suspected because the trees around my house have a greenish cast. Beneath the thin, white outer bark layer is a thin green photosynthetic layer that allows the tree to create sugars and grow when other deciduous trees are dormant. This characteristic is unique among deciduous trees. During hard winters, the green, sugary layer provides necessary nutrients for deer and elk. Throughout the year, young aspens provide food for a variety of animals including moose, black bears, beaver, porcupine, grouse and rodents.
Perhaps most impressively, quaking aspen along with sister species (distributed in Europe and Asia) occupy the broadest range of any tree species in the world. Why is that so? Possibly one reason might be because the bark carries out photosynthesis all year long allowing the stands to expand their geographic range.
Aspen drops its leaves in winter but, of course, remains alive and thus requires metabolic energy. The soft tan to greenish hues often visible in aspen bark mark an important photosynthetic capability provided by the differing levels of chloroplasts. Stem photosynthesis contributes significantly to aspen’s over-wintering survival capabilities. The disadvantages of this type of bark include low fire resistance, ease with which people carve their initials in these trees creating space for disease to enter, and that the trees are an excellent food source for elk, deer etc. Numerous insects and fungi that attack the bark are also considered potential problems.
Aspen form individual patches comprised of numerous stems, each with its own trunk, branches, leaves and a shared root system. All of these structures originally arose from a single aspen seed that germinated in the distant past; The patches remain connected via root systems, and if the root system between patches is severed, the patches form physiologically separate clumps but are generally still considered part of the same clone because they are composed of genetically identical parts. The boundaries of different clones stand out most clearly in the early spring when flowering and leafing-out occur (in that order). Aspen have male and female catkins, unlike the majority of tree species, which support both male and female reproductive parts on each individual.
During early spring in an aspen clone the reproductive male catkins produce pollen while the female produces eggs. Huge numbers of viable, tiny seeds mature and float off from the female on a tangle of cotton-like seed hairs that catch air currents, sometimes traveling great distances. Immediately after shedding their pollen and seeds, the clones then produce leaves that usually appear at the same time in all stems of a given clone revealing the boundaries between clones most visibly. In the fall the striking leaf colors do not mark clonial boundaries as reliably because the chemical processes that produce the colors tend to be very sensitive to local micro-climate conditions. A single clone may also exhibit multiple colors simultaneously. In aspen, all the pigments that give rise to these glorious colors can be found in the leaf from spring all the way through fall when the aspen begin to first break down the green chlorophyll molecules that are responsible for spring and summer color. Other pigment molecules then become more and more visible.
For many years, most western forest ecologists thought aspen reproduction from seeds was rare. However, it turns out that successful establishment of aspen via seeds occurs more frequently than previously believed. The ability of aspen to produce whole stands of “trees” vegetatively provides yet another key element in explaining the species’ ability to occupy huge geographic ranges.
There are several benefits of asexual expansion. The clones share resources. One part of a clone might be near an important water source and share its water with drier parts of the clone while those in a drier area may have greater access to the vital soil nutrient, phosphorous, and share that resource with those that are low in this nutrient.
Quaking aspen also tends to be a disturbed habitat species, meaning it often lives where avalanches, mudslides and fires occur frequently. So both the regenerative capability and the clonal reproductive capability allow aspen to initially establish or to re-establish into an area after disturbances like mud slides occur. Similar patterns often follow forest fires. Rarely will a fire burn hot enough to kill the entire root system from which these stems arise, so an individual clone may occupy a given space and be completely wiped out on the surface but re-grow from the root system many times.
If an aspen stand does not experience periodic disturbances such as natural fire or avalanche, more shade-tolerant conifers tend to establish and shade out the high-light-requiring aspen stems. If the disturbances are too frequent, then the clone cannot spread.
Clone structure varies with geography but also varies due to the strong influences of rainfall and relative humidity. The largest clones generally occur in semi-arid environments such as the mountains of the western and southwestern US. Clones tend to be smaller in areas where the climate supports seed germination.
The last particular attribute of quaking aspen important in contributing to its ability to occupy huge ranges derives from the comparatively high level of genetic variability among clones. These interclonal levels of variability provide raw material for evolutionary change across generational times. The large number of seeds produced from genetically variable sources generates an enormous array of potentially successful genotypes for establishment in newly opened areas and probably takes place at higher rates.
One of the most enchanting aspects of aspen is their ability to quake and tremble in the slightest breeze. Why do quaking aspen leaves quake and tremble? This is due to the physical structure of the leaf stem which traces a flat, oblong, elliptical pattern when viewed in cross section so it has strength in one dimension and minimal strength in the second dimension, so even a gentle wind causes produces movement and a hypnotic sound. Plant physiologists have pointed out several reasons for the trembling leaf behavior. They include minimizing the risk of too much sunlight as well as reducing the risk of overheating in intense, high elevation. Trembling also improves photosynthetic rates by keeping a fresh supply of carbon dioxide near the leaf surface where the plant takes up that compound. Insect damage may also be reduced by fluttering leaves.
This tree species seems to almost have it all: powerful, opportunistic, sexual reproduction, long-distance seed dispersal, effective vegetative spread, clonal reproduction, regeneration from roots, high levels of genetic variability, living bark and a potentially enormous life span.
And in my mind, Pando certainly represents one of the most remarkable “families” among all living organisms.
The language used to describe these remarkable organisms that cooperate by share root systems, information, and resources seems woefully inadequate. To call a phenomenal organism like Aspen a “clone”, although grammatically correct, is to reduce it to its lowest denominator. These trees live harmoniously within a family that supports the continuation of life, not just for one tree but for all its relatives. Most important this species has been self- sustaining for millennium.