Quaking Aspen: The Keystone Species that Clones Itself

By Lambert Strether of Corrente.

This may run a little thin for a biosphere perambulation post, since I have a household event to contend with. So here’s an hour-long YouTube of aspens in the wind for you to listen to, as compensation:

(One of my main use cases for my iPad is listening to soothing white noise, which the quaking aspen (populus tremuloides) certainly provides; the slightest breeze causes its leaves to flutter.

Since there seems to be no precise definition of a tree[1], I will happily skip over taxonomy and go straight to the aspen’s range:

The quaking aspen (Populus tremuloides) is the most widely distributed tree in North America (Map 1). The species ranges from Alaska to Newfoundland and from Virginia to northern Mexico. It is found at sea level in the northern end of its range and at 1,500 feet elevation in the southern end. In Oregon, the brilliant spring green and magnificent golden fall foliage of quaking aspens can be seen in the Cascades, the East Cascades Slopes and Foothills, and the Blue Mountains ecoregions. Only a few aspen stands occur in the Coast Range and the Klamath Mountains. More can be found in the high ranges of the Oregon desert such as Steens Mountain, Hart Mountain, and others.

Here is a handy map:

Purple prose like “brilliant spring green and magnificent golden fall foliage” is not rare in the literature on aspens, not even from case-hardened Federal bureaucrats (Forest Service: “glow in a patchwork quilt of these different hues”). Perhaps that’s because aspen groves are ludicrously beautiful:

(This is an photograph of the Pando clone, of which more below.) Here is a drawing of a single fallen yellow aspen leaf (by Sarah Drummond, winner of the American Museum of Natural History Young Naturalist Award for an essay on the aspen):

But why do aspen leaves tremble? Nature answers, at length:

[D]ue to the physical structure of the leaf stem (petiole) which traces a flat, oblong, elliptical pattern when viewed in cross section (i.e., perpendicular to the stem itself) so it has strength in one dimension (the long part of the ellipsis) and minimal strength in the second dimension (the narrow part of the ellipsis), so even a gentle wind causes shaking, quaking and trembling. We understand this phenomenon very well mechanically, yet a deeper question can be posed: Why does the petiole develop this way? Plant physiologists have pointed out several consequences of the trembling leaf behavior to include minimizing the risk of too much sunlight on the photosynthetic apparatus (photoinhibition), reducing the risk of overheating in intense, high elevation sunlight and improving photosynthetic rates by keeping a fresh supply of carbon dioxide near the leaf surface where the plant takes up that compound. Taking a different approach, one of our students did a small scale independent study several years ago where she identified matched pairs of aspen leaves and stabilized one with tubing to reduce its ability to tremble, then measured the amount of leaf damage due to insects near the end of the summer, comparing the leaves which could tremble with ones that could not. She found the insect damage to the ‘fixed’ leaves was, on average, about 27% higher in the stabilized members of the pairs.

So the trembling behavior is not so much like whispering, as it is like brushing away insects.[1]

Now we turn to aspen’s most salient feature: how it reproduces. From Nature:

Aspen form individual patches comprised of numerous stems, termed ramets, each with its own trunk, branches, leaves and a shared root system (Figure 2). All of these structures arose from a single aspen seed, often in the very distant past; while these patches remain connected via root systems, they comprise a single clone. If the root system between patches is severed, the patches form physiologically separate entitites but are generally still considered part of the same clone given that they are composed of genetically identical patches and parts, having been produced vegetatively. The boundaries of different clones stand out most clearly in the early spring when flowering and leafing-out occur (in that order). Aspen occur as males and females separately (dioecious), unlike the majority of tree species, which support both male and female reproductive parts on each individual (monoecious or hermaphroditic).

A diagram (we’re skipping the catkins):

In Utah, there is an aspen clone called Pando (Latin: “I spread”[3]), which weighs 13,000,000 pounds and might be the oldest living thing on earth:

Until recently, rough estimates had [Pando] at something like 80,000 years old; this has been revised down significantly in light of the fact that the area of Utah in which it lives was covered by an ice sheet 20,000 years ago, a problem even the most badass tree would have trouble surviving. Pando’s now clocking in at a mere 15,000 or so.

Unlike the bristlecones, which can be dated accurately through tree rings, it’s pretty hard to get a fix on Pando’s exact age. This is because Pando is a tree colony. Quaking aspens have the curious property of being able to reproduce themselves clonally, sending up new seedlings from their roots. These ‘ramets’ function, for all intents and purposes, as baby trees, living and dying just like you’d expect any stand-alone aspen to.

But they’re all the same organism. Pando is a 13 million pound collective of more than 40,000 stems, genetically identical and all grown from the same massive (100+ acres, easily) root system. It cares about individual stems just about as much the average person might fuss about individual hairs on their head.

I had no idea I would discover the world’s oldest (and heaviest?) living being when I wandered into this topic area! That said, aspens in general and Pando in particular are threatened, first by cows[4]:

Our results show that the brief but intense cattle grazing appears to be a major contributor to the decline of the Pando Clone, as well as other aspen groves in the immediate vicinity, in addition to the much lighter continuous herbivory by mule deer. Based on comparisons of the exclosures with the area open to both livestock and mule deer that this high level of use in the unfenced areas effectively eliminates regeneration. A previous study (Rogers and McAvoy 2018) attributed the failure of the Pando Clone to regenerate solely to mule deer, but our results indicate that cattle are also having a major impact on understory vegetation. Our results suggest that livestock herbivory may be having a synergistic interaction with mule deer foraging to suppress aspen sprout growth, and that trampling of soils by livestock may also play a role in depressed aspen recruitment in unfenced portions of the Pando Clone and adjacent aspen stands. Based on our results, we recommend removal of livestock from the Pando Clone area to protect this globally significant organism, and also recommend that livestock be removed from public land pastures elsewhere where aspen groves show signs of depressed regeneration.

The cow problem (“cow-bombing“) is general:

In the American West, the original 9.6 million acres of quaking aspen is down to just 3.9 million acres. The decline of quaking aspen in the American West has been severe and is mostly attributable to the onslaught of domestic livestock. Any amount of grazing by domestic livestock is harmful to aspens, as the bovines trash the understory and nip off all the replacement shoots from the root system.

Fortunately, cow-bombing is fixable:

But saving [Pando] may be as simple as putting up a good fence.

To test their idea, [Paul Rogers of Utah State University and the Western Aspen Alliance] and his colleagues fenced in 7 hectares of the grove. They also tried to stimulate tree growth by burning vegetation, clearing juniper bushes growing among the trees, and cutting mature aspens, all of which can cause new trees to sprout.

After three years, the part of Pando that was inside the fence contained more than eight times as many stems per hectare as an unfenced area. Though the burning, clearing and cutting enhanced growth, simply excluding browsing animals drove most of the change….

“It was a neat surprise that we can get pretty good results with fencing alone,” Rogers says.

What works for Pando might not work elsewhere, however. Installing barriers around large regions of the American west or whole mountain ranges would be impractical, Rogers notes.

I dunno. Nuking 9.6 million – 3.9 million = 5.7 million acres of a keystone species (more below) is practical? Really?

Aspens are further threatened by fire suppression. From the Forest Service:

Fire suppression has allowed conifers to spread into aspen groves where they shade the aspen. Since it is intolerant, the aspen begins to decline. Fire suppression also means that the above ground aspen are not killed, so the clone does not sprout. Aspen trees are short lived and need the sprouting response to keep the clone healthy. As the above ground aspen decline in health so does the vigor of the clone and this creates a cascade effect in the tree’s decline. As the clone becomes weaker the sprouts are fewer and grazing animals have more of a negative effect on the tree.

Drummond describes the process that ought to be happening, but isn’t:

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.

Drummond has more faith in dormancy than the Forest Service does. And then there is drought, which also affects dormancy:

More recently, however, “sudden aspen decline” (SAD) has been reported. As of 2007, widespread, severe, rapid dieback and mortality had affected about 13 percent of the aspen in Colorado. SAD is different from what is traditionally referred to as aspen decline. It is occurring on a landscape scale, versus the stand-level changes that are typically noted with aspen forest related to disturbance and succession. The mortality is rapidly occurring over a few years, versus the typical changes occurring over decades. In addition, pathogens and insects associated with SAD are different from those usually associated with aspen mortality in Colorado. Evidence indicates that warm drought conditions earlier in the decade played a primary inciting role, that certain stand and site factors predisposed aspen to damage, and that the pathogens and insects are killing stressed trees.

An accelerated dieback of aspen was also observed across northern Arizona following two defoliation events and several years of drought. A secondary disease and/or insect was found to be associated with the dying trees as well, including canker fungi, wood borers, and clear wing moths. Aspen regeneration by suckering was observed after the dieback, but little of it remained after heavy ungulate browsing. The extensive dieback and ungulate browsing of the aspen suckers is expected to result in forest type conversions, from aspen to conifer, in many of these sites across the state.

Aspen reproduce primarily by sprouting from root systems, rather than spreading seeds. Each “clone” can live hundreds or even thousands of years. A stem may die, but beneath the soil, the root sends out fresh shoots, and the cycle begins again. The aspen stands with dying trees do not seem to produce shoots to replace old trees as they normally would, which may be related to years of drought that inflicted deep damage.

Yikes.

Finally, let’s consider the aspen’s role as a keystone species (i.e., one that it would be really good not to kill off by cow-bombing, fire suppression, and drought, because so many other creatures depend on it. Many, many conceptualizations of “keystone species” focus primarily on predator prey relations, with predators as the keystone. For example, from Nature:

Keystone species have effects on communities that far exceed their abundance. That is to say, the importance of keystone species would not be predicted based upon their occurrence in an ecosystem. Dominant and keystone species influence the presence and abundance of other organisms through their feeding relationships. Feeding relationships — eating or being eaten — are called trophic interactions.

Nature provides a diagram:

Elsewhere, I have urged that diagrams like this:

[remind] me of academic politics, although some departments (***cough*** economics ***cough***) are no doubt far worse than others. Speculating freely, one might wonder whether these academics find predator-prey relations so powerful and illuminating as an analytical tool because they mirror the relationships that matter most to them in their professional lives. Department chairs, red in tooth and claw, as it were.

Silicon Valley, too, whose use of the word “ecosystem” is transparently predatory. It now strikes me that Nature’s diagram is nothing more than an org chart. And if you know anything about institutions, you know that the org chart does not necessarily relate in any way to the real functioning of the institution it purports to represent. I mean, there’s already one dotted-line relation in the Nature diagram; and where there’s one, there are more.

I prefer a concept of keystone species that permits a broader variety of relationships than predator-prey. After all, trees can hardly be predators, not even Ents! The Natural Resources Defense Council provides such an alternative:

Keystone species maintain the local biodiversity of an ecosystem, influencing the abundance and type of other species in a habitat. They are nearly always a critical component of the local food web. One of the defining characteristics of a keystone species is that it fills a critical ecological role that no other species can. Without its keystone species, an entire ecosystem would radically change—or cease to exist altogether. It’s important to note that a species’ role can change from one ecosystem to the next, and a species that is considered a keystone in one environment may not be considered the same in another.

NRDC provides five types of keystone species: “Predator, Prey, Ecosystem engineer, Mutualist, and Plants.” (We considered the role of the beaver as ecosystem engineer here and here). They write:

[T]he boreal forest forms a ring around the majority of the world’s northern latitudes just below the Arctic Circle. In the Canadian boreal, the snowshoe hare is an example of a keystone prey species, serving as food for the threatened Canada lynx (which relies on snowshoe hares for more than 75 percent of its winter diet), and other predators. In the boreal, keystone species also include trees like aspen and willow (these provide critical habitat for myriad organisms like lichens, fungi, insects, and birds) and plants like wild red raspberries, which are a critical food resource for animals from bees to bears.

Drummond writes:

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

It seems, then, that there are many more types of keystone species than the five proposed by NRDC, and that a broader focus on teasing out these types (“versatile roles”) would be interesting and useful. I think that the concept of “ecosystem services” — which I really have to gird myself to investigate — is trying to occupy that space, but my spidey sense tingles, because to me a synonym for “service” is “can be given a price and commoditized.” What price Pando?

And that YouTube should be just about done by now!

NOTES

[1] How to identify quaking aspens:

An easy place to start with identification is the bark. Young trees often have a creamy white (or even light greenish) bark that is fairly smooth. Because of this, it is often confused with paper birch. Unlike paper birch, however, aspen bark does not peel away from the trunk. As aspens age, their bark becomes furrowed and dark gray or even black, especially toward the base. Quaking aspen leaves tend to be almost circular. Again, the flattened petiole is a dead giveaway. If it is difficult to roll the petiole between your thumb and finger, it is probably an aspen leaf.

[2] Fortunately, there is an actual account on Twitter run by an aspen tree, @iamthepandotree, which provides other true facts. About “eyes”:

My “eyes” are actually branch scars, the places where branches fall off as my stems grow. According to legend, we aspen trees use our eyes to watch over children. #pando#pandoforest#aspenclone #fishlakenationalforest#worldslargesttree#aspeneyes#treeeyes#theyrewatching pic.twitter.com/34KP60Dwvl

— iamthepandotree (@iamthepandotree) April 2, 2021

And about bark:

We aspens have a nifty trick for making energy in the winter-time. We have chlorophyll in our bark which lets us photosynthesize year-round, even without our leaves.#pando#aspenclone #fishlake#Treelife#worldslargesttree#aspen#photosynthesis#aspenbark@AmForestFndn pic.twitter.com/Q0WiWcfsJg

— iamthepandotree (@iamthepandotree) April 7, 2021

Aspen bark has also been used for medicinal purposes:

The bark of quaking aspen was used by pioneers and American Indians as a fever remedy, as well as for scurvy. It contains salicin (similar to the active ingredient in aspirin). A substance similar to turpentine was extracted and used internally as an expectorant and externally as a counterirritant.

[3] From the same source: “Decadent stasis might be an interesting fantasy, but nature is built from Pandos, systems that masquerade as individuals and persist into what might as well be eternity.” Hmm.

[4] Contrast the more polite and less destructive behavior of elk. Once more from Sarah Drummond:

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.

And from Drummond’s notebook