All trees require sunlight.
Sunlight provides the energy trees need to rip apart molecules of water and carbon dioxide and rearrange them into carbohydrates. The trees use these carbs for growth and maintenance, and they store any surplus for later use.
But some trees require more sunlight than others.
Young oaks and magnolias, for example, only need the dappled light penetrating the forest canopy to grow. At least during their early years. Meanwhile, sycamores, red cedars and others thirst for sunlight. Deprived of enough, they’ll struggle to survive, never mind grow.
Quaking aspens are another sunlight-guzzling species that thrives best in wide-open habitats, where there aren’t many other trees blocking their light. That’s part of the reason aspens are pretty common in the tree-poor west, yet rare in the forests of the eastern U.S.
In fact, like several other smooth-barked trees, aspens even possess chlorophyll – the green compound responsible for the capturing sunlight and performing the molecular magic we call photosynthesis – in the smooth bark of their trunks and branches. This enables the trees to capture scraps of sunlight that other trees would miss.
These types of sun-harvesting traits are characteristic of many trees known as pioneers.
Forests are not static features of the landscape; they change over time.
This not only includes things like forest spread and canopy height, but it also includes the forest’s very composition. The combination of species present will change, with some growing more common and others slowly disappearing. Some species set up shop quickly, others take a while to get their footing.
Pioneers are the first species to colonize areas left barren by fire, disease, avalanche or clear-cut. In fact, pioneers typically thrive in these wide-open habitats, despite the inclement weather, predators and other threats they must endure to do so. And there’s one important reason they can: These tree-free areas provide all the juicy sun rays the aspens want.
But as forest age, the canopies pioneers create end up shading the forest floor below. This makes it difficult for the next generation of pioneers to keep up with the shade-tolerant species – known as climax species – who show up to the party later.
Climax species don’t require as much sunlight as pioneer species do, so they thrive in the dim light of the forest floor. It only takes them about 100 years or so to take over the joint and elbow the pioneers out completely.
Pioneers do have a few tricks up their sleeve that help them stay in the game a little longer. For example, most pioneer species grow very rapidly. This helps keep their canopies above those of the invading climax species and cling to life for a little while longer.
Aspens may grow 2 feet or more each year while they are young. Other pioneers grow even faster.
But this rapid growth comes at a cost. Aspens – and most other pioneers — live relatively short lives. Healthy aspens easily outlive humans, but they never survive for thousands of years like some pines and sequoias do. One-hundred-fifty years marks a healthy lifespan; 200-year-old specimens are quite rare.
But this isn’t a big problem for pioneer species. After all, the habitat will have changed drastically by the time the trees have been around for a century or two. At some point, it will have stopped being suitable for these sun-loving trees and the climax species will take over.
So, pioneer species embrace a live fast, die young evolutionary strategy. They mature as quickly as possible and then cast a metric butt-ton of seeds into the wind. With luck, some of these seeds will land in entirely new sun-drenched and disturbed habitats, where the process will begin anew.
Some plants and trees are noteworthy for having particularly resilient seeds.
Oak seeds (acorns) typically remain viable even after being collected and cached by squirrels and jays. River birch seeds float down rivers and streams for miles until they wash up somewhere conducive to germination. For that matter, coconuts cross entire oceans, only to germinate on some faraway shore.
Many seeds also remain viable for unthinkable lengths of time.
In 2005, Israeli scientists planted a 2,000-year-old date palm seed, collected from an ancient military outpost. It not only germinated; it thrived. A few years later, Russian scientists achieved similar success, only this time the seed in question was 32,000 years old and retrieved from an ancient rodent burrow.
But aspen seeds are at the opposite side of the spectrum, and they lack this type of resilience.
Tiny and lightweight, aspen seeds are typically borne in clumps of 10 or so. They’re cloaked in a ball of cotton-like fibers which helps catch the wind and keep the seeds aloft for great distances.
But while aspen seeds often spread far and wide, they only germinate under a very narrow range of conditions. They must land in bare, moist soil to germinate, and they must do so within the tiny window of time during which they remain viable.
A few do manage to run this survival gauntlet, but the overwhelming majority accomplish nothing at all. Even commercial tree growers find aspen seeds challenging – most grow their stock from cuttings.
This is pretty unusual for a pioneer species. Their very claim to fame is showing up first and getting started quicker than their competitors can. They are the proverbial early bird enjoying a delicious worm.
But it turns out that aspens don’t rely entirely on seeds to perpetuate their species. Aspens, it turns out, have devised a way of hitting the reset button and cheating death.
Aspen seeds are the result of sexual reproduction. The pollen from male flowers fertilizes the ova located within female flowers, which causes the ova to transform into a seed.
But they also reproduce asexually, through a process botanists call vegetative propagation. This occurs when they birth new stems from within their existing root system.
This is a pretty common phenomenon among plants. Stress – be it caused by illness, insufficient water or overly aggressive pruning – can cause a tree’s root system to produce a bevy of new stems. This way, if the original stem dies, one or more of these new stems can continue to survive.
These individual stems – which you would normally think of as individual trees – are all genetically identical.
So, when you are looking at a grove of aspen trees, you are often seeing a collection of stems that represent a single organism, rather than multiple individuals.
This also has important implications for the way we think about the lifespans of aspen trees.
The 100-odd-year lifespan discussed earlier is accurate, except for one small detail: Instead of saying that aspens live for a century or two, we should have replaced the word “aspen” with “stem.”
The aspen stems come and go in relatively quick fashion, lasting a century or two at best. But the greater organism – including, most importantly, the root mass – lives for much longer than this.
Organisms evolve different life strategies to adapt to their surroundings. Frogs and fiddler crabs take very different approaches to reproduction, as do yeasts and apple trees.
But in the 1970s, ecologists Robert MacArthur and E. O. Wilson noticed that these strategies tended to fall on either end of a continuum. Those at one end emphasized things like rapid growth, early maturation and prolific reproduction, while those at the opposite end took their time maturing and invested considerable resources in the production of only a handful of young.
They termed those in the former category r-strategists, while those in the latter were called K-strategists (r and K refer to the algebraic formulas used to describe population dynamics – r refers to the growth rate of an organism, while K refers to the carrying capacity of the environment).
Rats, rabbits and roaches are good examples of r-strategists, while elephants, humans and redwood trees are classic K-strategists.
However, as time went on and ecologists collected more data, they determined that the real world can’t be explained in such a neat-and-tidy fashion. Organisms don’t always occur at either end of this spectrum, and the r/K framework has largely been discredited by modern scientists.
But for the average nature-lover, the model can often prove helpful. Some organisms do appear to reside at one end of the spectrum or the other, and that lets you draw conclusions and make predictions about their needs, strengths and weaknesses.
Once again, we turn back to aspens.
Aspens are clearly pioneer species, and most pioneer species tend to exhibit r-selected traits like rapid growth. Most also produce an abundance of offspring. But while aspen trees grow quickly, their seeds are a woefully inefficient mechanism for creating new aspens. They shouldn’t be very good at colonizing new habitats at all.
How can we resolve this seeming paradox? How can such a poor colonizer become an effective pioneer?
The answer lies, in part, in the episodic nature of disturbance that characterizes the North American habitats in which aspens thrive. And the primary agent of this disturbance – at least before humans colonized the New World – has long been wildfire.
Wildfires are rarely one-off phenomena. They tend to occur repeatedly in a given area, areas in which aspens not-so-coincidentally thrive.
In fact, it turns out that aspen-dominated forests require periodic wildfires to survive. Deprived of periodic fires, the aspens eventually lose out to oaks and other late-successional species and disappear from the forest.
Aspens don’t burn as readily as the young climax species lurking in the understory do. So, when a fire tears through an aspen forest, it effectively wipes out the coming competition, while leaving the mature aspens largely unharmed.
Sure, the fire will kill a few aspen stems, but they’d only live 150 years or so anyway. Additionally, as we’ve already seen, the root system is the important part. The stems are largely replaceable. Once the fire has passed, the root system will produce a new army of stems to replace any lost to the flames – just like it would do for those killed by predators, disease or fungi.
Obviously, aspen seeds do occasionally land in hospitable locations, where they can germinate and grow. This represents their initial colonization of an area.
But it doesn’t happen very often, so once an aspen organism establishes itself, it clings on tenaciously. In the ensuing years, decades and centuries, the aspen stems will endure fires and other assaults, but the root system will remain ready to reclaim its territory, once the threat has passed.
It’s an elegant solution. In a way, aspens create a whole new niche. They are pioneers who have already colonized the land before – they are serial pioneers.
This strategy is not limited to aspens – they just provide one of the best examples of the phenomenon, as they respond to such disturbances with incredible vigor. Within two years of a stand-clearing event, some aspen colonies produce as many as 30,000 new shoots per acre, although 5,000 new shoots per acre represents a more typical effort.
One particularly resilient aspen clone has achieved an absurd level of success.
Nicknamed Pando, this 47,000-stem-strong clump of aspens has been growing in central Utah’s Fishlake National Forest for the last 80,000 years. None of the individual stems are anywhere near this old, but the root system has survived all of the fires, pests and invaders it has faced over this time.
To get your head wrapped around Pando’s age, consider that when “he” (Pando is a male plant) first emerged from a tiny seed, at least three different human relatives of the genus Homo still walked the earth. Our own species – Homo sapiens – had already appeared (although we had not yet colonized North America, much less begun growing our own crops), but Neanderthals (Homo neanderthalensis) were still around, as was Homo erectus.
Some evidence suggests that Pando may be even older than this. It turns out that 80,000 years is a conservative estimate of the ancient tree’s age.
But unfortunately, Pando has fallen on tough times and appears to be dying off.
In the context of a clonal forest, this means that new stems are not being produced quickly enough to offset the colony’s losses. The root system cannot live without enough stems to produce the sugars the organism needs to survive. Important though the roots are, they cannot produce the food they need.
Current fire-suppression strategies may be part of the reason Pando is struggling, but the decline is likely due to a combination of complex factors – the lack of fire is simply one of the most obvious.
Actions to save Pando are underway, and some portions of it have been clear-cut in an effort to trigger the root system into producing new sprouts. Should he perish, it may be some time before a fire rolls through, allowing a tiny aspen seed to gain purchase and start the process anew.