All trees require sunlight to power photosynthesis, but some require more than others do.

Flowering dogwoods (Cornus floridana), for example, are able to get by with only the dappled light that penetrates the forest canopy. But many other species perish in shaded habitats; only the most sun-bathed lands will do.

Take quaking aspens (Populus tremuloides), for example.

Aspens are strongly intolerant of shade and require many hours of full, direct sunlight to thrive. Aspens even possess chlorophyll – the active ingredient necessary for photosynthesis – in the smooth bark of their trunks and branches. This enables them to harness what little sunlight avoids their leaves.

When you consider the aspens’ place in the process of forest succession, this preference for intense sunlight makes perfect sense.

Succession Simplified

Forests are not static features of the landscape; they change over time. This is not only reflected by the forest canopy height, but also in the composition of species that create the forest.

Shade intolerant trees, such as quaking aspens, Virginia pines (Pinus virginiana) and black willows (Salix nigra), are among the first species to colonize areas left barren by fire, mudslides, avalanches or clear-cuts. They are the pioneers, who dominate the early days of a forest’s life.

But as forests age, they become increasingly shaded.

The shade-tolerant counterparts to pioneer species – also known as climax species – appear later. They begin to sprout as the forest canopy closes in, throwing shade across the forest floor. They move in, set up shop and get ready to take over, once the pioneers have come and gone.

To stay ahead of the shade, most pioneer species have evolved extremely rapid growth rates – aspens may rise 2 feet or more each year. But this rapid growth comes at a cost, and most pioneers live relatively short lives. Few aspens live longer than 150 years.

This makes perfect sense for pioneer species. The habitat changes drastically over a century or two, and by the time the climax species are ready to take over, the land no longer suits the sun-loving pioneers. The colonization of new lands is the only way for the species to survive.

To this end, pioneers tend to grow fast, mature quickly and cast a ton of seeds into the wind. With luck, some of these seeds will land in newly disturbed habitats, where they will start the process all over again.

Substandard Seeds

Aspens may grow quickly and produce an abundance of tiny, wind-carried seeds, but they aren’t especially effective colonizers.

The problem is, their seeds only germinate under a rather narrow range of conditions. They must land in bare, moist soil, within the tiny window of time in which they remain viable. Obviously, some of them are successful, but the overwhelming majority accomplish nothing at all.

This is somewhat unusual for a pioneer species. After all, their claim to fame is showing up first and releasing an abundance of seeds before their short lives end. By colonizing new lands, the species perpetuates.

But it turns out that aspens don’t put the future of the species in the hands of their seeds. Aspens have turned the entire problem of finding newly disturbed lands on its head – at least in part.

In addition to their seeds, which are the product of sexual reproduction, aspens also reproduce asexually, through a process botanists term vegetative propagation. They do so by birthing new stems from within their existing root systems.

This is a pretty common phenomenon among plants of various species. Plum (Prunus spp.), pear (Pyrus spp.) and beech trees (Fagus grandifolia), also produce new stems from existing root systems. Each new stem looks like an individual tree, but it is really just a single stem produced from a common root mass.

As they are part of the same organism, they share the same genetic code.

They are clones.

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A Perplexing Paradox

So, wait a second.

How does this type of asexual reproduction help aspens colonize newly disturbed lands?

It allows them to wait.

Wildfires are rarely one-off phenomena. They tend to occur repeatedly in a given area. In fact, aspen forests that don’t experience frequent wildfires become climax forests.

When a fire tears through an aspen forest, most of the invading climax species become nothing more than cinders and ash. Because aspens don’t burn easily, the fire effectively “cleanses” the forest, thereby ensuring that aspens will continue to dominate the habitat — it effectively rewinds the clock, ensuring the aspens retain their dominant position.

Sure, fires kill some aspen stems, and, as stated earlier, even in ideal conditions, relatively few stems live longer than 150 years, but the plants cope with this easily enough. The root system is continually producing new stems to offset those lost to predators, disease, fungi and fire.

Therefore, instead of relying on an influx of new aspen seeds, the root system of the colony persists. When the next fire comes, the process repeats. So, instead of trying to colonize a new disturbed habitat, aspens simply wait until their own habitat is disturbed, when they will be in a uniquely advantageous position.

It’s only a matter of time.

If you think about it, it is a really elegant solution — you could think of an aspen’s root system as a serial pioneer.

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Pando

One particularly resilient aspen clone has taken this strategy to an absurd extreme.

Nicknamed Pando, this particular 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 fires, pests and invaders that have threatened it through the ages.

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 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 on the scene and Homo erectus would survive for about 10,000 more years.

Some evidence even suggests that Pando is even older – 80,000 is a conservative estimate of the ancient tree’s age.

Pando is the largest organism in the world, tipping the scales at more than 6,000 tons. It’s spread is nearly as impressive – Pando’s root system encompasses an area more than 100 acres in size.

Unfortunately, Pando has fallen on tough times, and appears to be dying off. New stems are not being produced quickly enough to offset the colony’s losses. Current fire-suppression strategies may be playing a part in this, but other factors may also be influencing the colony.

Actions to save Pando are underway, and some portions of it have been clear cut in an effort to induce new sprouts. Should he perish, it may be some time before a fire clears the area and a tiny aspen seed manages to gain purchase in the area to start the process anew.

 

Photo Sources

Main Image: Pixabay

Photo 1: Wikipedia

Photo 2: Free Images

Photo 3: Pixabay

Photo 4: Wikipedia

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