Aspen or aspen tree is a common name for a group of tree species that are found mostly in cold regions of the Northern Hemisphere. However, they are also quite common in high-altitude and mountainous areas in the extended south. In extreme cases, aspen trees are found as far as the Tropic of cancer.
In scientific terms, Aspens belong to genus Populus of family Salicaceae, which include cottonwood, poplar, and other species of deciduous plants. They are often called aspen poplars.
These trees typically reach between 15-30 meters in height can be recognized by their thin (up to 2.5 meters in diameter), whitish-green trunk. Aspens are usually found regions that lack other deciduous tree species.
Different Species of Aspen Tree
There are about five known species of aspens, each of which is found in different parts of the world. Populus adenopoda, a species of poplar which is found exclusively in China, can reach up to 30 meters in height. An entirely separate species is found in Japan.
Populus tremula, or Eurasian aspen, is a transcontinental species of aspen tree native to a vast region of Eurasia stretching from the British Isles in the west as the way to Kamchatka (Russia) in the east. The European aspen is also found in the high-altitude mountainous region in Algeria.
In North America, two different species of aspen trees are found, namely, quaking aspen (Populus tremuloides) and Bigtooth aspen (Populus grandidentata).
Quaking Aspen Tree
Quaking aspens during autumn in Lamoille Canyon, Nevada | Image Courtesy: Wikimedia Commons/Famartin
Populus tremuloides is perhaps the most widely distributed aspen species in North America and is popularly known as quaking or trembling aspen. The species is found in almost every Canadian province. In the United States, it occurs as far north as Alaska to the southern state of New Mexico.
These trees are usually found in elevation between 1,500 m to 3,700 m above the sea level and are rarely seen below 460 m elevation. In high altitude regions, Populus tremuloides are also found in central Mexico.
A fast-growing tree, quaking aspen reaches about 20-25 m in height by maturity. The highest record, however, is 36.5 meters. The bark is smooth and greenish-white in color. It also features visible black knots and scars.
Populus tremuloides grow relatively slower and live much longer in western North America, where weather conditions are dry compared to humid east. The average age of quaking aspens in western North America is about 120 years. Many individuals are known to live as much as 200 years.
The Name “quaking” Comes From Flickering Motion Of Its Leaves
A bunch (foliage) of aspen tree leaves | Image Courtesy; Wikimedia Commons
The species obtained its name “quaking” aspen due to the characteristic shaking or trembling of its leaves in the wind. The shaking is caused by flattened and flexible petiole (stems that binds leaf blades to the stem).
One advantage of a flattened leaf petiole is that it reduces aerodynamic drag, which allows trees to withstand high winds without severely damaging its branches and stem.
A typical quaking aspen leaf is heart-shaped (gets rounder in mature trees) and grow as much as 7 cm in length. During autumn, the leaves change color from glossy green to golden-yellow and, in rare cases, red. The underside of the leaves is dull and grayish.
Bigtooth aspen leaves | Image Courtesy: Wikimedia Commons
Bigtooth aspen (Populus grandidentata) is found exclusively in the northwestern United States and southeastern Canada. In the US, its range extends from Minnesota and Maine in the north to Missouri and Virginia in the south. Several isolated populations can be found in Kentucky, Tennessee, and North Carolina.
Though bigtooth aspens are not as widespread as quaking aspens, they both share many qualities, including trembling of leaves.
Trees of both species have almost identical leaves (bigtooth leaves are slightly bigger). The bark of younger trees is smooth and whitish-green. It turns rough and grayish as they mature.
Bigtooth aspen trees are fast-growing but live relatively short lives. By maturity, they reach 18-24 meters in height and usually live up to 60-70 years of age. These trees have adapted well to flourish in different types of soils, but they are mostly found on sand and can tolerate rocky landscape.
Aspen Species Have Both Male and Female Population
Aspens are dioecious species, meaning it has separate male and female populations. They produce seeds from catkins (wind-pollinated flower clusters). Trees may begin seed production as early as three years of age, but any significant growth is not registered until the age of 20 years.
After being dispersed by wind, aspen seeds remain viable for about two to three weeks and require a moist environment to sprout.
They Also Reproduce Asexually Through Cloning
Another mode of reproduction for aspen species is asexually through a complex mechanism known as cloning. As we have mentioned earlier, individual trees can live between 60- 200 years of age, their roots, however, lives much-much extended lives.
Trees in a large aspen colony can be found interconnected (underground) to each other via an ever-spreading root system. After an environmental disturbance (fire, forest clearing), new sprouts appear in place of dead/fallen trees, and genetically identical aspens replace the older ones.
They are Highly Susceptible to Forest Fires
Even the slightest of forest fires can cause severe damage to aspens. Their thin bark can be easily injured by fire, which allows the fungus to take over and gradually make them hollow, weak, and unable to withstand powerful windstorms.
On the other hand, forest fires play a passive yet vital role in their life cycle. They allow aspen individuals to grow in an open landscape without any competing species.
World’s Largest Living Organism Is an Aspen Clone
Pando Aspen colony | Image Courtesy: J Zapell
A colony of trembling aspen trees located at Fishlake National Forest in south-central Utah, United States, is considered to be one of the largest living organism in the world.
The clonal colony, known as Pando or a trembling giant, is located over a massive area of 43 hectares. Its extensive root system is estimated to weigh close to 6 million kg and is nearly 80,000 years old.
In a recent study, however, it was found that Pando is actually not regenerating the way it used to in the past, and it’s dying. The research concluded that most of the colony now has much older trees, while younger and middle-aged ones are missing.
One of the main causes of its decline is over-grazing by cattle and mule deer, which are feeding off newly spawned sprouts at extremely fast rates giving them limited opportunity to mature.
Uses of Aspen Tree/Wood
The aspen wood is soft but is reasonably strong and has a low inflammability. Due to such characteristics, it has several possible applications.
The wood is widely used as excelsior, or wood wool, in various industries. A wool wood is a small, thin piece of wood sliced from logs. It is utilized in the packaging industry (for cushioning) and cooling pads for evaporative or swamp coolers. Shredded aspen wood is also a popular choice for animal bedding due to its effective odor control and minimal dusk. It is also used to produce oriented strand boards.
Due to its low inflammability, wood from aspen trees are ideal for making pulp and matchsticks. Other common uses include furniture production and construction.
These trees play an essential role in their respective local environments as well. Unlike most other trees, the aspen bark is food and forage for many animal species including, butterflies, European hare, elk, deer, and moose. They are also an important host for bryophytes or non-vascular plants such as hornworts and mosses.
Climate Change’s Effect on Aspens
Scientists believe that if climate change continues at the current rate, most aspen forests will likely die by 2050. In the last decade alone, thousands of aspen trees have died due to rising temperatures and drought in the western United States.
William Anderegg of Princeton University and the lead researcher of this study describes aspens as a wet-loving tree that lives in dryland, and they appear to be getting drier. He further explained, when the ground gets too dry, air bubbles appear in xylem (tissue transporting water and nutrients) that disrupt water supply in trees.
The research team was able to predict the mortality of aspen trees with the help of computer simulations. At the time when the report was published, their model had about 75 percent accuracy.
In 2019, a team of researchers from the University of Michigan explored possible ways to make aspen forests more resilient to climate change. Under UMBS Adaptive Aspen Management Experiment, the team will remove some of the old and mature aspens in a forest and replace them with trees that are much better at handling climate change.
The idea behind this approach is to enhance the overall capacity of forests to withstand global warming and other stress.