041 Aspen: The Soft Hardwood That Helps Rebuild a Forest

Know Your Woods: The Softer Eastern Hardwoods Series

When you see an aspen tree growing in the woods, there are several clues that can help identify it. But once the tree reaches a sawmill and becomes a stack of freshly cut boards, most of those clues disappear.

Now we have to look at the wood itself.

How heavy is it? How hard is it? What does the grain look like? What color is it when freshly cut? How does it behave when it goes through a planer? Does it cut cleanly, or does it leave behind torn and fuzzy fibers?

Aspen is particularly interesting because it is one of our lighter and softer eastern hardwoods, and at first glance it can resemble several others, including basswood.

But once you start working with it, aspen begins giving us some very useful clues of its own.

What Do We Mean by Aspen?

In northeastern North America, the name aspen usually refers primarily to two species:

• Quaking aspen (Populus tremuloides)
• Bigtooth aspen (Populus grandidentata)

Both belong to the genus Populus, which also includes cottonwoods and true poplars.

This is where common names can become confusing.

Aspen may be called poplar, popple, trembling poplar, or other regional names. Quaking aspen alone has been called trembling aspen, golden aspen, mountain aspen, popple, poplar, and trembling poplar.

Bigtooth aspen is also commonly called largetooth aspen, poplar, or popple.

So when someone at a sawmill says, "That's poplar," it is worth asking exactly what they mean.

In another article in this series, we will look at yellow-poplar, which is an entirely different tree. Yellow-poplar belongs to the magnolia family and is not a true poplar at all.

Quaking Aspen

Quaking aspen is one of the most remarkable trees in North America because of its enormous natural range.

In fact, it is considered the most widely distributed native tree species on the continent. It grows from Newfoundland and Labrador across Canada to Alaska, throughout much of the northern United States, and south through mountainous areas of the western United States and even into Mexico.

Quaking aspen gets its familiar name from its leaves.

The leaf stalk, or petiole, is flattened rather than round. Even a light breeze can cause the leaves to twist back and forth, producing the characteristic trembling or "quaking" appearance of an aspen grove.

Bigtooth Aspen

Bigtooth aspen has a much more eastern distribution. It grows primarily through northeastern and north-central portions of the United States and southeastern Canada.

Its leaves provide one of the easiest clues when the tree is standing. Compared with quaking aspen, the leaf margins have noticeably larger, coarser teeth—hence the name bigtooth aspen.

Bigtooth aspen also tends to favor somewhat drier upland sites than quaking aspen.

At the sawmill, however, distinguishing the lumber of the two species can be considerably more difficult, and commercially their wood is often treated together simply as aspen.

How Soft Is Aspen?

Aspen gives us another excellent example of why the terms hardwood and softwood can be misleading.

Aspen is botanically a hardwood because it is a broadleaf flowering tree. Physically, however, its wood is quite soft.

Quaking aspen has a Janka hardness of approximately 350 pounds-force. Bigtooth aspen is somewhat harder at approximately 420 pounds-force.

That gives us an interesting comparison with the first wood in this series:

• Quaking Aspen: approximately 350 lbf
• Basswood: approximately 410 lbf
• Bigtooth Aspen: approximately 420 lbf

Those numbers are surprisingly close.

And yet anyone who has carved good basswood knows that hardness alone does not determine how pleasant a wood is to carve.

Softness Is Only Part of the Story

This is where grain structure and fiber behavior become important.

The Janka hardness test measures resistance to indentation. It does not tell us how cleanly fibers will sever under a knife, chisel, planer blade, or other cutting tool.

Basswood has a remarkably fine and uniform structure. With sharp tools, it can cut very cleanly and hold fine carved details.

Aspen is also soft, but its fibers can behave differently. Instead of always being cleanly severed, they may bend, compress, pull, or tear.

That difference becomes especially noticeable when the wood is green.

From the Sawmill: Aspen Can Get Extremely Fuzzy

One characteristic I've noticed while working with freshly cut aspen is just how fuzzy the surface can become. Some fresh cut boards come off the planer when making special pallet pieces with masses of loose, fuzzy fibers that can approach 1/4 inch thick.

When those boards are run through the planer again while still green, all of those flexible fibers can create considerable resistance, making the boards surprisingly difficult to push through.

That is an excellent example of something a hardness number cannot tell us.

The wood may be soft enough to dent easily, yet those long, flexible surface fibers can make machining surprisingly troublesome.

For someone trying to identify an unknown stack of freshly processed lumber at a sawmill, that behavior can become another clue.

Why Does Fresh Aspen Get Fuzzy?

Green wood contains considerably more moisture than dried lumber. The cell walls are saturated and the fibers are much more flexible.

When planer knives encounter fibers that do not shear cleanly, some may bend over or pull partially free rather than being completely severed. The result can be the fuzzy or woolly-looking surface sometimes seen on freshly machined aspen.

Sharp knives, grain direction, feed rate, moisture content, and the individual board can all influence the result.

This also helps explain why simply describing aspen as "softer than basswood" does not tell us which wood will carve or machine more cleanly.

Identifying Aspen Lumber

If I were looking at unknown rough lumber and wondering whether it might be aspen, I would look for several characteristics together.

• Color: Usually very pale. Sapwood can range from nearly white to pale yellow, while the heartwood tends toward light brown.
• Weight: Light. Quaking aspen averages roughly 26 pounds per cubic foot when dried.
• Hardness: Quite soft and relatively easy to dent.
• Grain: Usually straight.
• Texture: Fairly even and generally medium rather than strongly coarse.
• Pores: Small and distributed throughout the growth ring.
• Figure: Usually subdued rather than dramatic.
• Fresh machining: Green boards may produce considerable fuzzy or torn fiber.

Like basswood, aspen is diffuse porous. It does not have the conspicuous rows of large earlywood pores found in ring-porous hardwoods such as oak or ash.

That means the end grain can be particularly useful when separating aspen from some hardwoods, while distinguishing it from other pale diffuse-porous woods requires several clues together.

Aspen Versus Basswood at the Sawmill

This is where our Know Your Woods series begins becoming especially useful.

Suppose there are two piles of pale, lightweight hardwood lumber sitting beside each other.

One is basswood. One is aspen.

Both can be light colored. Both are lightweight. Both are soft. Both generally have straight, rather subdued grain.

So how do we begin separating them?

I would start by looking closely at texture and machining behavior.

Basswood usually has a very fine, even appearance and has earned its reputation among carvers because a sharp cutting edge tends to move through it predictably.

Aspen can have more of that fibrous or fuzzy behavior, particularly when freshly cut and machined green.

Weight alone may not solve the mystery because dried quaking aspen and basswood are remarkably close in average density.

That makes this a good lesson in wood identification: never rely on just one characteristic.

Working With Aspen

Aspen is generally easy to saw and machine because of its low density and softness, although fuzzy surfaces and torn fibers can require additional attention.

Sharp cutting edges become especially important.

It can be worked with hand tools, glued, nailed, screwed, sanded, painted, and finished. Because it is soft, however, surfaces can dent relatively easily.

Aspen also has little natural resistance to decay, so it is generally much better suited to protected or interior applications than to exposed outdoor projects.

What About Carving?

Aspen certainly can be carved, and its softness makes cutting relatively easy.

But this is where our Basswood comparison becomes particularly interesting.

Quaking aspen is actually softer than basswood according to the Janka test, yet basswood is generally the better-known carving wood.

Why?

The likely answer is not simply hardness. It is the way the wood's fine structure and fibers respond to the cutting edge.

A good carving wood needs to do more than be soft. It should cut predictably, resist unwanted tearing, and hold small details without crushing or pulling fibers away from the surrounding surface.

Basswood's remarkably even structure gives it an advantage for fine carving.

Aspen's softness still makes it useful for carving and whittling, especially for less intricate work, but it demonstrates beautifully that carve-ability and hardness are not the same thing.

Finishing Aspen

Aspen's pale and relatively quiet grain gives it a very different finishing personality from woods such as walnut, cherry, or oak.

Oil will generally warm and slightly deepen the color, but there usually is not dramatic figure waiting to jump out when oil is applied.

Because softer woods can absorb stain unevenly, staining should be tested on scrap before committing to an entire project.

Aspen's pale color and relatively unobtrusive grain also make it a useful wood when paint rather than the natural grain is intended to become the finished surface.

A Wood With Many Practical Uses

Aspen has historically been valued more for utility than for dramatic furniture grain.

Uses have included:

• pulp and paper,
• flakeboard and composite panels,
• boxes and crates,
• pallet and utility lumber,
• matches,
• interior components,
• turning and carving,
• and other lightweight wood products.

The Forest Service notes that large quantities of quaking aspen are used for pulp and flakeboard, while higher grades have also been used for lumber and wooden matches.

The Tree That Moves Into an Opening

Aspen's story becomes even more interesting when we leave the sawmill and return to the forest.

Aspens are pioneer trees.

When fire, logging, storms, or another disturbance opens the forest canopy, aspen is exceptionally good at taking advantage of the new sunlight.

Bigtooth aspen can rapidly occupy disturbed ground, helping rebuild forest cover while improving and protecting the site for slower-growing trees that may eventually replace it.

Quaking aspen has an even more remarkable strategy.

One Tree Can Become an Entire Grove

Quaking aspen frequently reproduces through its roots.

When a tree is cut, burned, or otherwise killed above ground, surviving roots can send up large numbers of new shoots called suckers.

Those shoots may appear to be separate trees, but many can remain genetically identical and physically connected through the same root system.

What looks like a grove of hundreds of trees may therefore be one enormous clone.

This ability allows aspen to respond very quickly after disturbance.

Instead of beginning life entirely from seed, a new stem can draw upon an already established root system below the ground.

Aspen and Wildlife

Aspen forests are also valuable wildlife habitat.

The foliage, twigs, buds, bark, and young shoots provide food for numerous animals. Deer and moose browse young shoots, while aspen buds and other parts of the tree are particularly important to ruffed grouse.

That ecological role is another reminder that the value of a tree cannot be measured only by the lumber it produces.

The Bark Gives Us Another Clue

Standing quaking aspen is often fairly easy to recognize.

Younger trees typically have smooth, pale bark that may appear whitish, gray, or greenish. Older trees develop darker and rougher bark, particularly toward the base.

Bigtooth aspen also develops increasingly rough and furrowed bark with age.

Once the bark has been removed at a mill, however, that easy identification clue disappears—which brings us back to studying the lumber itself.

Learning Aspen From Both Ends of the Sawmill

One of the fascinating things about working around lumber every day is that eventually you begin connecting two different forms of identification.

At one end is the tree:

Leaves. Bark. Site. Shape. Branching. Growth habit.

At the other end is the lumber:

Color. Weight. Grain. Pores. Hardness. Moisture. Smell. End grain. Cutting behavior.

The better we become at connecting those two worlds, the less often a rough board is simply an unidentified piece of wood.

Aspen gives us another clue to add to that mental library:

Light-colored, lightweight, very soft hardwood—and when freshly cut and planed, sometimes surprisingly fuzzy.

Basswood or Aspen?

After looking at our first two woods, we already have an interesting comparison.

• Basswood: approximately 410 lbf Janka hardness; lightweight; fine, uniform texture; outstanding carving characteristics.
• Quaking Aspen: approximately 350 lbf; lightweight; straight grain; can be quite fibrous or fuzzy when machined green.
• Bigtooth Aspen: approximately 420 lbf; still lightweight and relatively soft, but slightly harder on average than quaking aspen.

That comparison teaches us something important.

The softest wood is not necessarily the easiest wood to carve or machine cleanly.

Hardness is only one characteristic. Fiber structure, grain direction, moisture, density, sharpness of the cutting edge, and the way those fibers separate all contribute to how a wood behaves in our hands and in our machines.

Aspen: More Interesting Than It First Appears

A freshly sawn aspen board may not immediately command attention. It is pale. The grain is usually quiet. It is light and soft.

But follow the wood back to the forest and the story becomes much bigger.

This is a tree capable of moving rapidly into disturbed ground, sending new stems from an established root system, creating entire colonies of genetically identical trees, feeding wildlife, providing enormous quantities of useful fiber, and growing across an astonishing portion of North America.

Then bring it back into the sawmill and it teaches another lesson.

A wood can be extremely soft and still fight you in unexpected ways.

Those fuzzy fibers coming from a freshly planed board aren't simply a nuisance. They are another clue—another opportunity to understand how the internal structure of a tree becomes the working personality of its lumber.

The more woods we work with, the more we realize that every species has its own personality.

And that is exactly what this Know Your Woods series is about.