More than two decades of decline had transformed the trunk into an empty pillar and undermined its structural integrity. Like the property’s collapsing barns, the tree’s removal edged the remnants of another Michigan farm into a forgotten past. This softwood maple tree was never suffocated by urban fumes and was a friend to countless birds, squirrels, raccoons, and humans in its life.
As my mom, dad, uncle, and I spent the post-Christmas vacation period splitting and hauling truckloads of wood to my grandpa’s the old chicken house for dry storage, I was intrigued.Lignin is of interest to soil scientists because it is a source of soil organic matter, and one that is often slowly decomposed. With the density and strength of these logs tiring my back and scratching my cheek, I began to wonder about lignin-rich structures on a larger scale. How do “wood scientists” think about wood? This blog post investigates the physical properties of wood.
A softwood hardwood
All maple trees are classified as hardwoods. And some maples, like the one that shaded my grandpa’s front porch for decades, are soft hardwoods. A soft hardwood is not as paradoxical as it may sound. The distinction between softwoods and hardwoods is a botanical one and not based on the actual hardness of a tree’s wood. Thus, some softwoods are actually harder than a “hard”wood and visa versa. The botanical distinction is based on a tree’s reproductive structures. Angiosperm trees are hardwoods while gymnosperm trees are softwoods. Most gymnosperms do have softer wood than angiosperms, but that is not a hard and fast rule.
|Anatomical distinction:||Porous structure||Nonporous|
|Reproductive structure||Seeds enclosed in flowers||Seeds released by cones|
|Typical foliage:||Broad leaves||Needles and cones|
In the soft maple category are: silver (Acer saccharinum), red (Acer rubrum), boxelder (Acer negundo) and bigleaf (Acer macrophyllum). The hard maples are sugar (Acer saccharum) and black (Acer nigrum). Unfortunately I’m not sure which type of soft maple we dismantled.
The strength of a giant
To say that one type of maple tree is harder than another is only expressing one quality of the wood’s strength. Hardness of wood is tested by shooting an 11.28 mm steel ball at its surface. The force needed to embed half of the ball’s diameter in wood is the wood’s hardness. That procedure is called the Janka hardness test and is really just the tip of the iceberg for measuring the strength of wood. Wood is compressed, bent, pummeled, and more to determine its robustness. To measure its ability to absorb impact without structural failure, a hammer is dropped from increasing heights onto a board until it either breaks or flexes 6 inches. In its 509-page Wood Handbook, the USDA’s Forest Products Laboratory (based right here in Madison) gives the strength of all commercially important woods based on the nine most common tests. Soft maple is a middle-of-the-road wood when it comes to hardness. Here’s how soft maples compare to hard maples on three of the tests:
|Test||Hard maples||Soft maples|
|Elasticity modulus||11.9 GPa||9.7 GPa|
|Impact bending (hammer test)||1120 mm||753 mm|
|Janka hardness test||5050 Newtons||3700 Newtons|
The elasticity modulus indicates the amount of force it takes to deform a material. At around 10 GPa (gigapascals), a softwood maple is 20 times easier to deform than steel. Even hickory wood, which is “exceptionally hard and strong” according to Forest Products Laboratory, does not have an elasticity modulusvgreater than 15 GPa. The modulus of elasticity for a diamond is 1200 GPa.
Still, even 1 GPa is one thousand million pascals, which is quite a lot. To put the entire weight of an elephant, ~7,700 lbs/3,500 kg, onto a piece of wood would generate 1 GPa of force. Getting into units of force is dizzying for a non-physicist, but resisting up to 10 GPa of force is no small task.