Hardwood is hard...and softwood is soft. Right?
No, not necessarily. This is by far the most common misconception people have about wood when they browse the aisles of the local lumber yard or big box building products store. So, you might think..."Well, why do they call it hardwood, and softwood, then?."
The most likely reason has to do with logging back in the old days. Farmers clearing their land in the east back in the 18th and 19th centuries would have encountered a great range of deciduous trees, scientifically categorized as angiosperms, those that have broad leaves, true flowers, have their seeds enclosed in a fruit, and shed their leaves in the fall (they are deciduous). The soil of the eastern part of North America was typically thick and rich in the valleys, because of the ancient age of the Appalachian mountains and the temperate climate that inhibited frequent and large wildfires. The result was a widely ranging deciduous forest, and the varied species that made them up consisted of a large percentage of oak, hickory, and maple. The oaks and hickories were spread far and wide by animals that loved the mast (nuts) produced by them, and recycled them periodically in their ramblings.
Maples, on the other hand, are prodigious self-seeders; their seeds are encompassed in a light fruit sack that has wings which take the seeds on a flight of the wind's fancy. They are also powerful stump sprouters, and reproduce themselves easily even when the farmers cut them down for timber or firewood. So, maple trees are everywhere in the Northeast, and provide it with lots of syrup in the early spring and color in the fall.
Now, the folks that were out there clearing all these oaks, hickories, and maples with axes and two-man saws, and shaping them for utensils with draw knives, found them pretty tough customers. The oaks and hickories, in particular, are heavy woods, going from 60 to 70 pounds per cubic foot (960 to 1120 kilograms per cubic meter). And the folks down south, who were harvesting live oaks for ship timbers and bows, really had a chore...live oak is the heaviest hardwood in North America, running well over seventy pounds per cubic foot (1120 kilograms per cubic meter) when green.
Apparently, these "heavy" species (technically, the ones with the highest "density") left enough of an impression on these pioneers that they generally thought of the deciduous angiosperms as "hard" wood, even though other species, such as cottonwood, aspen, American elm, and American chestnut, which were common back in those days, were quite a bit lighter. The aptly named cottonwood weighs less than sixty pounds per cubic foot (960 kilograms per cubic meter) when green, and whittles easily with a dull pocket knife, as I found out as a kid, a long time ago. For this reason, cottonwood has always been one of my favorite trees...its leaves fan the air on hot summer days when there is no air, and thereby help perpetuate the state of mind that "it's not so hot out here..."
Now, those old-timers generally didn't talk about different woods like scientists. They didn't have time or mental energy to waste thinking about the relative variability of wood properties expressed in different angiosperms at different moisture contents or growth rates. They just knew that the deciduous trees really wore out their saws and axes, and their muscles...so they got in the habit of calling them "hardwoods".
As opposed to the gymnosperms, which are those cone-bearing (coniferous) trees that have needles and retain them in winter; that is, they stay green when the other trees drop their leaves. Most of the coniferous trees in the Northeast are fairly light species; the famous Eastern white pine, which was the favorite of the King's navy back in colonial days for its straight, light, yet strong wood, and made perfect masts for their ships, slightly lighter than cottonwood at about fifty to fifty-five pounds per cubic foot (800 - 880 kilograms per cubic meter) when green. Eastern hemlock, the state tree of Pennsylvania, is just a tad lighter, at 48 - 50 pounds per cubic foot (775 - 800 kilograms per cubic meter), as is Eastern red cedar, at about 45 - 52 pounds per cubic foot (700 - 830 kilograms per cubic meter). Naturally, then, when the heaviest Northeastern conifers were compared to hickories or oaks, the conifers seemed light by comparison, and so became "softwoods" in the common vernacular.
As we spread out into the rest of the country, our common man's classification system began to break down. As we harvested the Lake States to build Chicago, Milwaukee, and Detroit, we found that aspen was a pretty "soft" deciduous tree, even though we had already mentally tagged the leaf-shedders as hardwoods. And early settlers out west found abundant red alder, a light-weight hardwood that has somewhat the look, weight, and feel of Western red cedar.
But the folks harvesting the southern U.S. were really confused, because not only did they find the super-light "hardwood" species basswood and cottonwood, but they found some of the continent's heaviest softwood, of which four species, longleaf, slash, shortleaf, and loblolly pine, are now marketed under the unifying moniker of Southern Yellow Pine. Southern pine not only has a relatively high density when dry (try driving a nail into a southern pine stud with ten or more growth rings per inch, and you'll bend a few nails) but it also has a resinous "pine tar" that served navies well in the wooden ship days. "Naval stores" were buckets of pine tar and turpentine that were used to caulk seams and cracks in hulls, and seal wood from moisture, and this pine tar, or "pitch" retained moisture in the stem and added even more weight to the wood. As a result, old southern pine trees could yield pitch-filled logs that could weigh as much as oak even though the specific gravity (the weight of a wood species relative to the weight of water) is quite a bit lower than those tough old oaks and hickories.
As you can see in the picture of southern pine cell structure, softwoods are comprised of long, thin tubular cells, and it is these that carry the water through the stem of the tree. It is this uniformity, in addition to the density of the wood, which makes softwoods seem relatively soft when being sawn or machined.
On the other hand, the moisture is transported in hardwoods through larger diameter pores, or vessels. These come in different shapes, sizes, and locations in the different hardwood species, and this variation contributes to the woodworker's sense that certain hardwoods are rough, or "hard" to saw and machine.
Nowadays, wood "hardness" is complicated (or, depending on your point of view, simplified) further by hardness standards developed and adopted for wood grading for different products. The most commonly used hardness metric used in the various wood industries is the "Janka-ball" hardness test, which is the amount of pounds-force (lbf) or newtons (N) required to imbed a .444 inch (11.28 millimeter) diameter steel ball into the wood to half the ball's diameter, as specified in ASTM Standard D143. These standardized results are then used as a relative measure of the hardness of a wood, the results of which are fairly easy to find on the web. If you browse a Janka hardness table, you'll see that the hardest woods are tropical hardwood species, but then below that, softwoods and hardwoods are relatively randomly mixed.
So, now you know the rest of the story...that hardwoods aren't necessarily hard, and softwoods aren't necessarily soft..and why.