This year, I think we need to explore more the science of wood. I'm going to be continuing the series "Getting Technical" that started with Chris Knowles of Oregon State University (OSU) discussing stable isotopes and DNA. This week I'm turning to David Jones of Mississippi State University for more on wood identification. He's moving us from DNA in cells to the actual cell structure.
David, talk to me about cells!
Like a tree's leaves, its cells and cell types are distinctive. Different cell types make it possible to identify wood long after all of the leaves and bark have been removed. The size, type, shape, and distribution of these cells allow the trees to transport water and nutrients to the crown and then food back to the cambium and roots, from the leaves.
Wood (also known as xylem) serves two functions in a standing tree. One function is to keep the tree standing tall and to withstand wind. The second function is to move water and nutrients from the soil to the leaves of the tree. After a tree has been harvested, water will continue to move in and out of the wood freely.
There are several wood types within a tree. They are named based on when the wood was formed or where the wood is within the tree. Heartwood is the center of older trees and is often darker because of the chemicals deposited there as the tree ages. Sapwood is the younger wood in a tree and is active in transporting water and nutrients. It generally has a lighter color and is closer to the bark.
Each year trees add one growth ring. In some species, the ring can be further separated into earlywood (wood formed in the Spring) and latewood (wood formed during the summer).
What about the differences between hardwood and softwood?That's right--trees are generally classified into two different types: hardwoods and softwoods. This classification has nothing to do with the wood itself, but with the type of leaves and flowers the tree has. However, we will also note significant differences in their cell structures as well.
About 90 to 95 percent of softwood cells are called longitudinal tracheids. They transport water. Because there are so few other cell types in softwoods, it can be difficult to distinguish between types of softwoods.
The structure of hardwoods is much more complex. There is also a lot of variation from one species of tree to another in hardwoods. Hardwoods contain vessel elements, or pores, that softwoods do not have. Pores vary greatly; they can be very small, very large, present in great numbers, or almost completely absent. If pores are present, the wood is a hardwood. If no pores are present, it is likely a softwood. Tropical hardwoods have their own distinctive look, so I'll cover them separately.
Got it. So how do we use this to identify wood species?
There are three surfaces, or planes, that we look at to identify wood. These surfaces allow us to see different cells and structures in the wood.
When a tree is cut down, the flat surface of the stump is the cross-sectional surface. The cross-sectional surface shows most of the cell types needed to identify wood. The tangential surface is the next most important surface, followed by the radial surface.
That's a great start. Next week, can you walk me through the identification process?
Absolutely. We'll prep some samples for you to study!