In our final post on wood identification with Part 2 of our species ID discussions.)
David, again, thank you so much for doing this. To wrap up this basic review of wood identification through cell structure, tell us about tropical hardwoods.
Tropical hardwoods have their own unique challenges when trying to identify them using standard methods. The first challenge is that there are several species groups that utilize a common name that bears no relationship to the actual species of trees that they come from.
They are simply grouped together based on gross anatomical similarities, usually color, and because it makes some type of marketing sense. The best example of this is mahogany. True mahogany is part of Swietenia genus, but most of the legally traded mahogany is from species that are not considered true mahogany from a botanical standpoint. There are approximately 15 species that are traded now that are called mahogany. Most familiar would be the "African mahogany" grouping, which are various species in the Khaya genus, the use of "Philippine mahogany" to describe the Shorea genus (which is known most frequently as meranti in other countries) and, of course, the flooring industry identifies Myroxylon balsamum as santos mahogany.
Of course, this can happen in some temperate species, too, for example, the group of poplar, which can contain yellow-poplar or tulip-poplar (Liriodendron tulipifera), eastern cottonwood (Populus deltoides) and white poplar (Populus alba), of which the latter two are the only ones that are in the actual poplar genus. But it's easier to identify temperate hardwoods than tropical.
Why is it difficult to identify tropical species?
Tropical hardwoods often do not have the growth patterns as temperate trees. Because tropical trees have the ability to grow continuously throughout the year, growth ring boundaries are not present. Because we often rely on seasonal changes from earlywood to latewood or marginal parenchyma at the end of the growing season to help with identification, tropical hardwoods can be difficult to deal with.
Further, tropical hardwoods can vary in color, hardness and strength based on where they were grown, what the growing conditions were, and how they were handled after they were cut and turned into products. The sapwood of purpleheart (Peltogyne) is actually white before the heartwood is formed, and the heartwood is purple but oxidizes to brown after being exposed to air for an extended time. Greenheart can vary from brown to green. In both cases using color as an indicator is not recommended.
Isn't that true for temperate hardwoods and softwoods, too? Is there a greater difference between, say, ipé in Brazil and Paraguay or northern/southern red oak?
It is certainly true that the color of our temperate hardwoods can vary. Red oak is a good example of that-it can be a pinkish red or it can be completely white. However, I believe that the variation in color of tropical woods is much greater than that, often to the point that without looking more closely, you would think because of the color difference that the wood could not possibly be the same species.
Any other issues?
Finally, because tropical woods can also have silica deposits and gums in the pores, the hardness of two identical pieces of wood may be different. This difference is more often detected in the machining process rather than when a processed sample is being identified.
The bottom line is that, like many domestic woods, some identification can be done on gross anatomical features, but the majority of tropical woods need to be identified utilizing a microscope to look at the ultrastructure of the wood, and, even then, many of the species can only be identified down to genus or species group.
How then do you really know what the tree is-is it through leaves and bark and something else if you can't tell from a wood sample?
The only true way to know what species a wood/tree is is to examine the leaves, bark, and ultimately the flowers. Many of the different species are separated out on the characteristics of the reproductive parts of the tree. That's why telling the difference between red oak lumber that comes from different species of red oak is impossible to do without seeing the bark, leaves, buds, or flowers. Just looking at it through a microscope might tell us that it is red oak, not white, but I usually won't know which red oak it is without seeing the tree itself. The challenges are magnified with many of the species in the tropical regions of the world.
So what does that mean for identification for legality purposes? Does that mean it's not always possible to know if the ipé is from Brazil or Paraguay? And would a DNA study possibly answer that?
With very rare exceptions, it is impossible to specify the location from which a sample of wood has come from using tools of wood identification. This is one reason why the potential of DNA testing is so important, although it hasn't met the expectations that we had, at least not yet. And DNA, of course, tells us more where the wood was from, rather than answering that basic question of what the wood is.
Thank you, David.