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Wednesday, February 5, 2014

What's Ambrosia Maple?

Wood can have a variety of figured grain types, including the familiar birdseye, striped, curly, quilted, etc. These are all the result of anomalies or peculiarities in the physical structure of the tree's wood tissues. All are desirable to woodworkers, and trees that contain such figured wood are uncommon or rare when compared to the number of trees there are growing on the planet. Furniture and musical instrument makers especially seek out these prized figured woods, which can occur in many, if not all, tree species.
Tiger Striped Apple Wood Vase

The causes of these different figure types are not completely understood, but generally are the result of variations in the tree's vertically oriented tissue cells; that is, the cells that normally grow longitudinally in the tree have some shape or orientation other than what is typical. When the lumber is sawn, the figure is exposed, depending on the plane of the cut made through the log.
Multi-figured Bastogne Walnut

Maple? Go figure.

 There is another "figure" commonly found in various species of maple trees, known as "ambrosia" maple. It is not a grain figure formed by the structure of the wood itself, but rather is a stain within the wood resulting from the interesting activities of the Beatles, er, that is, the ambrosia beetles. That moniker describes a group of weevil family beetles who dine on the food of the gods, as it were. But first, a short discussion, titled ...

Wood borers aren't boring

Generally speaking, female adults of the myriad number of wood boring beetle species deposit their eggs in the crevices of bark, or chew a tunnel into the tree's wood where the eggs are laid. The eggs hatch and the larvae tunnel into the wood to feed, eventually emerging as adults to repeat the cycle. In some species, it's an annual cycle; in others, it may take years for the larvae to become adults and emerge from the wood. In some cases, the tunneling can be so extensive that the tree's water conducting vessels are seriously damaged, resulting in the tree's death.

Many species of wood boring beetles exist. Some feed on the wood itself, and may or may not be significant pests. Most are important parts of our forest ecology, helping to decompose dead trees. A few non-natives are invasive and highly destructive to live trees though, such as the Asian longhorned beetle and the Emerald Ash borer, which kill trees by their tunneling activities. These two species are currently wreaking havoc on the eastern deciduous forest.

Other species inadvertently carry various fungi and/or bacteria into the trees, or at least make pathways for these organisms to enter; but ambrosia beetles put a slightly different, peculiar slant on this ...

Weird things in the woods

Although there are thousands of species of ambrosia beetles, one thing they have in common is their curious feeding strategy. As with other wood boring beetles, the adult female lays eggs in a tunnel she excavates in the wood (usually of dying or dead trees). But here's where it gets more interesting...

Ambrosia beetles have special organs ("mycangia") within their bodies to carry the spores of symbiotic ambrosia fungi; upon chewing a tunnel in the wood, the beetle releases some of these spores from her body, which quickly grow on and into the walls of the chamber. The fungus digests some of the wood tissue, and makes nutrients available on the tunnel walls. The female essentially cultivates her own "garden"... she's a fungus farmer!

She deposits her eggs in short side galleries, and the hatched larvae are raised there, feeding on the fungus the female has been growing for them. She tends to them while they grow; both adult and larvae feed on the fungus growing on the tunnel walls, not on the wood. The tunnels are kept clear of wood dust and excrement produced by the larvae, the female often pushing it out of the tree (if you examine the small beetle holes in ambrosia lumber, you'll usually find that they're clear of wood dust; the tunnels created by other boring insects will often be packed with wood dust). When the larvae have grown to become adults, they gather fungal spores and emerge from the tree to start their own colonies. These critters don't tunnel voraciously through wood, eating all the wood they can; rather, they exploit the moist environment within the wood only to grow their main food source, and raise young.

Who knows what weevil lurks in the hearts of ...

Some researchers have found that there are a few species of beetles that have short-circuited the system: these abominable ambrosias can locate the fungus garden galleries of larger ambrosias, drill smaller holes next to theirs, then steal the other's food supply! Some have become so successful at this that they no longer have spore-carrying organs, since they don't have to transport spores to start their own gardens. Hmm, I could make a political comment here, but let's keep this civil.

Isn't this fun, Gus?

Like the beetles, there are many kinds of ambrosia fungi, but they're mostly found only in these symbiotic associations with the beetles, typically nowhere else. That's why it's a symbiotic relationship- the fungus is propagated by the beetles, and the beetles rely on the fungus as their principal food source. The fungus is passed on to successive generations of beetles, as described above, which spread it through the forest.

An Ambrosia Maple Log

With the fungus now introduced into the tree, it grows and spreads into the wood tissues, staining it with color. That's what creates the attractive figure we call "ambrosia". A look at the end grain of a cut log that's infected with ambrosia fungus often reveals a somewhat starburst pattern of staining, typically radiating from the center of the log.

A number of tree species can be infected with ambrosia fungus, but here in New England we often find it in Red Maples (Acer rubrum), where it produces a gray and/or brown stain in the wood.

Here are some photos of a Red Maple that was cut down on a golf course this winter. It lay about 100 feet from the road, largely snow-covered. As I was driving by, I could immediately identify not only the fact that it was an ambrosia log, but the species as well, just by the appearance of the end grain staining. Once you become familiar with the look, it's easily recognizable at once. 

Several pictures of the cut ends of the logs show typical Red Maple ambrosia stain patterns.

Ambrosia Red Maple log

Ambrosia Maple Stain Pattern

Ambrosia Maple Staining

More Ambrosia Maple Staining

Ambrosia (Red) Maple Bowl

Ambrosia staining in Boxelder log
And Now, a More Colorful Story

A more spectacular example of ambrosia stain is found in another maple species, boxelder (Acer negundo). When found in this species, it produces a vivid, unforgettable red stain, sometimes called "flame boxelder". The effect can be stunning. The photo at right shows a freshly cut boxelder log end, with  a fair amount of staining.

Here are two (not yet completed) hollow vessels turned from ambrosia boxelder. As you can see, the red staining is quite beautiful, especially once a finish is applied.
Ambrosia Boxelder vessel

Pattern running down sides of Boxelder vessel

In these photos, you can see small holes that are evidence of the beetles' tunnel boring. When you see them, they're almost always within a streak of the stained area.

Beetle tunnel holes visible
Beetle holes in Boxelder
Although boring beetles are often considered to be pests that damage trees and the lumber we get from them, in some instances they certainly do enhance the results, and we woodturners appreciate their efforts!

Starburst Ambrosia pattern in Boxelder vessel


The vessels shown here, and others, will be available at Bowlwood when they're completed.

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