Fluorescent property in the heartwood of black locust

The fluorescent properties of wood

Every now and then I make an observation that leads to a wall, a gap in the scientific literature, some void where I want there to be an answer and the answer lies just beyond. And as much as I like answers, I love mystery. And so here we are, at the edge of understanding, meeting some trees that fluoresce. We describe what we find – the species that fluoresce, the parts of the trees that glow – and we’re left in the dark when it comes to why this phenomenon occurs, what adaptive significance there might be behind buckthorn sapwood, but not heartwood, fluorescing.

A bunch of years ago I learned the basics of woodturning and made several bowls from cherry blanks. I never really got comfortable with doing the process on my own and pretty quickly put woodturning on pause. And then, back in December, I came across a massive dead staghorn sumac – maybe 12″ across, 30′ tall. It’s rare to find a sumac more than 6″ across so I decided I’d take advantage of the boon and get back into woodturning. Thanks to all around handyman, woodworker, and Youth Programs Director at Crow’s Path, Ross Doree, I was back at the lathe and ready to transform the log into a bowl. Part of the reason I was so excited to turn a sumac bowl was because of its fluorescent properties.

After turning the bowl, I got out my trusty UV flashlight, and sure enough. Wow! I’ve obsessively been shining my light on all manner of objects, wood and otherwise, and have found fluorescent pitch, fungi, lichen, and and insects (the pillbugs caught by cellar spiders in my basement fluoresce, the spiders do not). Here are some other surprising things that fluoresce:

Porphyrin a pigment in the feathers of saw-whet owls (and other owls) flouresces under a black light. Porphyrin in feathers breaks from the tips to the base. New feathers (like the ones above) appear pink under the black light as the porphyrin has yet to break down. Hatch year owls have a single "cohort" of feathers. Second years (SY) have two different ages of feathers and After second years (ASY) have more than 2.

“Visible” Light

We see in the visible spectrum, from red to violet. The gradient from one color to the next represents a shift in the light’s wavelength (reds are low energy, long wavelength at 700nm while violets are high intensity and shorter at 400nm). Just beyond the margins of the visible spectrum are infrared (long wavelengths) and ultraviolet (short wavelengths). While most species have a rather narrow band of light that they can see, there are quite a few species that can see beyond our visible range. To them the world looks quite different than it does to us. A mosquito sees the infrared signature of body heat and CO2 (e.g. some snakes too), while a bumblebee sees ultraviolet patterns embedded on the petals of a flower that guide it to the sweet nectar reward at the heart of the flower. So what we call the “visible spectrum” is merely what is visible to us. Fortunately we have developed many different tools to “see” these other wavelengths.

The electromagnetic spectrum showing the very small portion that is the visible spectrum.

Seeing ultraviolet radiation

When we look at a green leaf, we’re seeing the light that’s reflected off the object. Each object (all other colors are being absorbed). Black objects absorb, white objects reflect all the visible wavelengths. It’s why on a sunny day snow is so blindingly bright and a black coat is so pleasantly warm. The black locust shavings in the image below may be reflecting a broad range of wavelengths, but we’re only able to see those in the visible spectrum: greens, browns, and yellows. By using a black light we can “see” the ultraviolet light that it reflects. Well, we’re not actually seeing ultraviolet radiation, but we are seeing the result of ultraviolet light hitting an object and reflecting back as visible light.

In my adventures with the black light, I’ve found the following woods to fluoresce:

  • Staghorn sumac: brilliant white sapwood and yellow green heartwood
  • Barberry: brilliant yellow
  • Buckthorn: most of the sapwood
  • Red oak: faint streaks in heart wood
  • Conifers: the pitch from conifers tends to fluoresce (seen in white cedar, Norway spruce, white pine, Scots pine)
  • Boxelder: the red fungal stain causes the wood to fluoresce

(Note: the above list is still very much a work in progress)

Wood shavings of black locust and sugar maple under fluorescent light
Wood shavings of black locust and sugar maple under UV light

So why fluoresce?

I first read about fluorescent wood in Hoadley’s excellent text, Understanding Wood. He aptly describes the physical phenomenon of fluorescence, but doesn’t give an evolutionary or adaptive reason why some trees might have this property. It’s a little easier to see how fluorescence might serve animals. And indeed, there are a range of hypotheses for why animal fur might fluoresce: predator avoidance, parasite deterrent, mate attraction. These all rely on visual cues to other organisms that can see the visual phenomenon. But with wood, which is hidden behind a protect layer of bark, the fluorescent properties aren’t associated with any visual cues available to animals (as would be the case for flower petals that reflect UV light to guide pollinators).

There’s some evidence that the fluorescent property is the result of the presence of flavonoids, phenolic compounds frequently found in wood. There are more than 6,000 different flavonoids, and their functions are quite diverse, from protecting plant tissues from UV radiation to deterring herbivores from munching unripe fruits. If true, then fluorescence is only the byproduct from another adaptive purpose. Here’s a good synopsis of what flavonoids are: link. Be forewarned, this all gets pretty technical pretty quick.

Many of these plant-derived compounds fluoresce under UV light. Hoadley says that wood tends to lose its fluorescent properties through time if exposed to sunlight, and I can personally attest to this as the sumac and black locust spoons that I carved between 2009 and 2013 no longer fluoresce. If it is the flavonoids responsble for the fluorescent effect, it’s possible that weathering and the breakdown of these flavonoids Hoadley also writes that early (spring) wood and late (summer) wood may differ in their reactivity. So a tree may produce fewer defensive compounds early in the spring when herbivorous animals are less active.

So not an answer, but certainly an observation that warrants more attention!

Gallery of fluorescent woods

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