The Wood Detective

Alex Wiedenhoeft has spent the past 7 years answering the same question about 10,000 times. When foresters, lumber dealers, crime investigators, and museum curators really need to know, “What kind of wood is this?” Wiedenhoeft is one of the few people they can go to. “A man called to say he’d bought an end table at an auction for $15,000, which I found flabbergasting just on general principles,” remembers Wiedenhoeft of the U.S. Forest Service’s Center for Wood Anatomy Research at the Forest Products Laboratory in Madison, Wis.

The caller had thought the table looked European, but after showing it to friends, he had second thoughts. Aspects of the construction seemed early American. If he sent some small samples, would Wiedenhoeft identify the wood and see if that narrowed the possibilities?

Wiedenhoeft studied the samples and reported that they came from one of several kinds of white pine. He stated the limits of his evidence carefully. Yet his report, combined with factors such as the size of the wood pieces and the table’s age, suggested that the man had unknowingly bought eastern-white-pine furniture made on this side of the Atlantic. Not exactly bad news: The potential value of the little table, now recognized as a rarity, jumped to more than $100,000.

The 26-year-old Wiedenhoeft represents the new generation of wood anatomists. They’re familiar with scanning electron microscopy and molecular genetics. Yet most of the cases Wiedenhoeft gets, even the exotic ones, still depend on spotting classic quirks of wood at the tissue and cellular levels. Optics may have changed somewhat since Sherlock Holmes’ day, but Wiedenhoeft still takes his first look at a problem with a handheld magnifying glass.

Wooden past

Wood was among the first things that Anton van Leeuwenhoek looked at as he pioneered microscopy in the 17th century. Since then, close study has revealed that tree trunks are great columns of plumbing. Strings of hollow, dead cells–bundled into a massive structure called xylem–carry water from the roots upward to thirsty leaves.

Counterpart pipelines of living cells, or phloem, carry liquids the other way, bringing the sugary harvest of photosynthesis from the leaves down to feed the rest of the plant. Generally, outer bark and a layer of phloem surround the thicker xylem, which makes up the wood. Trees tend to deploy several forms of xylem cells in distinctive patterns, so wood anatomists can often narrow tree identification to a group of species.

Much as the O.J. Simpson trial popularized the concept of human-DNA fingerprinting, the sensational 1935 trial of a man accused of kidnapping Charles Lindbergh’s baby showcased the power of wood analysis. When Lindberg’s 2-year-old son disappeared in 1932, the kidnapper left behind a broken, homemade ladder. During the 2 years that investigators struggled without finding a plausible suspect, that ladder came to represent what little hope there was of cracking the case. Arthur Koehler, one of Wiedenhoeft’s predecessors at the wood-research center, worked with police to identify the wood, picking out distinctive marks left by a flawed blade at a sawmill.

Police found their suspect, Bruno Hauptmann, by wood-free means: He tried to spend some of the ransom money. Yet wood had a role in the trial when prosecutors charged that a board incorporated into the ladder had been torn from Hauptmann’s house and matched boards still in place there. Koehler testified at what was described as “the trial of the century.” To the end, Hauptmann insisted on his innocence. He was a carpenter, he said, and if he’d made the ladder, it wouldn’t have broken. Nevertheless, he was convicted and electrocuted.

In more recent years, forensic botany has lagged behind other biological disciplines in crime fighting, laments Jane Brock of the University of Colorado in Boulder. “You can go to any forensic lab, and you can find experts in forensic entomology,” she says. Yet she doesn’t find in-house botanists.

She’s a plant anatomist and has worked with investigators several dozen times, identifying food in victims’ stomachs and intestines. Plant cell walls survive digestion in better shape than animal cells do, she says. At the 2002 meeting of the Botanical Society of America in Madison, she convened a special session to encourage academic botanists to offer their expertise to crime fighters. Most current work for wood identifiers lies in commercial disputes rather than in criminal cases, says Wiedenhoeft.

Wood eye?

To identify wood, Wiedenhoeft says he can deal with samples as small as half the size of a kitchen match. In exceptional cases, he’s made do with splinters, though he does not speak of them fondly. The smallest ones he’s ever managed to identify were samples that a curator had taken from Italian wood sculptures at the Metropolitan Museum of Art in New York.

As an example of his technique, Wiedenhoeft describes working with a Texas crime lab last year in studying a splintered pool cue that had been used on someone’s skull. The lab shipped samples of the murder weapon–”from the nonbusiness end, so it wasn’t gory,” says Wiedenhoeft–as well as wood slivers recovered from a suspect’s car.

The question was whether the slivers were the same kind of wood as that in the cue.

First, Wiedenhoeft took a close look at the wood, noting a cross-section and tangential view of the plumbing of each sample. “There are 18,000 species of woody plants in the world, but after 10 seconds with a hand lens, you’re usually down to less than 1,000 possibilities,” he says. Wood anatomists often feel a rush of recognition when they first look through the hand lens, he reports. Microscopic characteristics frequently confirm that impression.

Wiedenhoeft checked first for dots representing large-bore examples of the structures called vessel elements mixed in with more slender cells. If he finds these big structures, he probably has a broad-leaf tree such as a maple or oak instead of a pine or other conifer. The pool cue samples and splinters all had dots.

Next, he looked at the arrangement of these pores. They were dispersed diffusely throughout the wood, so he eliminated hickory, oak, ash, and other trees with vessels clustered in bands.

Wiedenhoeft shaved ultrathin slices from the samples and studied them under a light microscope. He worked his way through choices of cell characteristics, narrowing the possibilities as he went. The vessel cell walls had spiral thickenings. Both pool cue and sliver might be maple then, but not birch. Finally, he arrived at a combination of characteristics unique to sugar maples.

He reported to the Texas authorities that both the cue and the slivers had come from sugar maple. Texas sits well south of the natural range of that tree, he noted, yet it’s a common wood for making mid-grade pool cues.

Had the analysis proceeded differently, Wiedenhoeft might have turned to the collection of more than 100,000 wood samples stored in cabinets next door to his office. Or he might have performed a variety of special tests. Turn on a black light, and some species such as black locust will glow. Or apply a special reagent similar to a component of color film. If the wood turns blue, it’s been sequestering metal and probably belongs to the unusual Vochysiaceae family of South American trees, which accumulate aluminum in their tissue.

Wiedenhoeft rarely gets fancier, however. “Using a scanning electron microscope is not particularly efficient,” he says. “You go through a lot of trouble preparing samples.”

Sometime in the future, Wiedenhoeft says, wood identifiers might be able to use DNA in cases like the pool cue murder to see whether the pieces came from the same tree. Those methods aren’t practical yet although researchers have made progress extracting DNA from wood, a difficult challenge in itself because wood is made up mostly of empty dead cells.

The bigger problem is interpreting results of DNA comparisons. Someone arguing that, say, a splinter and a pool cue came from the same tree based on certain genetic markers would have a tough time showing that those markers don’t occur frequently in any population of sugar maples. Wood geneticists haven’t yet built the huge databases that would be required.

Wood works

Wiedenhoeft’s cases these days feature a lot of commercial outrage. He mimics a typical query: “I paid for sugar maple, and I’m getting Chinese maple. Is it the same thing?” For the record: It’s not.

Asia and Eastern Europe offer plenty of less expensive wood from sister species to popular North American trees. Wiedenhoeft explains, “We have a beech, and they have a beech. We have some maples. They have some maples.” With increasing frequency, he’s running across lumber dealers selling the bargain imports at the same high price as the familiar North American species.

Wiedenhoeft also gets calls about potential treaty violations. In a current case, he’s studying samples from a U.S. company that’s trying to sell lumber and veneer that it claims are at least in part African mahogany. Government inspectors contend that it’s actually mahogany from South America, a species banned from international trade by the Convention on the International Trade in Endangered Species.

And then there are the patent-infringement cases. An attorney in Minnesota asked Wiedenhoeft if he’d identify the wood in some samples of cat litter. The lawyer represented a company holding a patent that it contends prevents any other manufacturer from using the same wood for that purpose. Although the wood particles were “abysmally small,” Wiedenhoeft says, with dimensions of only a few millimeters, he managed to identify some of them. He declines to reveal the results of his analysis because the case is still pending.

Evidence turned out to be unhelpful for Drug Enforcement Administration agents who asked Wiedenhoeft to assist with a case against a man caught with materials for making the drug Ecstasy from roots of sassafras trees. The man had a bag of roots he’d thought were sassafras, but Wiedenhoeft found they weren’t from that tree or any other. He then turned the samples over to the botany department at the University of Wisconsin–Madison, which determined that the roots had come from sarsaparilla plants. The drug agency’s case crumbled.

Collectors and archaeologists also consult Wiedenhoeft, though not always to happy effect. Wiedenhoeft examined a purported vintage war club, “which looked like a thin dowel with a rounded knob on top,” he says. It had been attributed to the Seneca, who had lived in what is now New York State and Ohio. So, Wiedenhoeft was surprised to discover that the wood didn’t come from any tree species native to temperate North America.

Just what the wood really is, Wiedenhoeft’s still not sure. “We had so little material to work with,” he says. “Also, when I told the owner it wasn’t authentic, he lost interest in talking to me real fast.”

Wiedenhoeft has even analyzed truly ancient plant samples. A University of Wisconsin archaeology team is consulting him about spear-shaft remnants from the site of a copper workshop some 5,000 years old in the northern part of the state. He’s told them that the shafts aren’t wood but come from some reed or other tough plant stem. His analysis fits the team’s musings that the spears might have been ceremonial pieces rather than weapons.

He did manage to identify the genus of some bits of wood, perhaps 10,000 years old, extracted from the La Brea tar pits in California. They were preserved because they were impregnated with tar. These samples include plenty of junipers, information that the paleontologists studying the find will use in analyzing ancient climate conditions.

Wiedenhoeft also periodically gets wood samples from an ongoing underwater excavation of a sunken ship off North Carolina that might have belonged to the pirate Blackbeard. The plant anatomist has so far confirmed that the samples are white oak, a sensible and popular choice for shipbuilding in the 18th century, when Blackbeard terrorized the Mid-Atlantic seaboard.

Shipbuilding also came up several decades ago when the center received a bit of wood from what was described as “a boatlike object” found above the tree line on Mount Ararat in Turkey. This sample, too, is white oak, and the center still displays it as–perhaps–a piece of Noah’s ark.

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