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CALIFORNIA WILD

Horizons

Shades of Redwood

Blake Edgar

Redwoods don't become sexually mature until they're 20. It's not like crossing fruit flies," says Chris Brinegar, a patient molecular biologist who has set himself the task of deciphering the genetics of coast redwoods. Brinegar has been finding tantalizing clues about the diversity beneath the bark of trees separated by even a short distance, but, he says, "Redwoods are a tough nut to crack genetically.

While we get 23 chromosomes from each parent for a total of 46, the coast redwood (Sequoia sempervirens) has 66 chromosomes, divided into three sets of eleven from each parent. And, whereas a human cell nucleus contains two sets of chromosomes, as do the cells of most plants, animals, and other kinds of redwoods, each coast redwood cell contains six sets. In that sense, it's more like a grass than a towering conifer. Having multiple gene sets permits much greater combinations among them–more work for the geneticist, but possibly a vital strategy for the redwood's endurance.

"Being a hexaploid is a real advantage in terms of being able to maintain genetic diversity," says Deborah Rogers of the Genetic Resources Conservation Program at the University of California at Davis. For her doctoral study Rogers examined patterns of genetic variation among more than 650 old-growth trees in Humboldt Redwoods State Park. Looking at certain enzymes as an indirect measure of DNA diversity, she found a wide range and noticed that these markers changed with elevation and with distance from a river.

Brinegar sampled genetic diversity among redwoods from two gardens planted with small trees collected from Oregon south to Big Sur. He clipped branchlets, froze them in liquid nitrogen, and brought them back to his lab for DNA fingerprinting. The fingerprints showed that no two trees were alike. And there were hints that the relatively scattered redwood populations south of San Francisco Bay had quite distinctive genes from the more densely distributed trees further north.

These studies are expanding on pioneering work by Bill Libby and others that first noted the redwood's genetic variety. Brinegar finds the degree of diversity somewhat surprising. For one thing, redwoods are confined to a narrow coastal strip of their former range. And while redwoods reproduce sexually, an individual redwood can also sprout several new seedlings–all identical clones of the parent–from its trunk.

These stump-sprouts begin as dormant buds, or burls, that bulge from the base of a tree until fire, logging, or another disturbance stimulates their growth. The frequent phenomenon of "fairy rings," a circular array of adult redwoods, had been thought to be a mature cluster of identical sprouts growing around the site of a long-decayed parent tree. But Rogers found that in the circles she sampled, at least one redwood was not a clone of the others; often the clones were a minority. Something more complicated must be going on.

Maybe seeds from other trees managed to germinate and survive, shading out some stump-sprouts. Or maybe, given enough time, individual trees accumulate distinctive mutations. Conspicuous evidence that they can mutate comes in the form of occasional albinos spotted in the Santa Cruz Mountains. Unable to make chlorophyll, the albino trees remain stunted, and says Brinegar, "They look like artificial Christmas trees. They're white as snow...and right next to them are sister sprouts that are perfectly green."

Brinegar wants to learn how close in space trees need to be before differences disappear between their genes. At Castle Rock State Park, trees only 50 feet apart still showed clear differences. Next, Brinegar wants to choose a plot, probably in Butano or Big Basin state park, and examine every redwood–the approach that Rogers took in her Humboldt study--to see how local geography affects genetics.

But first he's refining his lab technique. Rather than grind up a branchlet with about 20 needles on it, he'll be extracting DNA to fingerprint from a single needle–cutting down the time it takes to prepare enough samples for population-level analysis from a few weeks to a few days. Brinegar hopes that with the new technique, he and his students can sequence specific portions of the redwood genome. These regions could then be targeted for study, and the same region of DNA located in each needle sampled–not possible with Brinegar's current method–in order to make more meaningful comparisons.

For now, our understanding of redwood genetics remains so basic that all Brinegar and Rogers can say is that redwoods exhibit considerable genetic diversity; how much, and what's due to mutation versus sexual reproduction or other sources, is still anyone's guess.

So if you're out hiking among redwoods this winter and you stumble upon a suspicious group wearing latex gloves and carrying pole pruners and containers of liquid nitrogen who, in Brinegar's words, "look like homicidal proctologists on a field trip," don't be alarmed. It's probably just Brinegar and his team taking the next small steps in their daunting task.

"It's hard for us to think in terms of the immense timeframes over which these trees reproduce and change," says Rogers. Echoes Brinegar, "I wish I could live as long as a redwood tree so I could nail this down."

Dan Chure had a dilemma. As paleontologist for Dinosaur National Monument in Utah, he oversees one of the richest dinosaur graveyards anywhere. But the famous fossil quarry had yielded mostly the big bones of long-necked, long-tailed Jurassic giants like Apatosaurus and Camarasaurus and precious few bones from their fearsome predator, and Chure's speciality, Allosaurus. Then, in 1990, came the chance discovery, a half-mile from the park's visitor center, of a few toe and tail bones–the rear end of a carnivorous dinosaur. Perched 20 feet up a 70-degree slope, Chure and colleagues began a painstaking excavation.

"After three and a half years, we got to the second vertebra of the neck, and there was no skull, which was disappointing," Chure says with understatement. He had the skeleton of an immature, 18-foot-long Allosaurus, but the skull had either eroded long ago or remained hidden in the hillside. There was no way to tell, and further digging would require risky tunneling.

Chure had all but given up on the skull, when a colleague put him in touch with Ramal Jones, an amateur fossil hunter and radiation analyst at the University of Utah. Jones had recently combined both areas of expertise to develop a promising tool for finding buried dinosaur bones. Using a shielded gamma scintillator, an instrument that he has patented, Jones can locate radiation from uranium trapped inside fossilized bone. The minute amount of radioactivity, comparable to that from a household smoke detector, would normally be swamped by natural background radiation present in soil and rock; so Jones shielded his detector in lead, leaving only a dime-sized opening on the bottom for gamma rays emanating directly from bones beneath the device.

Jones had put his detector through its paces in southeastern Utah at the Carol/RJ Quarry, named for his wife, who found the first bone fragments there. Once Jones realized that the bones were radioactive, he hit upon the idea of using a scintillator to map the site and see if he could pinpoint where other bones lay. In 1993, accompanying an excavation team from the College of Eastern Utah Prehistoric Museum, Jones located hot spots beneath some bones on the surface at the Carol site. Digging revealed the hot spots to be much of the skull, vertebrae, ribs, and limb pieces of a 100-million-year-old hadrosaur, a duckbilled dinosaur not thought to have existed in North America that long ago.

The following year, Jones detected radiation coming from the partial skull and skeleton of an armored nodosaur three feet beneath the surface. No bone fragments covered the surface in this corner of the site, so without Jones's survey the beast would probably still be buried. (Both dinosaurs have since been named for the Joneses, respectively Eolambia caroljonesa and Animantarx ramaljonesi.) Because the fragile bones broke easily upon exposure to air, Jones's detector was designed to help guide excavators to dig around each bone and to carefully remove them onto a plaster-coated pedestal of dirt.

Given that track record, Chure says, "We figured, what the heck, the worst we could come up with was still no skull." In the summer of 1996, Jones measured the radiation level of the Allosaurus skeleton, encased in plaster at the Dinosaur National Monument lab, and then measured the excavation site. A spot in the wall about six feet from where the skeleton had been removed gave an equally strong radioactivity reading, telling Jones that something lay near the surface. The second blow from a rock hammer struck the back of a skull. "There are very few happy endings like this in dinosaur paleontology," says Chure. The skull's left side was intact and exquisitely preserved, and Chure had the evidence to describe the specimen as a new species of Allosaurus.

Jones the dinosaur diviner next took his tool to Hagerman Fossil Beds National Monument in Idaho, where paleontologists prepared to reexcavate a bed full of bones from three-million-year-old horses. Radioactivity readings were taken every two feet along a grid covering the site. Jones recalls, "Where we said there was bone there was bone, and where there was no indication of radiation, there was no bone." The strongest readings occurred at spots with dozens of fossils packed together up to a foot below the surface.

The radiation detector had proven itself in finding buried bones from the Pliocene, Cretaceous, and Jurassic, from relatively small mammals to relatively big dinosaurs. It predicted the location of single fossils and dense deposits. No wonder Jones attracted a lot of interest last fall at a presentation to the Society of Vertebrate Paleontology. No other remote method for pinpointing the location of fossils in the ground has met with such success.

There are still some caveats. The bones must contain uranium, and they can't be buried more than a few feet deep. The technique's sensitivity varies with the amount of uranium present, the size of the bones, and how they came to be buried. But a paleontologist pressed for time or money, or excavating a remote site, needs to know where to expend the most effort. "This isn't the great panacea for vertebrate paleontology, but it's still a very useful tool," says Chure. "Even if Ray never found another bone, I'd swear by it."

Blake Edgar is Associate Editor of California Wild.

Winter 1999

Vol. 52:1