Sizing up a Grove of Giants (Sequoias)
By Carrie Pepper
November/December 1992
For American Forests LINK:  Publication 

The Placer County Big Tree Grove in California's Tahoe National Forest offers researchers opportunities to study the mystery behind isolated groves. Discovered in 1855, it was only in 1995 that foresters observed a young Sequoia growing within the area, suggesting reproduction.

Standing in regal isolation, these few giant sequoias are the subject of much study and wonder.

Six trees - even enormous ones - may not seem like a big deal, but these six, known as the Placer County Big Tree Grove, have made quite a name for themselves around these parts. Not only does Placer Grove hold the distinction of being the smallest outpost of giant sequoia in California, but its unique genetic makeup has made it the focus of much study and wonder.

The entire grove - six living trees and two fallen giants - exists on about two acres in the middle of a 160-acre recreation area of the Tahoe National Forest. With such a "low profile," it's no surprise that after its discovery in 1855, the two recommendations made for finding the place were a guide and good horses!

Some 70 to 80 natural groves occur here along the Sierra Nevada's western slope. John Muir was the first naturalist to theorize about isolated groves and how they came to be. Glacial action during the Pleistocene Era, he believed, caused these trees to be severed from a continuous sequoia forest then covering the Sierras.

Growing much further north than other giant sequoia groves, this miniature relic of the past has been a popular case study for many years. No other gap between groves is as great as the 60 airline miles from the Placer Grove south to the next grove, Calaveras Big Tree State Park. So scientists ask. Could this one be different from the southern groves, a separate race of giant sequoia?

Dr. William Libby, professor of forestry and genetics at the University of California, Berkeley, has been studying Placer Grove for over 20 years. He says it differs genetically from over 20 other groves also studied. The population has a high inbreeding level, indicating this grove has always been very small; seeds here germinate with six cotyledons (immature leaves that sprout from the seed). "That's almost never seen in the main populations," Libby says. "The typical number is three. People get kind of excited when they see a five, but a six is just not seen."

Introducing hybrids would probably increase the health of the grove, but scientists agree that the purity of the gene pool would be lost forever. And, Libby says, if this stand is a distinct race of giant sequoia - as it might well be - its future scientific and practical value should be preserved.

Other big trees grow here, too. Douglas-fir - one measures approximately 70 inches in diameter at breast height (dbh) and is estimated to be 500 to 600 years old - and sugar pine are of unusual size. "This is what some of the forests around here used to look like," says Libby. "The giant sequoias are just icing on the cake."

All the trees can be accessed by an easy loop trail. The Joffre tree, the largest, measures 250 feet in height and has a 10-foot dbh; the nearby Pershing tree is a bit stouter at 12 feet in dbh and 225 feet tall. Both were named during a dedication ceremony to these World War I generals in August of 1920.

The two fallen trees are thought to have toppled in a severe storm in 1862. The largest, Theodore Roosevelt, measures 16 feet dbh and stretches over 200 feet along the forest floor.

An underground stream running through the draw provides a cool, moist climate and is thought to be one factor in the longevity and size of the trees here. Wild azalea and dogwood fill the understory with snowy blossoms in spring. Also occurring here is a unique association of other plants such as Sierra laurel, star flower, and fawn lily.

Until quite recently, natural reproduction in the grove was thought to be nonexistent. Fire, which prepares the seedbed by allowing seeds to contact bare mineral earth and removes understory competition, hasn't been allowed for many years. The Pershing tree displays an obvious burn scar, but no one quite remembers when fires last swept through. Prescribed burns have been discussed to clear out dense understory, but so far none have been scheduled.

Is Libby worried about the grove's lack of natural reproduction? "When you're looking at an average lifespan of one to three millennia, you really don't need much reproduction," he says.

Recently, Libby and a colleague came across a young sequoia standing only about 20 feet tall. Growing smack against a white fir that had died recently, it was barely noticeable. Was it a native or not?

Foliage samples were gathered and labeled. "Our job," says Libby, "was to go through a detective process that is not unlike what is used in forensic medicine." Isozyme analysis is a method whereby specific enzymes or chemicals within the tree can be used as a measuring device to determine genetic diversity. Within a species, these isozymes will vary depending on elevation or geographic location. Although the method isn't foolproof, other factors seemed to back up the fact that this young tree was a natural offspring.

A core sample taken by Mike Newman at the Foresthill Ranger District showed the tree to be approximately 85 years old. So it was a seedling well before people have been known to plant in this area. And both Libby and Newman agree it would be a strange place to plant a tree - next to a white fir and on a rather steep slope.

"When you have only six trees and they live in excess of 1,000 years, you need to reproduce them at the rate of only about once every 200 years by one tree. Now we've got our one tree for this century!"

To be on the safe side, two backup plantations have recently been established near the grove. Propagated from cuttings at the USDA Forest Service's Chico Tree Improvement Center, 10 tiny sequoias are just now pushing their roots into Sierra soft.

Two separate sites were planted. District Ranger Richard A. Johnson chose the first because of its similarity, to the original grove location, at the bottom of a steep slope in a damp, bowl-like depression. The other, just down the road, stands in full sun overlooking the Crystal Mountain range. Both sites will be maintained by the Forest Service. With care and luck, perhaps our ancestors will be walking among these giants 500 years hence.

COPYRIGHT 1996 American Forests

 

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