Delicate Giants : Warming Trend Adds Urgency to Study of Ancient Sequoias

Thriving in pockets on the western slopes of the Sierra Nevada, California’s giant sequoias have bucked the odds for millions of years, surviving raging fires, glacial ice and the violent upheavals that create mountain ranges.

The good news is that, despite their tumultuous history, the Sequoia groves are in good shape and growing vigorously.

But now the trees are facing a new peril, a relatively sudden global warming that may occur as a result of the greenhouse effect.

Scientists are discovering that the sequoias are sensitive to subtle temperature variations associated with global climate changes. To survive, as they have in the past, the trees must have time to adjust, the experts report.

Sense of Urgency

These new findings have added a sense of urgency to a study of sequoias under way here and in Yosemite. By analyzing tree growth rings, sediments cored from nearby meadows and the mummified droppings of pack rats that lived in caves along the Kings River 35,000 years ago, the experts hope to learn more about how the trees have lived so long and what it will take to ensure their continued existence.

The Sequoiadendron giganteum is one of the oldest and largest living things on the planet. The prime specimen is the 2,700-year-old General Sherman Tree, standing nearly 275 feet tall, with a base diameter of 36 feet and a trunk weight of 1,385 tons.

Fossil evidence of these giants has been found in Colorado, Wyoming and Nevada, indicating that the great forests existed there before the tectonic upthrust of the Sierra 6 million years ago. The towering Sierra blocked the flow of Pacific storms over the Great Basin, drying out the land, dooming the sequoias east of the mountains.

Today thousands of these trees are clustered in 75 groves on the pine-forested western Sierra slopes. The northernmost groves are in federal forests near Lake Tahoe, and they range south 250 miles to the Tule River drainage in Tulare County. Nearly half of the trees are in national parks.

Bypassed Groves

One riddle researchers are trying to solve is how the trees survived the Ice Age that ended 10,000 years ago. Originally, they theorized that perhaps the glaciers somehow had bypassed the groves.

Recently, however, scientists were surprised to discover that the present-day groves date back only about 5,000 years. That meant that the groves surviving today did not come into existence until 5,000 years after the glaciers had melted away.

New research, although far from conclusive, suggests that the groves survived by moving as the climate changed. Each new generation took root in a slow-step march up or down the mountains, seeking suitable climate, adequate soil moisture and nutrients.

As the glaciers advanced, the sequoias moved to lower elevations, and after the ice flows melted, the groves moved to their current locations.

The discovery that the trees may, in fact, move in response to global temperature changes was made by paleo-ecologist Scott Anderson, director of Northern Arizona University’s pollen laboratory in Flagstaff. Anderson studies the pollen found trapped in sediments in Log Meadow in Giant Forest to find out what kind of plants have lived in this part of the park since the glaciers melted.

By coring 30 feet into the sediments, Anderson discovered that until 4,500 years ago, the dominant plants were pines, huckleberry and chinquapin bushes--vegetation typical of a dry, open forest, a place without enough moisture to support sequoia growth.

“There was virtually no sequoia pollen in Log Meadow 6,000 years ago,” Anderson said.

From that time forward, evidence of the big trees began to accumulate in the sediments, indicating that the Giant Forest grove grew rapidly, expanding until it began to dominate the landscape. Meadow grasses and other plants requiring plenty of moisture also began to flourish, Anderson found.

The evidence suggests that immediately after the last glacial period, the area was warm and dry. This bit of information coincides with a known warming period that lasted several thousand years because Earth’s solar orbit and the tilt of its axis put the Northern Hemisphere closer to the sun.

During this time, summer temperatures rose enough to dry out the soils. When cooler, wetter climate returned, the soil moisture increased and meadow-like conditions made the area ideal for sequoia growth.

May Provide Clue

All this may provide some clue as to how the sequoias may react to the kind of rapid global warming many scientists expect as a result of the greenhouse effect.

“The altithermal (warm, dry period) is the best analog of what might happen if the greenhouse effect does occur,” Anderson said.

If temperatures do go up several degrees, as predicted, he said, “we would expect the type of vegetation that had grown in Log Meadow 6,000 to 10,000 years ago . . . but no sequoias.”

The question then becomes what would happen to the sequoias. The supposition now is that the groves have survived climate changes by slowly moving and adapting to new locations, a natural process that requires thousands of years, park scientist Dave Parsons said.

The latest information suggests that the sequoias move long distances in their struggle to survive. The evidence has come from an odd source: ancient pack rat nests found in limestone caves in a rugged, low-elevation canyon along the Kings River.

No sequoias grow in this hot, dry canyon now, but contents of the pack rat middens strongly suggest that the trees once grew here, according to ranger-scientist Ken Cole. The rats use the same nests, generation after generation, accumulating twigs, rocks, bones, nesting materials and anything else they find and can carry back to the den. The rat’s urine mummifies these droppings, preserving them for eons.

Pack Rats

Cole found several pack rat middens in the caves in Kings Canyon National Park, just north of Sequoia Park. In these ancient nests, there were mummified pieces of sequoia twigs and pollen, evidence that thousands of years ago the trees were growing close by, at elevations as low as 3,000 feet. The closest sequoias today are miles away, at much higher elevations.

Carbon dating revealed Cole’s findings were 18,000 to 35,000 years old. That meant the trees were here during the coldest part of the last glacial period.

This, according to Cole, is the first direct evidence that the sequoias grew at these lower elevations during the Ice Age. The supposition is that as the globe warmed and the glaciers melted, the trees marched back up the steep mountains to their present location at around 7,000 feet elevation.

Whether the sequoias can out-maneuver the kind of rapid global warming scientists now are predicting as a result of the greenhouse effect is open to question.

Release Gases

Nature’s slow-moving atmospheric clock has being speeded up by the impact of modern technology. As autos and industrial plants release more carbon monoxide and other gases into the air, more heat is trapped inside the atmospheric greenhouse. Some experts predict that temperatures could rise two to nine degrees over the next century.

There is general scientific agreement that this global warming will occur at a rate 10 to 100 times faster than ever before, according to Stephen H. Schneider of the National Center for Atmospheric Research in Boulder, Colo.

In addition to temperature changes, scientists also are studying how sequoias react to fire and drought.

It was not until the late 1960s that national park officials realized that fire was an essential ingredient in the sequoia ecosystem. By suppressing fires for a century, rangers had literally disrupted the sequoia’s reproductive cycle. No new sequoias were sprouting.

Now rangers systematically torch off small sections of the groves in control burns designed to return fire to its natural place in the ecosystem.

Fewer Seedlings

They set the blazes without fully understanding the role of fire, Ranger Bill Tweed explained. The new studies are showing that the hottest fires seemed to be followed by rapid regeneration of sequoias, while cooler, more deliberate burns produced fewer seedlings and a lower survival rate. The reason why is not known.

To learn more, scientists from the University of Arizona Laboratory of Tree Ring Research were given a $60,000-a-year contract to study fire and climate in the groves. The work is being directed by Tom Swetnam, one of the country’s top tree-ring specialists.

Armed with a chain-saw and an old truck, Swetnam has spent the last two summers prowling the Sierra, collecting cross-sections from sequoia stumps and downed trees, carting tons of wood back to the university’s lab in Tucson for analysis. From the annual growth rings, Swetnam’s staff can tell a tree’s age, determine when it sprouted and plot it’s year-by-year growth.

In good years, when there’s plenty of moisture, the rings are wide apart and full of growth, in drought years there is less growth and the rings compressed tightly together, like lines on a topographical map. Two- and three-year periods of drought are not uncommon, the most recent occurring in the mid-1970s and again between 1959 and 1961, Swetnam said.

Big Fire

In the years immediately after a big fire that scars a tree, abnormal ring growth patterns swirl like storm clouds around the wound, showing how the tree tries to heal itself. Many of the big trees have huge fire scars. Some have been hollowed by fire and blasted by lightning, but still they have survived.

One cross-section shows a tree had gone through 32 fires between AD 519 and AD 1297. In the AD 1100-1200 period, the sequoias document a long period of drought that gripped much of the West. During wetter times, there were fewer fires, but the burns were hotter and more widespread, burning off entire stands of smaller pine and fir.

“There was one huge fire in 1297 AD in Mountain Home (State Forest) . . . that seems to have killed off all of the understory (smaller pine trees),” Swetnam reported. “During the next few years, there was a tremendous growth release in the sequoias.”

Without the competing smaller trees, the growth rate among the sequoias tripled, and new stands sprouted in the burn scar.