Giant sequoias thousands of years old and so gigantic that they make us humans feel tiny by comparison grow in California – however, drought is threatening these behemoths.
The giant trees leave visitors speechless. They walk across the spring forest floor without a word, the beams from their headlights falling on mighty red pillars. Cathedral atmosphere. Any word, any sound would tear apart the reverent stillness. This place is sacred. Wendy Baxter glances at her hand-drawn map, then at the mighty tree trunks disappearing into the darkness above her. “That must be the one,” she says softly, indicating one of the giants, number 233. She carefully lowers her backpack with the climbing harness onto the ground.
Giant Forest in California’s Sequoia National Park is where the mightiest trees on earth are found. Thousands of giant sequoias, or giant redwoods, grow here, including the record holder, General Sherman Tree, which at just under 84 meters tall and with a trunk circumference of 31 meters, is by volume the largest tree in the world. It would take 17 adults with outstretched arms to embrace it. Around it, four more of the ten biggest trees stretch their branches heavenward. The most ancient of these was probably already 700 years old when the Greeks began building the Parthenon in Athens. Here in the Sierra Nevada, on an area measuring roughly 150 square kilometers, is where the last natural stands of sequoiadendron giganteum survive.
Continued drought is slowly drying out California. Even if there was more precipitation last winter than in the preceding years, the past six have still brought far too little snow and rain. At the same time, there are 40 million Californians consuming large quantities of water. According to the US Forest Service, 62 million trees perished in California in 2016 alone. More than 102 million trees have died of thirst during the drought that has prevailed since 2011, and millions more are doomed to go the same way. At first, it was only the lowland trees that died, pines, firs and oaks, but then death began to creep up the mountains. Now the giant redwoods at 1500 to 2200 meters are under threat. Trees that once defied storms, forest fires and disease are now showing signs of stress: brown leaves, dying branches. In January, a “tunnel tree” toppled, a mighty sequoia more than two thousand years old into which lumberjacks in the 1880s cut a tunnel wide and high enough to allow a man to ride through on horseback.
It’s not quite four o’clock in the morning. One-and-a-half hours ago, the obtrusive ring of an iPhone drove tree scientist Wendy Baxter, 37, and her colleague Anthony Ambrose, 49, from their sleeping bags. They fixed themselves bagels with cream cheese and drank a swiftly brewed coffee – it’s cold at 2000 meters. The scientists shiver as they climb out of the truck, but soon warm up on the half-hour’s tramp to tree 233. It stands 72 meters tall and has a diameter of just under five meters. “A sturdy guy,” says Ambrose. Last week, they selected dozens of trees and nylon lines over protruding branches with their crossbows, then pulled up heavier lines and finally brought their climbing lines into position. Now no. 233 is rigged, ready to climb.
The two forest ecologists from the University of California in Berkeley are running health checks on the giant redwoods. To find out how the giants are coping with their changing living conditions, they collect samples. “We have to get up there,” says Baxter, gazing up into the darkness. You can sense that the black of the sky will soon change to a deep blue. “We need small twigs from the crown of the tree,” she explains, “ideally a sample taken at dawn, when the trees are at their most relaxed, and one in the heat of the afternoon, when their metabolism is in full swing.” She hooks in a couple of cable clips and begins to pull herself further and further up the tree in a caterpillar-like motion sequence. Soon, all we can see of her is her helmet light, and after a couple of minutes, that too vanishes among the branches of the colossus.
The huge trees are like gigantic drinking straws drawing water from the ground – experts estimate that a single tree can manage up to three tons a day, more than any other tree species. The trees release the humidity into the air through their leaves. This process reduces the pressure in the capillaries along which the water rises up the tree. The drier the atmosphere and the less groundwater there is available, the greater the tension inside the tree. Under extreme conditions, the flow of water can break off like an overstretched elastic band. When that happens, gas bubbles form, and they can block the capillaries in the trunk. If this happens frequently, the pores of the leaves responsible for exchanging water and gas close, leaving the tree without the ability to absorb carbon dioxide, an important source of energy. The giant trees can live for a while on their vast reservoirs, but if the pores remain closed for too long, the plants starve.
One-and-a-half hours later, Baxter, now totally exhausted, is back on the ground. She takes off her purple helmet, revealing sweat-soaked hair clinging to her head. She has a plastic bag containing the small twigs she has brought down – and a large graze on her upper arm. She still has five climbs ahead of her today – trees 231 and 272, and, of course, the afternoon climb of tree 233. By comparing the samples, the scientists can see the extent of the “stress” already caused by the lack of water.
“I’ve been doing this for four years,” she says, her dark eyes shining, “but the climb is always a crazy experience. I feel like an ant every time I sit up in the treetops.” From her vantage point up in the treetops, the giant sequoias resemble colossal exclamation points rising from the forest. “You cannot understand them if you never climb onto the roof,” says Baxter. She wanted to become a doctor originally, but switched to ecology during her time at college. “I am really a kind of doctor now, anyway,” she grins, “only arriving at a diagnosis is a little more strenuous than it is with people.”
The oldest of the giant sequoias was probably already 700 years old when the Parthenon was built in Athens
The titanic trees have stood on the slopes of the Sierra Nevada for hundreds of thousands of years and are among the most resilient organisms on the planet. No one knows what age they can reach. The oldest known specimen to date was 3200 years old. In the 19th century, the explorer John Muir claimed to have found a stump with 4000 growth rings. Once a giant sequoia has reached “its teens” at a few centuries old, it is as good as indestructible. Its bark, up to one meter thick, is soft and fibrous. It contains little resin, over the decades enabling it to survive recurring forest fires from the tree’s core. Their highly aromatic tannins repel insects. “It’s a perfect design,” says Ambrose, “when they get to 2000 or 3000 years old, they really must have already survived some period of drought.”
But therein precisely lies the question: Is the prevailing drought harming the giant sequoias more than anything that happened before? And what role does Central Valley play? Just a few dozen kilometers west of the sequoia forests, it’s the area with the worst air pollution anywhere in the USA, a fatal blend of smog, smoke from forest fires, exhaust fumes and dust swirled up as a result of intensive farming.
Later in the afternoon, Nathan Stephenson joins the climbers. Also a forest ecologist, the bearded 60-year-old is a sequoia expert and has been living in the small town of Three Rivers on the edge of the national park for 37 years now. He’s dressed in green pants, green shirt, green jacket and green baseball cap. He was the one who raised the alarm three years ago. He had been on his hands and knees, studying the effects of the drought on young saplings just centimeters high, when he glanced upward. “I couldn’t believe what I saw: a mature sequoia with brown leaves. Then I found another sequoia with branches almost reaching the ground and dead leaves floating down.” That was something I had never encountered in over 30 years of tree research.
His questions: Was this the drought? And: What else are we in for if this drought is a kind of dress rehearsal for global climate change? After finding dozens more withering giants, Nate Stephenson notified the authorities. After all, the sequoias are also a symbol for the USA: big, strong, unique – and supposedly indestructible. Scientists were hastily assigned to investigate whether and to what extent the lack of rain is stressing the tough titans. They used a special airplane to measure the structure of the forests and the light reflectance of the leaves. In their labs, Baxter and Ambrose tied the data from their own measurements in with those gathered in the air. “To the human eye, everything looks green and healthy from above,” reports Baxter. But the spectrometers that measure the treetops’ water content paint a different picture. The images reveal extensive red, dry areas. “With many forests, we simply haven’t yet realized that they are dead,” says Baxter. Luckily, the images show most of the sequoia groves in a soothing blue. “They have sufficient water.” Baxter pauses a moment. “For now!”
A handful of tourists have gathered beside the tree. They regard the two disheveled figures suspiciously: Baxter and Ambrose look like they have been living in the wild for weeks. Ambrose has bits of bark sticking to his clothes, scratches on his face and large patches of perspiration. Baxter’s face and upper body are covered with charcoal marks made when she squeezed by a scorched trunk. “I look like a dalmation,” she laughs. One tourist wants to call the police and report “fun athletes” climbing trees – but her cell phone has no signal. Stephenson gets rids of the visitors, asking them to keep their distance from trees that are being climbed, as branches could snap and fall to the ground. He talks of a falling cone that once smashed a car’s windscreen. This has the desired effect. With dubious glances upward and a couple of quick smartphone snaps, away they go.
Giants of their species
“We know so little about the sequoias,” sighs Stephenson, the man who has been studying the trees for decades, “we know more about the depths of the ocean than we do about them.” He fingers a sequoia cone. “This here is the crowning glory of the plant world – and we are only scratching the surface.” For decades, research funding went into projects that studied the effects of forest fires on the giant trees. But now what they want to find out is whether drought and climate change are threatening the trees’ very existence. Analyzing the timber giants is difficult by reason of their sheer size. “We don’t know how deep the roots of these trees go,” says Stephenson, stroking his gray beard and adding with a grin: “You could dig one up to find out … good luck with the digging.” Question upon question: Is there a tipping point at which the heat stress becomes too great for the trees and kills them? Will it be easier for disease and insects to overcome the trees’ defense systems in a warmer future? And most important of all: How can the trees be protected?
Ambrose sits on the forest floor that is now giving off the heat of the sun it has been storing throughout the day. Exhausted, he stuffs an energy bar with peanut butter and chocolate into his mouth and swallows large gulps from his water bottle. Stephenson and a couple of helpers collect the hermetically sealed bags of twigs. This is the final tree for today. Baxter is still up in the treetop. It’s got late – not for the first time – and the light is fading on the ground, while 30 meters higher up, the trunk gleams red in the last rays of the setting sun. “Everything okay?” asks Ambrose via his walkie-talkie. He takes a step back, throws back his head and peers up. Long pause. The two-way radio crackles. “Yes. It’s just…” Ambrose doesn’t know what’s going on up there. “It’s just … what?” he asks. Long pause. “Oh, nothing,” comes the reply, “it’s just so incredibly beautiful.” Ambrose rolls his eyes, then gets serious. “I know what you mean,” he answers softly.