Amid California’s historic drought, ancient sequoias show signs of stress – ‘I’ve been studying sequoias for 35 years and have never seen anything like this’Posted by Jim at Sunday, September 06, 2015
By Cally Carswell
5 September 2015
(The Guardian) – Last September, US Geological Survey ecologist Nate Stephenson hiked into Sequoia National Park’s Giant Forest to look for dying seedlings. California was suffering through its third year of severe drought, and trees were dying in the park in greater numbers than usual. The roadside leading up to Giant Forest was pincushioned with trees faded brown – dead oaks, sugar pine, fir, incense cedar. But the forest’s namesake trees, which are among the world’s oldest and largest, were faring better. They’re tough – they have to be to live for thousands of years – and tend to grow in the wettest parts of the landscape.
Still, Stephenson thought the effects of the drought might have started to become visible on sequoia seedlings, which are typically more vulnerable to environmental fluctuations than mature trees. He searched the forest floor, but found nothing out of the ordinary. It was only when he looked up that he was startled: he saw a towering old sequoia loaded with tufts of evergreen foliage turned brown.
The tree wasn’t dead, but such foliage die-back is an uncommon sign of stress. “I’ve been studying sequoias for 35 years or so and had never seen anything like this,” Stephenson says. He deployed a field crew to hike through Sequoia and its sister park, Kings Canyon, to document the die-back. About half of the more than 4,300 trees they surveyed had lost 10% to 50% of their foliage, while 1 in 100 had lost more than 50%.
How likely giant sequoias are to survive as the climate changes, making grueling droughts increasingly common, is uncertain. “One of the big questions is, just how much drought can giant sequoias survive?” explains Koren Nydick, an ecologist and science coordinator for Sequoia-Kings Canyon.
The stressed trees may hold clues to their species’ future that can help managers figure out how to protect them, and this summer, Stephenson and others have been racing to document them.
“We’re treating the drought as if it’s a possible preview of the future,” Stephenson explains. “Our goal is to map completely for all sequoia groves in the Sierra Nevada where they’re most vulnerable to drought.”
Increasingly sophisticated technology has made such an ambitious proposition possible. Greg Asner, a professor at Stanford University and lead scientist for the Carnegie Airborne Observatory, has spent the summer flying over California’s forests to assess their condition. His plane is equipped with instruments that capture the chemistry of individual trees across entire landscapes, generating colorful 3D maps that allow land managers to identify hotspots of stress or resilience. “It’s like going in and getting a blood test, and the doctor saying you’re OK or you’re not,” Asner explains.
For the state as a whole, the results so far are sobering. “The only time I’ve seen it this bad is in the Amazon in 2010, when I mapped millions and millions of dead trees,” Asner says.
Worst off are the forests south of the Sierra Nevada, almost all of which are “in huge trouble,” according to Asner. [more]