SEQUOIA NATIONAL FOREST, Calif. (AP) -- In parts of California's Sierra Nevada, marshy meadows are going dry, wildflowers are blooming earlier and glaciers are melting into ice fields.
Scientists also are predicting the optimal temperature zone for giant sequoias will rise hundreds and hundreds of feet, leaving trees at risk of dying over the next 100 years.
As indicators point toward a warming climate, scientists across 4 million acres of federally protected land are noting changes affecting everything from the massive trees that can grow to more than two-dozen feet across to the tiny, hamsterlike pika. But what the changes mean and whether humans should do anything to intervene are sources of disagreement among land managers.
"That's the tricky part of the debate: If humans are causing warming, does that obligate us under the laws of the National Park Service to try to counteract those effects?" said Nate Stephenson, a research ecologist with the U.S. Geological Survey.
"How do you adapt to a changing climate if you're a national park?" added Stephenson, who is 30 years into a study of trees in the largest wilderness in the continental U.S., Sequoia-Kings Canyon National Park.
Since 1895, the average temperature across California has increased by 1.7 degrees, and experts say the most visible effects of that warming occur within the Sierra Nevada, where low temperatures are rising and precipitation increasingly falls as rain rather than snow. Some models show noncoastal California warming by 2.7 degrees between 2000 and 2050, one of many reasons President Obama pledged last week to use executive powers to cut carbon pollution.
The state's two largest rivers -- the Sacramento and San Joaquin -- originate in the Sierra. The range also is home to Lake Tahoe, the largest alpine lake in North America; Mount Whitney, the highest peak in the Lower 48; and the nation's only groves of giant sequoias, the largest living things on earth.
There are mounting concerns about the beloved sequoias, whose sprawling, 10-foot-deep root systems make them especially vulnerable to drought and heat.
Because the trees exist only in such a small region, scientists are debating whether to irrigate the 65 groves in the southern Sierra to help them endure warmer temperatures. Otherwise they fear the trees could die. During the last warm, dry period 4,000 to 10,000 years ago, their numbers were greatly diminished, according to pollen evidence collected by researchers at Northern Arizona University.
"Whether we would water them certainly comes up on our climate change scenario planning," said Koren Nydick, science coordinator at Sequoia-Kings Canyon National Park. "They are a very unusual species because they're also looked on as a social artifact."
Stephenson says his decades of studying conifers in Sequoia National Forest have shown they are dying at twice their historic rate, partly because the climate is warmer and dryer. The giant sequoias grow much more slowly than conifers over many hundreds of years so changes have been tougher to recognize, though researchers suspect seedlings already may be having a harder time taking root.
"That's always the million-dollar question," said Stephenson, director of USGS's Sierra Nevada Global Change Research Program. "We just don't have a big enough sample size to know what's going on with the giant sequoias, whereas we monitor thousands of pines and firs and have much more confidence."
So far, the dozens of changes researchers have noted, in everything from earlier songbird fledging dates to greater wildfire intensity, may point to a warming climate. But it's far from understood whether that would mean doom or adaptation for California's ecological heart.
"I don't want to say that because we're seeing one thing, that's how it will play out," said Rob Klinger who is studying alpine mammals for the USGS's Western Ecological Research Center. "The endgame of our study is determining whether there will be uniform change or will it be patchwork. If you look at evolutionary time scales, species have gone through these changes before, and they handle it."
As part of a Ph.D. project at the University of California, Merced, Kaitlin Lubetkin for five summers has hiked the backcountry taking inventory of 350 subalpine meadows formed when glaciers retreated eons ago. The marshy ground acts as a reservoir that eases flooding after snow melts, and the stored water feeds streams during dry months and sustains wildlife such as the endangered willow flycatcher songbird and the Yosemite toad, which is being considered for threatened species status.
Over the past decade of warmer, drier conditions, however, pine trees have begun to take root, acting like straws to pull the moisture out of the meadows, Klinger and Lubetkin have observed.