If you've hiked in the high Sierra, you've seen the wind-sculpted, shrub-like stands of whitebark pine that cling to the ridges at tree-line around 11,000 feet.
They are the last gasp of the conifer kingdom, seemingly capable of enduring whatever nature throws at them.
But now these hardy survivors are dying in places and scientists aren't sure why. But some speculate that it may be related to a shrinking snowpack and climate change.
"Our hypothesis is that in areas where there is more snowmelt than in previous centuries, they are actually being exposed to colder conditions that they would under snow," said Connie Millar, a research ecologist with the U.S. Forest Service who took this photo near Mt. Conness north of Tioga Pass last year. "The snow really protects them."
More work will be needed to prove or disprove the hypothesis. But the situation underscores how complex the impacts of global warming can be in the Sierra. "It's sort of counter-intuitive," Millar said. "Everybody thinks of heat. But as soon as you don't have the blanket of snow on, things could be experiencing a much more severe climate, and some super-cold temperatures."
The same phenomenon may also be involved in recent population declines of America pika - a small, mountain-dwelling, rabbit-like mammal - in some parts of the West. "It's a question to test," Millar said. "But it's certainly a fact that if you're exposed in the winter to the atmosphere, you experience extremes of cold and heat more than if there is snow cover.
"There is no question that is true in mountainous areas," she added. "The question is whether that is happening more and is related to the death we are seeing."