As the oldest individual organisms on the planet, Great Basin bristlecone pine trees are nothing if not resilient. But rising temperatures due to climate change are putting these trees, which can live to be over 5,000 years old, under a new and worrisome kind of pressure, according to a new study.
Bristlecones grow just below the treeline in the mountains of Nevada, Utah, and eastern California. As the planet warms, tree lines—the area beyond which trees won’t grow—are shifting higher in elevation. But the bristlecone is finding it difficult to keep up and its juveniles are being crowded out by another Great Basin species, the limber pine.
“It’s really kind of a bum rush to get to the very few micro sites where a tree can survive,” said Brian Smithers, a PhD candidate at U.C. Davis and lead author of the study, of the trees’ race upslope.
And so far, limber pines are winning. The study, published in Global Change Biology, found that there are two factors that contribute to limber pines outpacing bristlecones. First, young bristlecone pines require certain soil types and other ecological factors in order to thrive, but young limber pines have far fewer parameters on where they will grow—meaning they have a lot more choice as the treeline moves upward. Second, limber pines have an avian leg-up: the Clark’s nutcracker, a Great Basin bird, collects limber pine seeds and buries them as a winter food source, giving the tree a built-in dispersal agent for its seeds. These two factors are allowing limber pines to “leap-frog” bristlecones up the slope of a mountain, the study found.
Bristlecones are no stranger to stress, however. Their longevity is the product of the harsh conditions they live in: the subalpine regions of the Great Basin see driving wind and snow in the winter, high temperatures in the summer, and a short window of time when it’s both wet enough and warm enough for the tree to grow. These conditions contribute to the trees’ remarkable appearance, Smithers said.
“They’re really strikingly beautiful in the way that a boxer is beautiful after a fight,” he said. Because of the harsh conditions of their environment, bristlecones “grow into these really gnarled, twisted shapes, where half the tree is dead and half will keep living. Because of that, the dead wood has a bunch of hues of color to it.”
Scientists have long worked to determine how species will respond to climate change—which ones will successfully find new habitats, adapt to their old ones, or struggle to survive. The tree line, Smithers said, is a fantastic tool in this research. “It makes this really great literal line in the sand to look at climate change effects on species range and species range shifts,” he said.
But beyond trees, there is evidence of this climate-induced migration across the globe: Rising ocean temperatures are forcing fish to migrate to cooler waters, while some birds and tropical plants are moving upslope. This northward and upward migration isn’t consistent among all species: Studies have found evidence of bird species shifting their habitats downslope and southward in response to changing temperatures. But it’s safe to say that climate change is causing major geographic shifts among both plants and animals.
The trouble comes when these creatures aren’t able to move fast enough to outpace climate change or their competitors—or, if they find themselves at the top of a mountain with nowhere else to migrate. A 2014 report from the Audubon Society, for instance, predicts that 126 of North America’s bird species will lose more than half of their current ranges by 2050, and likely won’t be able to find new regions to colonize.
Thankfully, the bristlecone pine isn’t at risk of going extinct due to climate change, Smithers said. But there will likely be places—regions without much room between the top of the mountain and the top of the present-day tree line—where local populations of bristlecones could be lost.
And there’s another factor that Smithers is concerned about: wildfires. Thanks to drier weather, fires are increasing in the Great Basin subalpine forests where the bristlecones grow, and Smithers has noticed that, as forests burn in higher elevations, they aren’t being replaced by bristlecones. Instead, pinyon pines are populating the burned area—trees which typically grow far lower in elevation than limbers and bristlecones. That’s bad news for bristlecones.
“It looks like this could really be a big game changer,” Smithers said of the wildfires. “Instead of saying, oh, these changes will happen in a couple thousand years, [we may be] now saying, if fire comes in, it could happen in 15 years.”
Besides the stress climate change is putting on bristlecones, another key finding of the study was that models for where species will move as the Earth warms shouldn’t be created based solely on the ecological needs of the adults. And in the case of trees especially, it may be the needs of juveniles that conservationists should focus on the most.
“All trees are rooted to the ground, so the way their ranges shift, at least on the leading edge, is by seeds moving out,” said Janneke Hille Ris Lambers, a professor focusing on plant community ecology at the University of Washington. “So if the juveniles can’t make it, it doesn’t matter if the adults can make it.”
Smithers plans to continue to study bristlecones, but he’s not sure what the solution is to helping bolster the trees’ long-term survival.
“Frankly I’m not sure what can be done,” he said. “Stop driving, stop burning coal—you know, these are the obvious things. But these trees are not economically important. It’s not like there’s a budget to save pretty trees.”
There is the option of “assisted migration”—scientists helping the bristlecones get established by planting the trees themselves. But that’s a controversial move in scientific circles.
“There are plenty of people who say ‘let’s let nature respond how it will, or else we’re going to keep fighting what it wants to do anyway,’” Smithers said.
As always, more data on how climate change is impacting species’ ranges will help, both for bristlecones and for species across the globe.
“There have been a number of other studies that have found that there are a lot of complexities in terms of how these range shifts happen,” Hille Ris Lambers said. “We really need a lot more case studies, like this one, in different regions to try to get a sense of what we can actually predict.”