“Jeffrey was appearing thousands of feet above where it should be,” Safford said. “Now everything is moving uphill, but Jeffrey pines are jumping, and that’s our mystery. We’re looking at the result, without having looked much at causes.”

He has a hypothesis, however.

Climate models predicted that the highest-altitude trees would continue to migrate up mountainsides, climbing — slowly — until they couldn’t climb anymore. And that’s happening. Last summer Safford and his team found three trees above 4,000 meters (13,124 feet), the first trees ever found at that altitude in the U.S. and Canada, outside of the southern Rockies.

But Safford says those models haven’t accounted for the role of seed dispersal by birds and other species. His number one suspects are Clark’s nutcrackers (named for explorer William Clark but slated to be renamed), clever corvids who use their strong beaks to extract pinecone seeds. They often bury them to eat later, serving as accidental gardeners in the process.

“The Clark’s nutcracker is a huge part of the story,” Safford says.