Trees Tell of Warming Climate

What can trees tell us about climate change? Last September, Professor Hugh Safford, a forest ecologist in the UC Davis Department of Environmental Science and Policy, was hiking for pleasure in California’s High Sierra when he stumbled upon a Jeffrey pine at an altitude it shouldn’t exist.

“I walk over, and it’s a Jeffrey pine! It made no sense. What is a Jeffrey pine doing above 11,500'?” During his hike-turnedscience-expedition, Safford found and examined 14 Jeffrey pines above 11,800,' some of which were at least 20 years old. At least a dozen others were visible.

The discovery signifies a changing climate amid California’s highest peaks. As snow melts earlier and air temperatures rise, Jeffrey pine seeds are germinating on land they previously found frozen and inhospitable. Preliminary evidence suggests a bird, the Clark’s nutcracker, carries fleshy Jeffrey pine seeds up the mountain, storing them in the High Sierra’s “refrigerator” for an early summer snack, thus enabling some seeds to not only germinate, but establish themselves as a new population.

Jeffrey pines are not considered a traditional subalpine tree species, which inhabit the most extreme high elevations. Yet none of the six traditional subalpine forest species have been collected above 12,034.' Safford recorded Jeffrey pines as high as 12,657' (1,860' higher than the previous record for the species). This suggests the Jeffrey pine is California’s highest tree—at least for the moment.

Safford’s work indicates that other species are growing higher than commonly used databases suggest. Species attempting to stay ahead of climate changes by moving uphill are doing so far too slowly to keep pace, climate modeling literature suggests. Yet the models don’t account for the role of seed dispersals by birds and other species amid shifting windows of ecological opportunity.

“I’m looking at trees surviving in habitats where they couldn’t before, but they’re also dying in places they used to live before,” Safford says. “This crazy leapfrogging of species challenges what we think we know about these systems reacting as the climate warms.”

The discovery underscores a need for scientists to couple powerful technologies with direct observation. The trees Safford encountered were not detected by any available database, artificial intelligence platform, satellite or remote sensing technology.

“People aren’t marching to the tops of the mountains to see where the trees really are,” Safford says. “Instead, they are relying on satellite imagery, which can’t see most small trees. What science does is help us understand how the world functions. In this case, where you see the impacts of climate change most dramatically are at high elevations and high latitudes. If we want our finger on the pulse of how the climate is warming and what the impacts are, that’s where it will be happening first. We just need to get people out there.”

This summer, Safford and students from his lab will return to the southern Sierra Nevada to further research. n on a hike along Mount Kaweah in the High Sierra. (Photos: Hugh Safford, UC Davis)