Bristlecone pine trees dot the White Mountains in eastern California, giving the stark and rocky landscape one of its few highly visible signs of life. These gnarly-barked trees can survive at altitudes of up to 3,470 meters, although their growth rate at these heights is limited because of cold temperatures. But those limits have been loosening lately. In the past 50 years, as regional temperatures have warmed, the growth of bristlecone pine trees at high altitudes has been accelerating, whereas that of trees lower down the slopes has not, according to the results of a study published November 16 in the Proceedings of the National Academy of Sciences.

The research focused on Great Basin bristlecone pines (Pinus longaeva), which grow in six western U.S. states and are among the longest-lived organisms on Earth. Some pines reach ages of up to 5,000 years, which gave the study authors an opportunity to put together a record going back nearly as far that compares bristlecone growth rates at various altitudes.

To arrive at their findings, study lead author Matthew Salzer, a research associate in the Laboratory of Tree-Ring Research at the University of Arizona in Tucson, and his colleagues studied 678 tree ring radii of bristlecone pines, both living and dead, from a site on the White Mountains and two sites in Nevada—on Mount Washington and Pearl Peak. In the White Mountains the team sampled trees living at the maximum altitude, or tree line, which can be as high as 3,470 meters or as low as 2,805 meters. To measure annual tree-ring width, the scientists drilled out pencil-size sections from one side of living trees, or sawed-off cross-sections of dead ones.

Most living bristlecone pines sampled by the scientists were younger than 4,000 years old, so the ring widths of dead trees allowed the scientists to get earlier growth data. The ring patterns of old living trees were matched to those of dead trees to pinpoint years when the dead trees had been alive.

Among bristlecone pines at the tree line, the period from A.D. 1951 to 2000 saw a positive growth period, when the median ring width reached 0.58 millimeter, which was greater than any other 50-year median since 2650 B.C. Furthermore, the annual median growth of these trees between 1900 and 2000 correlated with the annual mean local temperature. Salzer and his colleagues did not detect any trend toward positive or negative growth of the pines living 150 meters and farther below the tree line on the White Mountains.
"Location is everything. It's where [they] are in relation to the climatic boundary that allows the trees to grow," Salzer says.

Trees below the tree line showed no increased growth, because other climatic conditions hold them back, he says. The limiting factor for these lower altitude trees is moisture. "Low trees grow wider rings in general during years when there's more precipitation and it's cooler," Salzer says. The amount of precipitation in the White Mountains has steadily increased in the period from 1900 to 2000, but the increase does not seem to have been sufficient to stimulate the growth of trees below the tree line. Given the warming temperatures, rainfall could be evaporating more quickly and offsetting the increase in precipitation, Salzer says.

This study suggests that the temperatures at tree lines today are similar to the temperatures that pine trees 150 meters below the demarcator were exposed to several decades ago, Salzer says. And because trees at the tree line are no longer living in the coldest thermal extreme they can withstand, he predicts that the tree line could be shifting higher. He and his colleagues recently have noticed new pines growing at altitudes above what had been considered the tree line. As the trees' seeds get dispersed, either via animals or by wind, those that land above the tree line are now apparently able to survive at these higher altitudes.

If the ceiling for Great Basin bristlecone pine growth does rise, these trees would join the ranks of organisms around the world whose habitats have shifted to cooler altitudes and/or latitudes in response to climate change. Pacific salmon, flowering plants in North America and algae in the Mediterranean have all been documented as having moved farther north to find conditions closer to their former habitats.