Bristlecone pine trees, which live for thousands of years, have been growing at a faster rate over the second half of the 20th century.
The world's longest-lived trees appear to be growing more quickly because of higher temperatures on the mountaintops where they grow, a finding that's a new "smoking gun" signal of increasing temperatures in the late 20th century.
"What we're seeing is just part of a bigger picture that people have been seeing elsewhere about warming at high elevation. Warming appears to be stronger with altitude," said study lead author Matthew Salzer of the University of Arizona Laboratory of Tree-Ring Research.
The melting of high-altitude glaciers is another example of this trend, he said. "With that will come changes in the ecosystems, like the treelines moving up."
The study was published in the Proceedings of the National Academy of Sciences.
Bristlecone pine tree rings near the treeline have gotten wider -- indicating faster growth each season -- during the period from 1951 to 2000, the research showed.
"The ring width at the treeline was much greater in the second half of the 20th century than it was in any previous 50-year period for the last 3,700 years," said Salzer.
Trees growing 500 vertical feet or more below the treeline did not show accelerated growth in the same period, the researchers found. They suspect that near the treeline, tree growth is limited by temperature, while at lower elevation, moisture limits how fast the trees grow. The faster growth appears to be associated with warmer temperatures at the highest points of the trees' habitat.
Bristlecone pines can live for thousands of years on the dry mountain slopes of the western United States. The oldest living bristlecone pine, known as Methuselah, is over 4,800 years old.
Trees grow in two phases each season, adding early wood in the spring and late wood in the fall. The late wood has denser, thicker cell walls. Because of this pattern, the trunks of the trees develop rings when looked at in cross-section. Counting these rings tells scientists the age of the tree. Wider rings indicate more growth in a season.
Because the dense, dead wood of bristlecone pines resists degradation, researchers can assemble tree ring timelines that extend back much farther than the lifetime of a single tree by aligning tree ring cores from dead wood with those from living trees.
Salzer and colleagues analyzed tree rings at three sites in California and Nevada. The team compared trees growing near the treeline with those below. Using dead and living trees and some archived data, they constructed a timeline of tree ring growth stretching back 4,600 years.
These are significant findings, said Peter Brown of Rocky Mountain Tree Ring Research in Fort Collins, Colo. Not only were the rings widest over the last 50 years, but the widest rings of all appeared in just the last few years of the study, he noted.
The past few years have been some of the warmest on record.