Ancient pines close to treeline have wider annual growth rings for the period from 1951 to 2000 than for the previous 3,700 years, reports a University of Arizona-led research team. Regional temperatures have increased, particularly at high elevations, during the same 50-year time period. Increasing temperatures at high altitudes are fueling the post-1950 growth spurt has been observed in Rocky Mountain bristlecone pines, including ones in Arizona's San Francisco Peaks.
Bristlecone pines live for thousands of years on dry, windswept, high-elevation mountain slopes in the western U.S. The scientists collected and analyzed tree rings from Great Basin bristlecone pines located in three mountain ranges in eastern California and Nevada that are separated by hundreds of miles.
The team analyzed the average and median width of tree rings for 50-year blocks of time, starting with the latter half of the 20th century, the years 1951 to 2000, and going backward in time to 2650 B.C. The analysis spans more than 4,600 years.
"We're showing this increased growth rate at treeline in a number of locations," said Matthew W. Salzer, a research associate at UA's Laboratory of Tree-Ring Research. "It's unique in several millennia, and it's related specifically to treeline."
Only trees growing within about 500 feet (150 meters) of treeline showed the surge in growth. In general, those trees were at or above about 11,000 feet (3,300 meters) in elevation.
"You can come downslope less than 200 vertical meters and sample the same species of tree, and it won't show the same wide band of growth," Salzer said.
Growth at the pines' upper elevational range is limited by cold temperatures. At the lower elevations, growth of the trees is limited by moisture more than temperature, Salzer said.
Co-author Malcolm K. Hughes said, "Something very unusual is happening at high elevations, and this is one more piece of evidence for that." One other example, he said, was the accelerated melting of small glaciers at high altitudes.
"There is increasingly rapid warming in western North America," said Hughes, a UA Regents' Professor of dendrochronology. "The higher you go, the faster it's warming. We think our finding may be part of that whole phenomenon."
Individual Great Basin bristlecone pines, Pinus longaeva, are the longest-living organisms known. The trees live at an elevation range of approximately 8,200 to 11,400 feet (about 2,500 to 3,500 meters). The oldest living bristlecone, almost 5,000 years old, is in California's White Mountains.
The trees' longevity coupled with the excellent preservation of trunks from even older dead trees has allowed some scientists to reconstruct regional climate 8,000 years into the past using tree-ring records from bristlecone pines.
The recent rapid growth of three species of pines at elevations close to treeline had been noticed more than 25 years ago by previous researchers from UA's Laboratory of Tree-Ring Research. The sudden growth surge was puzzling in trees hundreds to thousands of years old, well past their adolescence.
Source: University of Arizona.