Bristlecone pines, the world's oldest trees and growing at high altitudes in the American West, have been undergoing a growth spurt since 1950, fuelled by increasing temperatures, researchers say.
A dead Great Basin bristlecone pine stands on Sheep Mountain in the White Mountains of California. (Malcolm K. Hughes) The tree rings of living and dead bristlecone pines close to the treeline at three sites in Nevada and California show wider annual growth rings from the period between 1951 and 2000 than in the previous 3,700 years.
The researchers used an archive of climate data to map changes in temperature around the three sites.
"We've got a pretty strong pointer that temperature plays a part in this," said Malcolm Hughes, University of Arizona professor and co-author of the study, published this week in the Proceedings of the National Academy of Sciences.
However, the increase in growth wasn't seen in trees farther down the mountains.
"You can come downslope less than 200 vertical metres and sample the same species of tree, and it won't show the same wide band of growth," said Matthew Salzer, a research associate at the University of Arizona.
The researchers found that the growth of the trees at the upper altitudes is limited by temperature, and that growth is faster when it's warmer. The growth of trees farther down the mountain is limited by rain, so they aren't as affected by temperature changes.
The researchers said the findings of warmer temperatures have implications beyond the surge in growth of mountain pine trees.
"High mountains are our water towers," Hughes said. "That's where we store water as snow through the winter.
"If the snow melts earlier, the mountains won't be able to hold onto water for as long. They won't be as effective as water towers for us."
The Great Basin bristlecone pines are the longest-living individual organisms known. The oldest known bristlecone tree is nicknamed Methuselah and, based on a sample taken in 1957, is estimated to be 4,841 years old.
(Clonal plants, such as the quaking aspen, reproduce asexually and their offspring are genetically identical to their parents, so it's difficult to say when one individual dies and another is born. A quaking aspen colony in Utah is estimated by some to be 80,000 years old.)
