Forest fires, lightning strikes and insect pests also can affect annual rings. Other factors are the amount of sunlight and competition from other trees. These things have nothing to do with climate, so researchers have to take lots of samples in lots of places.
And while tree rings have proven valuable for climate studies in the Northern Hemisphere, they are nearly useless in the tropics because tropical trees don't live long and don't have annual rings.
Some of the best trees for tree ring study are at high elevations. Last summer, Biondi hiked to 14,000 feet among a range of active volcanoes in the central Mexican states of Mexico and Colima. Trees that grow at the tree line are especially sensitive to changes in climate, and can serve as a sort of early warning system.
Biondi is one of 14 international researchers examining the ecology and climate signals from high-altitude trees from Alaska to the tip of Chile in a study sponsored by the Interamerican Institute for Global Change Research in Brazil.
On the steep, rocky north slope of Baden-Powell, the afternoon's work is about to end. Biondi and Imsand, a Pasadena naturalist who helped collect tree ring samples last summer, have found some of the trees to be uncooperative.
The hard, compact wood fibers twist like a candy cane, all the better to resist strong winds and pests, as well as the coring tools used by researchers. Biondi drills the tool into the wood like a corkscrew, probing as far as he can.
He has already broken one tool by hitting a rock concealed by the tree's growth; another probe turned up rotten inner wood, useless for study. But Biondi hits pay dirt with a sample from a tree dating to AD 1241. "This is a beautiful core; it's a dream come true," says the Italian researcher. "It's in perfect condition."
After recording the tree's location, Biondi wraps the fragile, chopstick-sized cores in paper, and heads down the mountain.