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.