Earlier this year, National Geographic Daily News ran a story about dendrochronologists in New Zealand. In the 1980s a researcher named John Ogden and his students started what has become a truly significant tree ring record. By matching the thin-thick-thin patterns of wood samples taken from kauri trees of varying ages, they started to establish a chronology for the local area. More recently, dendrochronologist Gretel Boswijk has been updating and extending that record. The kauri trees of New Zealand include some quite old individuals. Using living trees and wood from buildings, Boswijk was able to record the patterns in the wood going back to the 1200s.
Using wood found in old — even ancient — buildings is a clever approach on the part of the tree-ring crowd. Here in the U.S., dendrochronologists were able to date the age of the Pueblo Bonito civilization in New Mexico. They did this by matching the old parts of living trees with the younger parts of the ancient samples in ruins, thus extending the record back in time.
Happily, archaeological samples are not the only ancient wood available. In parts of New Zealand there are swamps that preserve kauri trees that have fallen into the muck and been sealed off from air. Using those samples, Boswijk and people working with her were able to establish a record going back nearly 4,500 years. That’s a great record of local conditions over a long period of time, going back pretty far into what geologists call the Holocene Epoch.
Anthony Fowler, who works with Boswijk at New Zealand’s Tree-Ring Laboratory, specializes in looking at climate change, with part of his interest being how climate change has been recorded in tree samples. Specifically, some of the information he can deduce from the patterns of tree-ring widths in New Zealand relate to El Niños -- the recurring weather patterns related to changing ocean temperatures in the Pacific. Looking at the evidence of the wood samples, Fowler has determined that El Niños in the southern hemisphere have been getting more intense in the last 500 years. We don’t yet know why that might be the case, but that’s the evidence given to us by the trees.