The Rock Doc: Growth rings track more than the age of trees

As every schoolchild knows, counting the growth rings in a tree tells you how old the tree is. But some samples of wood can tell you even more than age. That’s because some trees live in difficult environments. They grow best only when there is a good year in terms of precipitation, temperature and the like, so they have growth rings that are quite uneven. Some are thick, representing good years for growth, while others are thin from when times were tough.

In the southwest U.S., a lot of work has been done with tree rings. Indeed, the whole science of what’s called dendrochronology was worked out in that region in the early and mid-20th century. But since then, scientists around the world have also used basic ideas about tree rings to do several different things.

Earlier this year National Geographic Daily News ran a story about dendrochronologists in New Zealand. In the 1980s, researcher 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 long chronology for the local area. More recently, dendrochronologist Gretel Boswijk has been updating and extending that record. The kauri trees of New Zealand include 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 the living trees with the younger parts of the ancient samples, 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. Some of the information he can deduce from the patterns of tree-ring widths in New Zealand relate to El Niños. Looking at the evidence of the wood samples, Fowler has determined that El Niños in at least one part of 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.

It’s impressive what specialists can deduce from simple samples of wood. It will be interesting to see what other natural secrets can be decoded with the help of tree rings.