Leonardo and the Tree Rings

I found a little gem in the library this afternoon in Isis, a journal “devoted to the History of Science and its Cultural Influences”. My old college mentor Joe Maier introduced me to Isis, so it felt a little nostalgic pawing through its pages.

What interested me was a little trifle published in the back pages of the December 1945 issue entitled “When was tree-ring analysis discovered.”[1]

I’ve been following back footnotes from George Webb?s scientific biography of A.E. Douglass[2], and Webb mentions the marvelous factoid that it was Leonardo (What was it an old colleague used to say? “Great story. Better if true.”) who first suggested the possibility that tree rings could be used for the study of climate.

(Click through to read the rest, it’s really long.)

Webb?s source for the assertion is the little Isis trifle, a brief disquisition by George Sarton, dean of the historians of science, on the question of the ?discovery? of the science of tree rings.

Sarton doesn?t question the primacy of my guy Douglass, who essentially invented the science of dendrochronology in the first few decades of the 20th century. A couple of 19th century researchers, Twining and Kuechler, had worked on the problem of crossdating between trees, but it was really Douglass who put it all together. But well before that, Sarton finds a number of antecedents. ?It appears,? Sarton writes, ?that in this case as in so many others the discoverer was Leonardo da Vinci.?

Leonardo observed the tree rings and recognized in them indications of weather and age; he noticed the differences obtaining between the northern and southern exposures of each tree (the rings are broader on the north side, and hence the center of the section is closer to the south side).

Sarton also quotes a delightful account by Michel de Montaigne, who traveled in Italy in 1580-1581 (this would be about a century after Leonardo). In his diary (translated by Sarton from the Italian in which Montaigne wrote it), the traveler describes a meeting with an Italian craftsman who seemed to have the tree ring thing all figured out:

The artist, a clever man, famous for his ability to make mathematical instruments, taught me that every tree has inside as many circles and turns (cerchi e giri) as it has years. He caused me to see it in many kinds of wood which he had in his shop, for he is a carpenter. The part of the wood turned to the North is the straightest, and the circles there are closer together than in the other parts. Therefore when a piece of timber is brought to him he is able, he claims, to tell the age of the tree and its situation (the orientation of the section).

Sarton also offers a tantalizing suggestion, though not well documented, that the ancient Greek Theophrastos had a go at the tree ring thing as well. ?It is probable,? Sarton quotes another scientist, Waldo Glock, as writing, ?that the use of tree rings for dating purposes has been rediscovered several times in the last 2,000 years or more.?

1. Sarton, G., When was tree-ring analysis discovered. Isis, 1954. 45(4): p. 383-384.
2. Webb, G.E., Tree rings and telescopes : the scientific career of A.E. Douglass. 1983, Tucson, Ariz.: University of Arizona Press. xiii, 242 p.