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Dendrochronology; Beavers


In 1946, I began the periodic observation of several beaver colonies located near my family's summer cabin, in Crow Wing Co., central Minnesota. From 1953 to 1956, a detailed study of the colonies was conducted as a Junior Academy of Science project. During this period, several new colonies were established and many of the older ones abandoned. A study of aerial photographs, followed by a ground check on foot or by canoe, turned up many other beaver colonies, some active at the time of discovery and many apparently long deserted.

In searching for a method of ascertaining when these workings had been started or last occupied by beaver, a Swedish Increment Borer was obtained and a study begun of the growth layers of trees growing near beaver ponds. Trees were chosen that were growing close enough to the water table so that aeration of their root systems would be reduced by very slight rises in water level. Many of the trees studied showed several years of very slow growth, preceded and followed by many years of much greater growth. A cross check with trees growing nearby on high ground showed no similarity in relative growth rates for this period, thus tending to rule out insects or climatic causes: A comparison of the ring pattern, of trees growing at low sites, with the records kept on these ponds since 1948, showed a perfect correlation between the known dates when the areas had been ponded and the years of very slow growth as shown by unusually small rings formed these years.

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