How old is that oak?

Published on
13 February 2012

When we say that we found an oak that is at least 400 years old, what do we base that conclusion on? Obviously, we would have to cut the tree down to count the rings, and unless it is already dead, we aren’t going to do that!

The short answer is that we look the size up on a table that Marlin Bowles at the Morton Arboretum created to help people estimate the age of various trees based on the tree’s diameter at breast height (dbh):

Inches dbh

Bur/Swamp white oak

Red oak

Shagbark Hickory

White Oak

10

66

76

102

84

12

79

89

116

100

14

91

102

129

115

16

104

115

142

129

18

117

128

155

144

20

129

141

167

159

22

142

154

179

173

24

154

161

190

187

26

167

179

202

201

28

179

191

212

215

30

192

203

223

229

31

204

215

234

243

34

219

230

246

260

35

229

239

254

271

37

242

251

264

285

39

254

263

274

298

The table was created using actual data from a large number of trees that were “aged” either after death or by using a coring tool that allows one to take a small core from the tree so that the rings can be counted without cutting the tree down. The various ages for the various diameters of the various species of trees were analyzed and a table showing the approximate age for different diameters of different species was created.

One important thing to notice, however, is that the table stops at 39” dbh. So, for the really large trees – like the 54 ½” white oak at Gateway Park or the 48” bur oak at Hennen Conservation Area, one has to extrapolate.

In the case of the 54½” white oak, we look at the table that tells us a 39” white oak is about 298 years old. Our specimen is 40% larger than that, which would mean 417 years old if it grew in diameter at the same rate when it was 300 as it did when it was much younger. To be a little conservative, we say the tree could be 400 years old.

In the case of the 48” bur oak, again, we know that a 39” specimen is about 254 years old. A 48” tree is about 23% larger than that, so we would calculate it to be 312 years old, and say the tree could be 300 years old!

We won’t be able to say for sure until the tree is dead. In the meantime, let’s not argue over a few years one way or the other!

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