How old are our bur oaks?

Upon viewing the oak savanna landscape at Pleasant Valley Conservancy it is natural to wonder about the ages of the trees. Many of the oaks, especially the burs, appear to be ancient. They have survived a lot of history and have still managed to keep alive and grow.

This bur oak savanna at the top of the ridge has been burned annually for the past 10 years. Some of these trees were cored for the climage/age study.

Fortunately for us, a U.W.-Platteville research associate Sara Allen and students under Professor Evan Larson were looking for old bur oaks for a project related to climate change. Our Conservancy was especially suitable because each tree has been tagged and has been mapped by GIS.

Last year in mid November Allen and student assistants came and took cores from 10 large bur oaks and 2 large white oaks. Recently they shared the results with us.

Finding a suitable tree

Coring the tree

Removing the core

Close-up of part of the core. The rings (54 in this small sector) can be easily counted

In the laboratory the cores are mounted for precise analysis of the tree rings.

The oldest bur oak  had a start date of 1735 for a current age of 279. The other trees sampled were 274, 267, 258, 238, 237, 226, 208, 184, and 169 years old. It is fascinating to consider that the oldest trees began life during the rein of King George II.  One tree (238 years old) started life during the time of the Revolutionary War (1776).

It is interesting to note that there was no correlation between the diameter of a tree and its age. This is understandable because growth conditions vary markedly from one site to another.

Two white oaks were also cored. Although their diameters were similar to those of the bur oaks (28-29.5 inches), their ages were much younger (129 and 145 years).

Obviously, with species to species and site to site variability in growth rates, there is no possibility of estimated an age of a tree from its diameter!