Blue Oaks Grow Slowly

Oaks ’n’ Folks – Volume 5, Issue 1 – June, 1990


How old are the oak trees that cover the rolling foothills of California? When did the solitary giant valley oaks scattered in the central basin of the state originate and how long can we expect these trees to remain alive? These are commonly asked questions which have been difficult to answer because little research has been done on aging native oak trees. A recent study, however, has shed some light on this subject for one of the most common species, Quercus douglasii , or blue oak.

As part of a study evaluating the sprouting ability of blue oak stumps, a number of trees were cut down at 5 locations, ranging from Mendocino County in the north to San Luis Obispo County in the south. After the trees were felled, disks from 25 trees per site were cut, sanded, and the number and width of annual rings evaluated to determine tree age and growth patterns. All of the trees were growing in fairly dense stands with 100 to 200 trees per acre.

The age analysis showed that, in general, blue oaks grow very slowly. On average, trees that were 7 inches in diameter were 100 years old. Although growth was slightly faster than this in some locations, even at the best site, it took an average of ten years for a tree to grow each inch in thickness.

Another important finding from this study is that tree size and age are often not very closely related and it is difficult to make age predictions based on diameter alone. While larger trees were, in general, older, it was not uncommon to find a 6-inch tree that was older than a 10-inch tree growing nearby.

These findings suggest that we should be cautious when making decisions about harvesting blue oak trees. Even if a stand that was cut today regenerated well, it could take several human generations before mature trees were reestablished.

Douglas D. McCreary

prepared and edited by John M. Harper, Richard B. Standiford, and John W. LeBlanc