Measuring Growth and Age of Second-growth Coast Redwood

By Thembi Borras

A while ago I was told the outrageous story of a man who cut down the oldest tree to the remove the increment borer he had gotten stuck attempting to determine it's age. It was a bristlecone pine tree in Southern California that was approximated to be 4,000 years old. An increment borer is a long cylindrical hollow steel tube with a drill on one end and a handle on the other. An extractor as long as the tube is used to remove the core. Foresters use increment borers to measure a trees total age and/ or radial increment, a means to ascertain volume growth. Total age is used to recreate stand management history, history and pattern of natural disturbance and with tree height determine site productivity. To measure total age, the increment borer must be long enough to reach the center of the tree. I carry a 12" increment borer, which is comfortable and adequate for measuring the increment of the last ten years and with it, I can also get total age on a tree less than 18" in diameter at breast height.

A tree grows by laying a cone of xylem, within the bark, each year atop the previous years cone of xylem. When viewed in cross section these rings are easily distinguishable by the growth ring boundary, which is where the previous years small, thick walled cells of the latewood meet next years large, thin walled cells of the earlywood. So it is logical to think when the rings of a tree core taken at the base of the tree from bark to center are counted that the result will be the age of the tree.

However, in a paper recently published in the Canadian Journal of Forest Research, authors Kristen Waring and Kevin O'Hara caution drawing conclusions about a trees age from increment cores of second-growth coast redwood due to discontinuous or missing rings. A total of 157 cross sections were analyzed from 22 trees to reach several conclusions including, 40% of the time the rings counted in a core of a codominant second-growth redwood will be less than the trees actual age. For a suppressed tree this number climbs to 85%. This error can be minimized, but not eliminated, by modifying collection methods, which include taking the core on the outside of a sprout clump where growth rings are usually larger. How many rings are missing in a core? According to Dr. Kevin O'Hara, in his experience, more than a few, but this number is more difficult to quantify. Ultimately the concern is that, "growth and yield estimates based on tree cores will overestimate growth, because ages or time intervals are underestimated."

A portion of this production was gleaned from the paper entitled, "Estimating relative error in growth ring analyses of second-growth coast redwood", written by Kristen Waring and Kevin O'Hara.