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Amateur Naturalist: How Young Is Young And How Old Is Old?

on November 9, 2017 - 7:29am
The dark color of one tree and brownish-orange of the other suggests that one is young while the other is mature. However both trees have similar diameters and both have their branches emerging further up their trunks. Are they different or similar in their maturity? Photo by Robert Dryja

Los Alamos

A ponderosa tree may live for as long as 700 years. This is 10 times longer than a person. This means there are some ponderosa trees that were already approaching 200 years old when Columbus discovered the Americas. Others were saplings before the renaissance that began in medieval Europe.

So what does it mean to be young, middle aged, or old from the perspective of a tree that may live 700 years? First, time changes slowly. It is measured by the season. A tree senses the changing seasons over a year rather than how many minutes, hours, or days have passed. Growth (wakefulness) and other biological activity starts with the arrival of spring. Dormancy (sleep) starts with the arrival of fall.

A ponderosa is considered to be young when its bark is dark in color and there are only narrow furrows in it. It is considered to be entering maturity when its bark changes to a brownish-orange color and the furrows are wider. The bark itself develops a layered texture. A tree is considered to be old when branches have fallen from its lower trunk and vertical growth has stopped. These descriptions may be good if you are ponderosa living for hundreds of years, but what happens when compared to a human time scale? Things then can become a bit frustrating.

The frustration occurs when walking through the forest on the downward slope away from the Aquatic Center. Two ponderosa trees are be growing adjacent to one another. Both have nearly the same diameter, indicating a similar age. The branches for each are growing further up their trunks, indicating they both are mature.

The frustration begins when you additionally consider the color of the trunks. One tree has the dark colored bark of youth but the other has the brownish-orange bark of maturity. Perhaps the tree with youthful dark bark will change to an orange color in another twenty years, matching the already maturely colored tree. Twenty years for a person represents a generation. Twenty years for the tree is like two years for a growing teenager.

A count of tree rings is an alternative way to determine the age of the two trees rather than using visual impressions. How well do ring counts associate with any of the visual impressions? It turns out that trees that have the same diameter can vary greatly in age based ring counts. One study showed that trees that were 20 inches in diameter typically were from 50 to 100 years in age. Further, a number of the trees were approaching 200 years in age. Proportionally, the difference of 50 or 100 years for a tree living 700 years is the same as 5 or 10 years for a person living 70 years.  Now consider a person maturing from 13 to 18 years old. These 5 years cover the change from youth to adulthood.

Diameter, branch height, bark color and ring count take on a different perspective when spread over decades and centuries. Time lapse photography provides a way to speed up the growth of plants to appear more like the way we perceive time. This video link shows a vine actively “searching” for support as it grows upward. What would we see if a time lapse video could be made over several decades of two ponderosa trees growing next to one another? Slowly growing trees may appear very active. Two trees may look very similar as they grow, having only brief visual differences as they grow.