The next time you go to the New Jersey shore (What other shore is there?), look closely at the clam shells. Or run your finger down the outside of the shell. If it's an ocean quahog (Arctica islandica), you could find as many as 405 small ridges. Yes, you guessed where this is going: A 405-year-old shell. It counts time just like trees, and just like trees, the annually added lines and segments show not only age but life history.

In the case of trees, the wood produced during spring's resource bounty is lighter and less dense. In hardwoods, large vessels are also concentrated in the early season's wood. (You can easily see these vessels in natural oak furniture. But as the season progresses, these vessels disappear, and in all woods, the wood becomes denser and darker. Using a tree's rings to date and study a tree is called dendrochronology.

If you just cut your own Christmas tree or even if you trimmed it so that it could absorb water, you can easily imagine the cross-section of a tree trunk. As a bonus, if you chop your own firewood, you can imagine the cross sections of branches. In either case, it is common for the rings to be off-center within the trunk or limb. In other words, the rings are uniformly thinner on one side of the trunk or branch. This "deformity" is actually not a defect, but is called response wood. Trees actually grow thicker on the "compression" side of the trunk. So if the tree is exposed to a prevailing wind, the rings on the opposite side of the wind will be thicker. And on a limb, the rings will be thicker on the lower side of a heavy branch.

Back to shells: Just like trees, shell rings show variation due to climatic and other factors. In other words, variables such as sea water temperature, length of growing season, and available food supply affect the size of the annual rings. Recently researchers have discovered that the combination of water temperature and density affects the oxygen isotopes of the molecular composition of these rings. Conch shells take the "ontogenetic" story to a different level. While the shells of conchs form by accretion, they do not appear to show annual markings. Instead, they visually preserve their life's ontogeny. So if this were you or I, we could look in a mirror to see what we looked like as a toddler.

Here let us imagine a quahog were found in Neptune's non-existent seas. It would take over 65,000 earth years for it to produce the 405 ridges on its shell. But although they live inside the time of their planet, quahogs and sequoias only carry time's markings. It is uniquely human to look into the mirror of time and make choices. Happy New Year!

Joshua Arp is an ISA-certified municipal specialist, Clarks Summit's municipal arborist and an operator of an organic lawn and landscape maintenance business. Reach him at


Recommended for you