Nature Bulletin No. 516-A   February 2, 1974
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

****:TREE RINGS

Every one of our native trees keeps its own diary of climatic changes 
or other events that affect its growth. Each year a page is added which 
faithfully records whether that was a lean year or a fat one. Each year, 
beneath the bark, the tree adds a layer of wood to its trunk which 
becomes that much larger in diameter. When conditions are ideal, the 
layer is thick. When there is a severe drought, or a plague of insects 
that destroy most of its leaves in early summer, or some other trouble, 
the layer will be thin. If the tree is cut down with a saw, those layers 
appear on the stump as a series of concentric rings called Annual 
Rings or Tree Rings.

Chicago's Museum of Science and Industry displays a section of the 
trunk of a California redwood cut in 1933. It is about nine feet in 
diameter and has 1384 rings. That tree started to grow only 549 years 
after the birth of Christ. In contrast, John Muir tells about a wind-
twisted stunted pine, stubbornly clinging to a rock ledge near the 
timber line on a mountain, that was barely three feet high and four 
inches in diameter but had 255 annual rings.

If you examine the stump of an oak or a walnut cut while it was alive 
and sound, you will see at the center a small core of pith formed when 
the tree was a sapling. Then comes a cylinder of dark dense wood -- 
the heartwood -- with annual rings which are often very narrow near 
the core because as a youngster it grew in the shade of older trees and 
did not get enough light. Surrounding that is a collar of lighter-colored 
wood -- the sapwood -- with a smaller number of rings. Beyond that 
and just inside the rough outer bark is a spongy layer of inner bark 
called the phloem.

But you will not see, because it is so very thin, the most vital part of 
all. Between the sapwood and the inner bark there is a single layer of 
living cells, the cambium, which has the magical property of 
producing, each year, a layer of sapwood on the inside and a layer of 
inner bark on the outside. The wood formed each spring consists of 
light-colored thin-walled cells. Toward the end of the growing season, 
the cells formed are smaller and have darker thicker walls. The 
springwood and summerwood form that year's ring and their 
difference in color distinguishes it from the similar one a year older.

In slow growing trees, with dense fine-grained wood, such as an oak, 
the annual rings are generally much narrower than those of fast-
growing species such as the sycamore and poplars. Further, in any 
tree, the thickness of each ring is affected, for good or bad, by one or 
more factors: (a) the precipitation of rain and snow; (b) the amount of 
sunlight it gets; (c) the fertility of the soil and whether it is aerated or 
badly compacted; (d) temperatures; (e) the length of the growing 
season; (f) fires; (g) insect infestations and diseases.

In a tree, it is the sapwood that conducts water and dissolved nutrients, 
taken from the soil by the roots, up through the trunk and branches to 
the leaves. The food, mostly sugars, manufactured by the leaves is 
carried down to the trunk and roots by the sieve-like inner bark or 
phloem. As the tree grows, the older rings of sapwood are gradually 
filled with a hard substance, called lignin, and become heartwood. It 
many species, such as the oaks, walnuts and sugar maple the 
heartwood contains other substances that cause it to be darker, tougher 
and more durable than the sapwood. In others, such as the sycamore, 
there is very little difference between the two.