They're round. They're full of fiber. But unless you're a termite, you
can't eat tree cookies!
Tree cookies are cross sections of tree trunks that foresters and teachers
use to illustrate how trees grow. Tree cookies reveal the many different
layers that make up a tree. And each layer can tell us something about the
tree's life and the climate in which it grew.
Item 1 is called the cambium. It is a layer or zone of cells,
just one cell thick, inside the inner bark. The cambium produces both the
xylem and phloem cells. This is where diameter growth occurs,
and where rings and inner bark are formed.
Item 2 is the phloem or inner bark. This layer carries sugar made
in the leaves or needles down to the branches trunks and roots, where it
is converted into the food the tree needs for growth.
Item 3 is the xylem or sapwood. This layer carries the sap (water
plus nitrogen and mineral nutrients) back up from the roots to the leaves.
Sapwood gives a tree its strength.
Item 4 is a growth ring. The lighter portion is called the "early
wood" (beacuse it grows in the spring), and the darker portion the
"late wood" (which grows in the summer). Together, they represent
one year of growth. (You can count the rings to see how old a tree is!)
Item 5 is the heartwood. Heartwood develops as a tree gets older.
It is old sapwood that no longer carries sap, and gives the trunk support
and stiffness. In many kinds of trees, heartwood is a darker color than
sapwood, since its water-carrying tubes get clogged up. The tree cookie
at right, like many of its fellow young pines, has not developed heartwood
Layer 6 is the outer bark. This layer protects a tree from insects
and disease, excessive heat and cold, and other injuries.
The rings of a tree give us a lot of information about the age of the
tree, its health, and the climate conditions during each year of its growth.
Just for fun, predict the number of rings on each of the tree cookies on
this page. They are about the same size, but are they of the same age?
Count the rings and find out. Hmmmm. How might you account for the differences?
(HINT: Think about all the things a tree needs in order to grow.)
Stumped (tee hee)? Here are some explanations to help you think about
it: The first tree cookie shown has a small number of wide rings, indicating
that it came from a young tree that grew in an area where it had little
competition for the things a tree needs to grow -- such as sunlight, water,
The second cookie (below) has many tight rings. It is from an older tree
that grew with more competition.
The fact that the center rings are offset
indicates that the tree either grew on a slope or had to grow around some
sort of obstruction.
If you were a forest manager, how could you use your
knowledge about tree growth to manage a forest for wood production? For
wildlife habitat? For water quality? Or for all three?
(Most forest managers
manage for all these benefits -- and more! What an interesting and challenging