Cross Section A:
The uneven growth shown in the rings could have been caused by a fallen tree leaning against the tree. The tree grew more on one side than the other, and curved up around the fallen tree. This uneven ring pattern could also belong to a tree growing on a steep slope.
Cross Section B:
The scarring on this cross section was caused by a forest fire during the tree's sixth growing season.
Cross Section C:
The mark beginning in year six is all that's left of a branch that died and fell off. Eventually the tree's trunk grew around the remains of the branch and covered it. (The branch could also have been broken or cut off.)
Cross Section D:
The narrow rings shown in this cross section could have been caused by several things such as drought, heavy insect damage, or damage from construction. If a tree loses all or most of its leaves because of an insect attack or drought, it is not able to make food and grows very little that year. Root damage from the construction of a house or sidewalk too close to the tree reduces the water and minerals the roots can absorb.
Additional Resource To make your own cross section of a tree, or "tree cookie" for educational activity.