Annual rings are formed by trees growing in temperate regions on an annual basis. The cambium layer of trees is responsible for the formation of growth rings. The cambium is the region of a tree where cell division takes place that adds new wood and increases the diameter of a tree. During the cold winter months, activity in the cambium layer of trees is inactive, but in the spring when temperatures begin to warm up, activity in the cambium will be reactivated. When cell division is initiated in the spring, it proceeds rapidly and then begins to slow down in late summer and finally completely stopping in the fall. This pattern of growth forms one growth ring per year. The two kinds of woods that appear as alternate concentric rings constitute an annual ring. The earlywood tends to be lighter in color than the latewood. Occasionally, a tree may create a false ring, which can occur when an event such as a severe drought or insect defoliation occurs. If the event is severe enough to cause the formation of latewood cells earlier than normal, and then the conditions change within the same growing season that cause growth to resume to normal, it is possible for the tree to develop a false ring.