Changing seasons produce changing growth in plants. In climates with cold winters where the plants become dormant, the tree's cells actually change the thickness of their walls to adapt to the changing weather. The changes in cell walls are rather obvious, because the color of the wood changes.
Find the darkest part of the stump's rings. The dark rings are from the tree's summer growth. During this time of year the tree's cell walls are very thick. This growth is known as "latewood" or "summerwood."
Find the lighter part of the rings. The light portion of the rings are from early spring when the tree is growing quickly. At this point, the cells walls are very thin, so the color appears lighter. This growth is known as "earlywood" or "springwood."
Observe the difference in the distance between the rings. At the points where the rings are spaced farther apart, the tree had a longer growing season. Growing seasons can be cut short by drought, fires, or an early winter.
Count the total rings inside the tree. The number of rings tells you the age of the tree as the pattern from light to dark signifies the annual transition through spring's fast growth to winter's dormancy.