Dendochronology: Looking at Tree Rings

Trees grow at different rates in relationship to the amount of rainfall. Wider tree rings represent greater growth during wetter years; narrower rings represent less growth during drier years. The outer ring corresponds to the year a tree fell or was cut down.

Trees that depend on rainfall, such as douglas fir, ponderosa pine, piñon and juniper, can show a clear picture of annual environmental conditions. Uniform rainfall within a region creates a unique pattern of wider and narrower tree rings. Scientists use combined growth ring sequences from living and dead trees to create reference sequences for a particular region. When archaeologists working in these areas excavate fragments of wood, they compare the patterned sequence of tree rings to the reference sequence in order to determine the year that a tree was cut down.

Scientists have not completed a reference sequence for the Mimbres region because tree rings do not give a clear picture of annual rainfall over the region. Intense, localized monsoon rains during the summer create variation of tree growth, sometimes from one hillside to the next.