No other organism but the tree provides a chronologically accurate record of the past in the form of growth rings. Archeologists use growth rings to date precisely buildings erected in the past 6000 years and physicists determine the isotope content of the cellulose in the rings to reconstruct the intensity of the earth's magnetic field and the solar wind. The practical forester can use tree rings to find the answers to many of the questions with which he is confronted every day. The effects of snow, fungi, insects and rodents, of fire and wind, forest operations and the climate are often manifested in stem discs as long-lasting growth changes or abrupt ones lasting only one year in the form of zones of reaction wood or wounds.