Listening with your eyes
A tree's trunk consists of several layers that envelope each other in a concentric pattern. The bark consists of two layers, an outer bark that protects the tree and an inner bark, or phloem, which carries sugar from the leaves to the roots.
The cambium, a thin layer between the bark and the rest of the trunk, produces both the phloem and the tree rings that can be seen in a cross-section view of a trunk. Each tree
The newest tree ring that the tree has created—called sapwood or xylem—carries food from the roots back to the leaves. The older rings, or heartwood, no longer carry sap and are often darker in color than the xylem because its water-carrying tubes are clogged. Depending on the tree, it can be many, many years before heartwood is created.
A tree never forgets
A tree can grow for hundreds or, sometimes even thousands of years. It will encounter overcrowding, fires, objects like rocks or logs leaning against it, and insect infestations. If the tree survives all of these, its rings will tell the tale.
The thickness of a ring will display the quality of growing conditions. Thick rings signify quick growth and tell of a year with plenty of water and good access to sunlight. Narrow rings could mean a drought or groups of narrow rings could signify that the tree was fighting with surrounding, taller trees for sunlight. When in doubt, a researcher counts the rings to determine which year's ring is in question. They then compare its thickness to weather records for that year.
Recorded weather trends can tell us about seasonal conditions for a large area. Tree rings can pinpoint these differences in much smaller areas. When studying a specific area, dendrochronologists can also compare the rings of newly cut trees to older logs and stumps in the area. Once they've found similar rings, they can use the older stump's inner rings to extend their research past the oldest living trees.
Reversely, dendrochronologists can study a wooden object and determine how old the object is based on the rings and the type of wood used. The most famous example relates to dating the Messiah violin. The controversy focused on determining if the famous Antonio Stradivari created the violin or if it was a counterfeit. Dr. Henri Grissino-Mayer led the review panel, which determined that the spruce wood used to make the instrument grew between 1577-1687. This matched the time period in which Stradivari created his violins and not when other known counterfeits were created.
Storm recorders
In Georgia, dendrochronology and geochemistry came together in a study of longleaf pines in an environmental preserve. The combined effort has provided a hurricane history for the past 220 years.
Rain has two major isotopes of oxygen—O-16 and O-18. Long rainstorms, like those associated with hurricanes, have an unbalanced isotope ratio with more O-16. As areas become saturated by rain, plants absorb the water and record the unbalanced isotopes as they create new cells. Because much of the water will run off before absorbing into the ground, plants with shallow roots, like the longleaf pines, rely upon rainwater more than groundwater for their growth. And they will record more isotope difference than plants with deeper roots.
Hurricanes often occur in the late summer when growing conditions are not ideal and trees create narrow rings. Pulling this isotope information from the narrow dark bands of the tree is not easy; but it's worth the effort. Weather records of hurricanes that hit the area matched the isotope ratios in the tree rings. However, the trees also recorded older hurricanes, which were not on the weather records.
Using this information, researchers hope to discover whether the recent increase in hurricane activity is part of a long-term trend or the result of pollution and global warming. The research continues, and by eventually studying multiple populations of trees from all the southern states that feel the effects of hurricanes, scientists hope to provide an important piece of the global warming puzzle.