Window on Nature
By Lowell & Kaye Christie, F47246
On one of our earliest trips to see coast redwood trees (Sequoia sempervirens), we discovered a huge redwood tree stump, crosscut so that the tree rings could be marked with the corresponding dates of historic events. To our surprise, the marker associated with the year Christopher Columbus discovered America was much closer to the outside bark than to the center of the stump. Since each successive tree ring develops right under the bark, it was quite easy for us to see that this tree was very old.
Even after spending much of our lives on the West Coast (Lowell was born in California, and Kaye arrived there as a teenager), it’s easy to forget how many of the trees in our extended backyard hold one type of record or another. In August 2000 researchers discovered a “new” tallest living tree, a coast redwood named the Stratosphere Giant, located in the Humboldt Redwoods State Park in northern California. It tops out at approximately 370 feet. To put this size in perspective, that’s the length of a football field plus another 23 yards.
If you’re more impressed by sheer bulk, head south to Sequoia National Park in central California and check out the 2,000-ton General Sherman tree. This giant sequoia (Sequoiadendron giganteum) is regarded as the world’s most massive tree. While it stands only 275 feet tall, it measures 102 feet in circumference at its base. That adds up to 52,600 cubic feet of wood.
But what impresses us more than size is age. According to Rocky Mountain Tree-Ring Research Inc., the oldest coast redwood is a mere 2,200 years old. One of the massive giant sequoias has reached the grand old age of 3,266. But to find the oldest tree, forget about height and girth and move to the eastern edge of California. There, on windswept mountaintops, in what seems like the most uninhabitable locations, you’ll find the bristlecone pines (Pinus longaeva) that seldom exceed 60 feet in height and 36 feet in circumference. But with many of these trees attaining ages in excess of 4,000 years, it is the oldest living tree species in the world.
Bristlecone pines grow in only six western states “” California, Nevada, Utah, Arizona, Colorado, and New Mexico. Researchers have found that one of them, nicknamed Methuselah, has lived in the White-Inyo Mountain Range of California for 4,790 years.
You may wonder how researchers could arrive at that age. Determining the age of trees is a part of the scientific field of dendrochronology, the study of tree rings.
Except in the tropics, most trees have seasonal bursts of growth during which time they produce new leaves and create new sapwood just below the bark to carry nutrients up to the growing points. In wet years, a lot of new wood is created “” in dry years, much less. Early in the season the new wood cells are large, while later, as the growing season ends, the cells become smaller and packed closer together. The end result is the series of concentric rings that are visible when a tree is cut down. By counting the rings, a dendrochronologist can determine the age of a tree.
Is it really that simple? Of course not. In certain situations, a tree may produce more than one ring, but these instances tend to occur in the tropics where there are multiple wet seasons within a single year. Occasionally a tree will skip a year if it is severely stressed by something such as a lightning strike, fire, or an extreme drought. But tree ring experts, in addition to being skilled at identifying false rings, have other ways to compensate for these problems. The most important is crossdating.
Crossdating is the comparison of multiple tree samples from a given area, often using computers to help match up the rings. The procedure enables researchers to determine the exact year when a ring was created. It also allows them to create a chronology that is longer than the life span of any one tree.
Here’s a simplified example. Assume you compare the rings on one tree that lived for 500 years with another tree that also lived for 500 years, but had died 300 years earlier. That creates a 200-year overlap when both trees were alive at the same time. With a perfect match in rings during those 200 years you could, with some assurance, use the combined rings to span 800 years.
Because of the extreme climatic conditions under which the bristlecone pine grows, deadwood can lie on the ground for thousands of years and still be suitable for tree ring research. By using this type of matching in a much more sophisticated manner, dendrochronologists have created a bristlecone pine ring map that extends back almost 9,000 years.
That’s 9,000 years of area history that can provide researchers with details about climate changes, wet and dry years, and the number of years between droughts. The cells of these ancient trees even contain evidence showing that sunspot activity, which has major cycles of 51 and 96 years, has occurred with the same regularity century after century.
Of course, you wouldn’t want to cut down an ancient tree just to discover the patterns stored in its rings. So dendrochronologists have developed a boring tool that can extract a “core” from the tree measuring 0.423 centimeters in diameter. But since tree growth is not always symmetrical, researchers often take several samples from a single tree to extract information from all the rings. The same technique can be used on dead trees, on deadwood that has fallen from trees, and even upon logs that have been used to construct old buildings. This makes dendrochronology particularly useful to archaeologists.
The Oxford Dendrochronology Laboratory in England specializes in the archaeological use of tree rings. The company has done extensive research on historical buildings in England, and even some work in the United States, most notably in Boston. In the Boston Dendrochronology Project, researchers dated a number of houses and churches for the Massachusetts Historical Commission, ranging in time from 1640 to 1769. Just remember that the date reported is when the tree that was used to build the structure died or was cut down, and not when the structure was actually built, although the two will likely be very close.
Although England may not have trees as old as some in the United States, they certainly have older structures. One of the projects contracted by the laboratory sought information on the Tower of London. Their research determined that construction dates for some of the earlier parts of the White Tower took place shortly after the year 1050.
Let’s do a little calculation here. The year 1050 was 954 years ago. At that time the bristlecone pine named Methuselah was only 3,836 years old. And although Methuselah is recognized as the oldest-known living tree, that distinction should belong to another tree.
Ancient bristlecone pines also are located on Wheeler Peak, near the border of Nevada and Utah. During the late 1950s and early 1960s environmentalists tried to give the area government protection through the National Park Service. Unfortunately, there also was plenty of opposition, and the legislation to protect the area failed year after year.
In 1964 a geography student from the University of North Carolina was conducting research on Ice Age glaciers in the area, and got permission from the U.S. Forest Service to take core samples from some of the local trees. After finding a tree that was more than 4,000 years old, his boring tool broke, so the student received permission from the Forest Service to cut down the tree. It was later determined that the tree, called Prometheus, had been the oldest living tree on earth.
If still alive today, it would be 4,902 years old. Just think: It may have been Methuselah’s grandpa.