Prairie wanderings: Tree rings, weather, and sunspots
By PAUL G. JANTZEN
Contributing writer
When chopping wood for a fireplace or a wood-burning stove, one probably doesn't think a lot about tree rings and the age of the fuel being processed.
Growth rings are easily seen whenever a tree or woody shrub is cut to expose a cross-section of the stem or when a core borer is used to remove a pencil-sized core without serious damage to the tree.
In a several-year-old stem, the pith is at the center consisting of food storage tissue. At the outside is protective bark. The inside layer of bark has several tissues including phloem cells of several kinds which conduct dissolved organic food from the leaves (where it is produced by photosynthesis) to various parts of the plant including the roots.
Immediately inside the ring of phloem tissue is a thin-walled cylinder of actively dividing cells that are undifferentiated in form and function and called cambium. The cambium cells that mature toward the outer part of the stem become phloem cells while those that mature toward the center of the stem form wood or xylem tissue which conducts water with dissolved minerals from the soil to the leaves. The cambium layer persists because one of the two cells formed at each division remains undifferentiated, available for further division. The cambium and any tissues external to the xylem are pushed outward by the production of more xylem cells. All of this results in an increase in the diameter of the stem and the circumference of the cambium layer.
Growth rings of alternate light and dark bands are caused by differences in the xylem cells produced during periods of rapid growth (as during spring rains) and those produced during periods of slow growth (as during summer and fall). During rapid growth, xylem cells are larger and have thinner walls than when growth is slow. The sequence of rapid growth, slow growth, and little or no growth is usually noticeable without the use of a microscope because the smaller thick-walled cells appear darker than the larger thin-walled cells.
Environmental factors such as periods of drought, flooding, fire, a nearby stream, or unseasonable rainfall can result in fewer or more than one ring per year. Two growth rings may be formed when a drought interrupts the growing season bringing the growth of xylem cells to an unseasonable halt and causing the tree to develop a dark layer in late spring or summer. Later rains then may produce another layer of lighter xylem. This, then, looks like two year's growth. The edge of the midseason dark ring, however, is fuzzy while the ring marking the end of the season always is sharp. In a very dry year, a tree may miss a growing season entirely, leaving little or no indication of the omission. For these reasons the term "growth ring" is more appropriate than the commonly used term "annual ring."
A century ago a series of growth rings in ancient trees was compared to the pattern of sunspot activity. Sunspots are dark specks of whirling electrified gases that are seen in the sun's atmosphere. They take about 25 days to make one rotation around the sun's circumference. The number of sunspots changes periodically reaching a maximum every so often. Arizona astronomer, Andrew Douglass, in 1904, wondered whether the sunspot cycle somehow related to weather and the growth rate of trees. So he analyzed the cross-sections of ponderosa pine trees that averaged 348 years of age. He identified certain sequences of wide and narrow growth rings which matched in different trees and reflected variations in the year-to-year moisture supply. He broadened his collection of tree cross-sections beyond Arizona and by 1915, cross-identified trees from Vermont, Oregon, England, Norway, Sweden, and Germany. Most of the growth ring sequences also correlated with sunspot cycles, even though rainfall often showed no such correlation. In areas of abundant moisture where the annual growth of trees was not so sensitive to variations in rainfall, tree growth still followed the rhythm of the sunspot cycle.
Until this time, reliable records of sunspot activity went back less than 100 years. During this period a clear 11-year cycle appeared. Douglass found the same 11-year cycle during most of the 500-year period covered by his tree records. During the period from about 1650 to 1720, however, the sunspot cycle disappeared completely. Even trees 2,000 to 3,000 years old showed the 11-year solar cycle, except for its disappearance in the late 17th century.
In 1922, Douglass was surprised by a letter from a meteorologist at the Royal Observatory in Greenwich, England. The meteorologist had compiled all known sunspot observations for the past several centuries and reported very little sunspot activity between 1645 and 1715. Douglass now was convinced of reliable correspondence between tree growth and the sunspot cycle.
Douglass also analyzed rings in a few ancient petrified trees providing 11,000 rings. They indicated the sunspot cycle existed back to 50 million years ago.
Meanwhile Douglass' work with tree rings led him into the archeology of the cliff dwellings of the Southwest Indians. The ponderosa pines he knew in Arizona and New Mexico were useful but living trees went back only 640 years. In 1916, he got sections of pine logs from prehistoric pueblo ruins near Aztec, N.M. He then obtained more specimens from house beams 50 miles south of Aztec at Pueblo Bonito from which he extracted tubular cores. By overlapping rings representing the same time periods, Douglass developed a continuous chronology of about 200 years from a period before 1300. But the dates of these samples were unknown. (A chronology not joined with the pattern from living trees and, therefore, our calendar, is a floating chronology.) But they did tell him the logs from the Aztec beams had been cut about 50 years later than those from Pueblo Bonito.
Eventually with continued investigation and many more wood and charcoal specimens, several floating chronologies were established linking Pueblo Bonito with 25 other ruins including Mesa Verde. But only a single, very poor specimen linked these runs with the modern sequence of live trees which went back to 1300.
Then, one evening, the charred end of a buried timber was found and carefully sawed to examine its growth rings. And that charred timber linked the dates of nearly 40 prehistoric communities and was the key to describing southwestern archeology as the "most precisely dated archeology without written record in the world."
Further analysis of tree rings offered a clue to the absence of timbers for a period of 23 years prior to 1300. From 1276 to 1299, a sequence of 23 very thin rings told the story of protracted drought. Many villages were abandoned and the building of new homes was delayed may years.
So Douglass' work, beginning in 1904, became a classic investigation involving the disciplines of astronomy, meteorology, botany, and archeology.
Note: For ease in counting growth rings for analysis or for display purposes, the sawed surface of a tree's cross-section should be sanded smooth and oiled. The dates of important events of a community or an individual's life can be noted on an old specimen's cross-section.
