A FAMILIAR RING: AN INTRODUCTION TO TREE-RING DATING

Transcription

1 A FAMILIAR RING: AN INTRODUCTION TO TREE-RING DATING Dr. Henri D. Grissino-Mayer Department of Geography The University of Tennessee Knoxville, Tennessee (office) (fax) INTRODUCTION Trees do more than offer shade and beauty to our landscape, a landscape that is all too quickly becoming more and more shaped by human activities. Trees also help maintain a delicate balance between the amounts of carbon dioxide and oxygen in the atmosphere, a service necessary for all life to exist. Trees may even act as a sink for the extra carbon dioxide that we humans are pumping into the atmosphere in amounts that may artificially be increasing global temperatures. Because trees are intimately bound to the environment, they are considered by some scientists to be Nature s ultimate recorder of the environment. As goes the environment, so goes the vitality and health of the tree in the form of its growth and its growth rate. Trees are just as vulnerable to the events happening in its environment as are we humans, events that trigger responses in the growth habits of trees. Weather patterns in any year cause a response by trees in the amount of wood that the tree produces. In some years, the environment is favorable for tree growth, allowing trees to produce greater amounts of wood than average. In other years, climate conditions may not be favorable for tree growth and less wood is produced. For example, a very wet winter followed by a cooler than normal spring and summer may be ideal for enhancing the growth rates of trees because less moisture is evaporated from the soil due to the cooler temperatures, making it available for tree growth. In contrast, a dry fall and winter, followed by a warmer than average spring and summer, may restrict the amount of moisture available to promote tree growth. 1

2 THE BIOLOGY OF TREE GROWTH A tree grows by forming a new sheath or layer of woody tissue each year, much like a stack of inverted cones one on top of the other. This growth occurs in a very thin layer of cells that completely shrouds the tree called the vascular cambium. Here, cambial cells divide, with the outer cells contributing to the formation of phloem tissue, which helps transport nutrients downward from the crown of the tree. As phloem cells become older and die, they contribute to the protective outer layer of the tree called bark. The inner cambial cells contribute to the formation of woody tissue called xylem. The xylem is divided into sapwood, which transports water and sap upward, and darkercolored heartwood to the interior of the tree trunk, consisting of xylem cells where waste substances have accumulated. During the spring, most trees come out of their winter dormancy, and begin growth for this new year by forming new cells of wood using the nutrient reserves stored from the previous growing season. In conifers, such as pine trees, these cells are large and have thin walls, producing a light-colored wood called earlywood, or springwood. In certain hardwoods, such as oaks, large cells called vessels are formed in the earlywood. Towards the end of the growing season, just before the tree begins its dormancy period, a pine tree will form smaller, thick-walled cells that create wood that is more dense. This denser wood is darker in color and is called latewood, or summerwood. Hardwoods will create a thin, denser layer of cells as well, marking the end of the growing season for that year. THE FORMATION OF TREE RINGS Let s examine this sequence of growth throughout the year for a pine tree. In the spring, a lighter band of cells was produced by the tree, called earlywood, while later in the summer and fall, a darker band of cells was formed called latewood. Taken together, these two bands of wood formed during one year s growth create a tree ring, so called because the layers of cells are produced completely around the circumference of the tree trunk (or branch or twig), forming a ring. We have all seen a stump of a tree just after it was cut down, and you may have noticed when looking down to the top of the stump those concentric bands of darker wood. These are the tree rings themselves. Each ring 2

3 therefore represents the amount of wood produced in one year by that tree. By counting the number of tree rings (actually by counting the darker bands), you can easily learn how old the tree was when it was cut down. This relationship - one ring per year - holds true for most trees formed in the temperate latitudes (between degrees north or south of the equator). Sometimes, a tree may begin to form latewood towards the end of the growing season, but a new flush of growth may again form earlywood cells. This band of latewood between earlywood bands, termed a false ring, would ordinarily be classified as a true tree ring. However, it is anatomically different from a true tree ring when magnified, and is therefore not counted. On the other hand, environmental conditions during some years may be so unfavorable that some trees may not form new wood completely throughout the trunk of the tree, and may result in a missing ring. (In reality, this layer of new growth may still be found in the upper portions of the tree where growth is initiated, so this ring is sometimes more appropriately termed a locally absent ring in the sense it is not evident in the trunk of the tree.) In most cases, false and missing rings cause few problems when counting tree rings. THE SCIENCE OF DENDROCHRONOLOGY At the turn of the century, an astronomer at the University of Arizona named Andrew Ellicott Douglass was curious about the relationship between sunspots and the Earth s climate. He surmised that any change in solar energy output caused by changes in sunspot activity would cause changes in the amount of energy received by the Earth, and therefore alter the Earth s climate. Because climate affects the growth of plants, sunspot cycles should be reflected in the growth of trees. He therefore hoped to see patterns in the tree-ring record similar to the cycle of sunspot activity. His investigations on tree rings lasted another 50 years, all the while formulating the principles that today serve as the basis of dendrochronology. Dendrochronology is the science that uses tree rings dated to their exact year of formation to analyze temporal and spatial patterns of processes in the physical and cultural sciences. Dendrochronology takes advantage of the fact that trees faithfully record patterns in the environment over time. Whatever is happening to the environment around the tree will likely be recorded at some level within the tree-ring record. For example, a pine tree located near a mine smelting operation will be greatly 3

4 affected by chemical pollution, both in the air and in the soil. By analyzing its tree rings, both chemically and physically, a quantitative record of the impact of this pollution on the environment can be assessed. But what exactly constitutes the tree-ring data and how is it collected? COLLECTING THE TREE-RING DATA The primary method to collect tree-ring data uses a device called an increment borer, which is a hollow metal tube with a thread on the end that is screwed into the trunk of a tree. A special extractor is then inserted under the tree core that now resides inside the borer shaft, and gently pulled from the tree. The hole left by the increment borer rarely does harm to a conifer tree because often the hole is rapidly filled with resin or covered by bark. However, the bore hole in a hardwood tree may eventually cause internal wood discoloration and lead to the formation of fungi that can decay wood. The pencil-thin core is then glued to a wooden slat, then sanded with progressively finer sandpaper (usually with a belt or orbital sander), until a high polish is obtained and the cell structure of the wood is easily discernible under a microscope. Occasionally, certain types of research require complete cross sections of the trunk of a tree, and a chain saw is most often used, especially on logs and smaller pieces of wood from a tree long dead. However, dendrochronologists rarely cut down living trees, as an increment borer can easily extract the tree-ring record from living trees. Scientists are primarily interested in the width of individual tree rings, and devices have been developed that allow the measurement of tree-ring widths to the hundredth of a millimeter called measuring stages. However, other properties of tree rings can also be measured and used, which include the average density of the tree ring, the minimum density of the earlywood, or the maximum density of the latewood, as well as the widths of the earlywood or latewood themselves. Density is most often obtained using X-ray images made of tree cores that have been sliced ultra-thin. Within the last 20 years, new scanning techniques and image analysis of tree rings have provided a new means to easily capture tree-ring data. But, how do dendrochronologists assign exact dates to tree rings? 4

5 THE PRINCIPLE OF CROSSDATING The guiding principle of dendrochronology is the Principle of Crossdating. Crossdating is the process by which patterns of wide and narrow tree rings from one tree can be exactly matched against corresponding patterns from another tree. Crossdating is made possible because climate is largely a regional phenomenon, affecting all trees in a like manner, so that they will produce similar patterns of wide and narrow tree rings. Think of this tree-ring pattern like your signature, which is unlike any other person s signature. A unique pattern of tree-ring widths during a 50 year period is unlikely to be formed during any other 50 year period because climate varies from year to year. Crossdating is the primary guiding principle because it allows scientists to accurately assign calendrical dates to tree rings by matching the sequence of tree-ring widths against a known reference chronology. Let s illustrate using an example. AN EXAMPLE OF CROSSDATING To begin the crossdating process, dendrochronologists often use reference chronologies, but where do these come from? A tree-ring chronology represents the collective information from numerous trees within a region, and is represented by indices of tree growth that represent departures from average. These indices are derived using complex mathematical expressions that capture the majority of variability in tree-ring widths during any given year. Indices have a mean of 1.0, which indicates normal or average growth for any given year, and a minimum value of 0.0. Any index below 1.0 would suggest an unfavorable year for tree growth (perhaps a drought year), while an index above 1.0 would indicate a favorable year for tree growth (perhaps a very wet year). The length of the reference chronology represents the maximum length in time of the oldest tree sampled. Literally hundreds of tree-ring chronologies exist for nearly all parts of the temperate regions of the Earth (i.e., between north and south latitude). Let s imagine that an archaeologist has presented a sample of wood to a dendrochronologist that was extracted from a room in a pueblo ruin in the American Southwest. The archaeologist is interested in the outer ring for this piece of wood because 5

6 it would tell them when that room in that pueblo was likely constructed. After surfacing the wood sample, the dendrochronologist will graphically compare the pattern of ring widths in this sample with a known reference chronology developed previously for the area. In the American Southwest, this graphical comparison is done with skeleton plots, which accentuate the importance of narrow rings in the semiarid region. Once the pattern of narrow rings is matched, or crossdated, against the reference chronology and dates are obtained, another dendrochronologist must independently confirm the accuracy of the crossdating. In most studies, the tree-ring widths are then measured, and statistically confirmed using specialized computer software. To ensure a high level of confidence in the dates assigned to the tree rings, dendrochronologists look for exceedingly high levels of statistical significance, much higher than those normally used in statistics to identify a probable match between two series of data. For example, European dendrochronologists most often use a statistical measure called a t-value to help confirm crossdating between two series of tree-ring data. While a value of 2.0 is usually considered statistically significant (in other words, you have less than a one in 20 chance of obtaining a value higher than this for 30 or more years of tree-ring data), dendrochronologists look for values of 3.5 or higher (in other words, you have less than a one in 1000 chance of obtaining a higher value). Only after a tree-ring series has been crossdated precisely both graphically and statically and independently confirmed can dates for a wood sample be announced. APPLICATIONS OF DENDROCHRONOLOGY Today, the science of dendrochronology is practiced by thousands of scientists worldwide on every continent. The value of dendrochronology is its use as a tool that can be applied to a variety of research questions, all of which concern to some degree changes in the environment over time. Interestingly, the practice of tree-ring dating would be impractical unless it was applied to help answer a research hypothesis. For example, dendroarchaeology is the science that uses tree rings to date the wood material found in archaeological sites or artifacts, and has been most often applied in the Southwestern United States and Europe. In fact, dendrochronology came of age on June 29, 1929, when A.E. Douglass was able to connect his ring chronology obtained from 6

7 living trees with those chronologies he had developed from trees used in archaeological sites in the American Southwest, effectively dating the construction to the year of many of the Southwestern pueblos. The use of tree rings is still a major application in the American Southwest today. In Europe, dendrochronologists are often employed to date the period of construction of barns, manors, cathedrals and churches, Roman bridges, wells and fountains, pile dwellings in lakes, and Neolithic settlements. In 1996, dendrochronologists from Cornell University announced the development of a multimillennial tree-ring chronology that helped rewrite Mediterranean archaeological history. Dendrochronology is also useful for dating the likely construction period of any piece of wooden artwork (e.g., statues and panel paintings) or musical instrument. Soon after A.E. Douglass dated the archaeological pueblo ruins, his student at the University of Arizona, Edmund Schulman, began investigating the use of tree-ring data to reconstruct past climate, a science known as dendroclimatology. Because temperature, precipitation, atmospheric pressure, and other climatic variables affect tree growth, it seemed logical that the climatic information contained in tree rings could be extracted and reconstructed back in time for the length of the tree-ring record. For example, the oldest known trees, the bristlecone pines growing in the White Mountains of eastern California, have provided information on both temperature and precipitation fluctuations for the western United States over the past 8000 years. Reconstructions of annual rainfall amounts for the American Southwest over the past 2000 years have shed light on the environment of the ancient Native Americans who lived there, and provided new clues concerning the role of droughts that lasted for decades that may have prompted the abandonment of the pueblos years ago. Forests are subject to a variety of ecological processes and disturbances that may maintain, create, or alter forest structure and composition. For example, periodic insect outbreaks (such as the eastern spruce budworm) can defoliate and/or kill individual or large groups of trees. It makes sense that any such defoliation event can be discerned in the tree-ring record as a group of continuous and progressively narrower tree rings. Dendroecology is the science that analyzes changes in ecological processes over time using tree-ring information. In addition to analyzing the effects of insects on forest stands, other applications that fall under dendroecology include analyzing: (1) the effects of rising carbon dioxide levels (i.e., the greenhouse effect ) in the atmosphere on tree 7

8 growth, (2) the effects of air, water, and soil pollution on tree growth and forest health, (3) the age, maturity, and successional status of forest stands, and (4) the effects of human disturbances and management on forest vitality. Another ecological process that greatly affects forest stands are wildfires that periodically occur in nearly all ecosystems of the world. Wildfires can occur as lowintensity surface fires that creep along the understory, clearing out leaf litter and other forest debris, or as a high-intensity crown fire that kills large swaths of forested areas. Low-intensity wildfires will kill a part of the living cambium on the lower portion of the tree trunk which subsequent growth will preserve, thus leaving a fire scar in the tree-ring record. Dating the tree ring will therefore also date the fire scar, thus dating to the exact year when a wildfire occurred. This science is known as dendropyrochronology, which reconstructs wildfires from the tree-ring record. This field has major implications because humans have greatly affected the natural course of wildfires, largely by suppressing fires at all costs, not realizing the beneficial nature of these wildfires to the forests. Many governmental agencies now seek to restore the vitality of ecosystems by restoring fire. Tree-ring scientists are able to reconstruct how many fires occurred in the past, thus helping these agencies plan and manage for fire in the future. Finally, dendrogeomorphology is the science that studies earth surface processes, such as landslides and snow avalanches, using tree-ring data. For example, a snow avalanche will usually occur down a drainage on the side of a mountain, and can either kill or bend the trees in its path or along the edges of the avalanche track. Tree-ring scientists can date when these trees were killed (by dating the outer ring of the tree) or when they were bent (by analyzing when dramatic changes in tree growth occurred). Therefore, a chronology of past avalanche or landslide events can be developed back in time to help assess the hazard potential for the low-lying areas at the base of these slopes. This type of analysis has major implications for creating hazard maps in areas where neighborhood development, road construction, or the creation of business districts may take place. 8

9 SUMMARY Dendrochronology has firmly taken its place among the many useful tools offered to scientists to better understand how changes in our environment may eventually affect humans. Tree-ring dating provides a perspective back in time of changes that have occurred in many environmental factors, so that we can be better prepared for any changes in our environment in the future. For example, do trees show changes in past temperatures that may be similar to the changes we are currently experiencing worldwide due to global warming? If not, this would argue that global warming is largely a humancaused effect. Dendrochronology also provides a useful tool for understanding the chronology of human events. In practically all human cultures, wood has been a primary resource used in the construction of so many commodities necessary for human existence and advancement: houses and other dwellings, furniture, statues and other artwork, churches and cathedrals, roadways and railways, musical instruments and other objects used in entertainment, and particularly in industry and business. And in this wood, you will find tree rings. Keep in mind that trees are, at this very moment, recording the varying factors that are part of our environment. Dendrochronologists simply read the language of the tree, knowing full well that trees are incapable of lying. They may make the truth sometimes difficult to learn, but they never lie! 9