Dendrochronology

Dendrochronology is the study of past climate change through examination of tree ring growth. Andrew Ellicott Douglass from the University of Arizona first used this specialized branch of science in the early 1900s. Douglass was the first to realize that the wide rings of certain species of trees were produced during years with ample rainfall and favorable growing conditions.

Every year, a tree adds a new layer of wood to its trunk and branches, which create the annual rings that are visible when viewing a cross section of a tree trunk. The new layer of wood grows from the cambium layer between the old wood and the bark. During new spring growth

Wood Growth Ring

Each year a tree adds a new ring, enabling scientists to determine the age of trees. Wet years result in thicker rings, dry years are associated with thin rings. By using these rings as a time line, climatologists can determine the past climate history of an area.

(Nature's Images)

when rainfall is abundant, the tree uses its energy to produce new growth cells. The newest cells generated are the largest of the annual layer. As the summer progresses, temperature rises, and moisture become less abundant, the size of the cells decrease. Then, in the fall, the growth stops and the cells die. The tree remains dormant during the cold winter months. It is the contrast between the smaller old cells and the following spring's larger new cells that creates the visible boundary that is referred to as a ring. Each year, the tree repeats this cycle, adding a new ring. Over time, the tree becomes a measuring stick of time because the rings can be counted in order to determine chronology (time). Trees at middle and high latitudes produce the annual rings necessary to analyze past climate. Patterns in the width, wood density, and hydrogen and oxygen isotopic composition of tree rings can be used to estimate temperature.

Analyze Wood Density

Because the same set of environmental factors influence tree growth throughout a region, the patterns of ring characteristics, such as ring widths, are often common from tree to tree. These patterns can be matched between trees in a process called cross-dating, which is used to assign exact calendar year dates to each individual ring. The calibrated rings from a number of trees in a region are combined to form a tree-ring chronology. (NOAA Paleoclimatology Paleo Slide Set, Laboratory of Tree-Ring Research, the University of Arizona)

Because the same set of environmental factors influence tree growth throughout a region, the patterns of ring characteristics, such as ring widths, are often common from tree to tree. These patterns can be matched between trees in a process called cross-dating, which is used to assign exact calendar year dates to each individual ring. The calibrated rings from a number of trees in a region are combined to form a tree-ring chronology. (NOAA Paleoclimatology Paleo Slide Set, Laboratory of Tree-Ring Research, the University of Arizona)

Climatologists study climate changes and climate patterns in geographic areas through analysis of tree rings. Samples from trees of unknown age can be studied to see if they match with trees that have been analyzed and dated. If cross-samples can be matched somewhere in their ring sequences and overlap in age with rings that have been dated, climatologists can look further into the past. This stair-step concept is referred to as extending the chronology using cross-dating techniques. Samples are usually taken from trees with a long slender boring tool without harming the tree. This technique has allowed scientists to establish some bristlecone pine chronologies in North America that

This is a bristlecone pine in the White Mountains of California. The bristlecone pines of the Great Basin region of the western United States are the oldest known living trees—reaching ages up to 7,000 years old, providing extensive historical records of climate. (NOAA Paleo-climatology Paleo Slide Set; Jonathan Pilcher, Palaeoecology Centre, Queen's University, Belfast, photographer)

Www Plicher Tree

date to nearly 9,000 years ago. Trees can grow to be hundreds to thousands of years old and can contain reliable records of climate for centuries to millennia.

It is important to keep in mind, however, that as with all proxy data, there are limitations to techniques. In dendrochronology, the most serious limitation is that more variables affect tree growth and health than just moisture availability. Tree growth itself is very complex, as is interaction with the surrounding environment. Growth of trees can also be affected by wind, soil properties (nutrients, physical structure), slope (steepness of the terrain), sunlight availability, temperature, winter snow accumulation, fire, and disease. All of these factors contribute in some way—big or small—to a tree's annual growth and therefore to the appearance of its rings.

When climatologists sample trees in the field, they do not base their conclusions on just one core from one tree. They usually take more than one core from each tree, and they sample many trees over a broad area.

When trees grow to become hundreds or even thousands of years old, they are a valuable source of information about climate change. When scientists can look hundreds of years into the natural record found in a tree's rings, they can unravel the clues necessary to answer questions today about the causes and effects of global warming. (NOAA Paleo-climatology Paleo Slide Set; Peter Brown, Rocky Mountain Tree-Ring Research, Fort Collins, Colorado, photographer)

Once they collect these data for an entire climatic region, they analyze them and take the average in order to obtain the best estimate of climatic conditions during a certain period. They then use computer programs to analyze the collected data.

The best trees to use for studying climate change are bristlecone pines, which can be thousands of years old, because as they age they are not as easily affected by environmental variables as are young trees. They are sturdier and can be successfully used to document the effects of global warming.

Statistical analysis allows climatologists to see overall changes in climate trends and address various environmental problems, such as global warming. Examining the sensitivity of trees to changes in climate allows not only past reconstruction, but also present-day monitoring, especially in high-latitude areas such as Alaska, Siberia, and Scandinavia, where global warming is already causing serious problems.

DENDROCHRONOLOGY

Andrew Ellicott Douglass was an American astronomer whose major accomplishment did not involve space: He discovered dendrochronology. When he was just 27 years old, he found what he believed to be a connection between climate and plant growth. He first began recording the annual rings of pine and Douglas fir trees around his home. Then, in 1911, he discovered matching records among trees 50 miles (80 km) away. He invented the term dendrochronology, which means "tree-ring dating."

His discoveries made it possible for many other scientists to advance in their own fields—archaeologists could use it to date past civilizations, climatologists could use it to pinpoint significant climate events, biologists could date insect plagues, geographers could study past droughts, and geologists could interpret past volcanic events. His techniques allowed cross-matching and linked together "living tree" chronology with "archaeological tree" chronology.

As his work grew in volume and in popularity, a tree ring laboratory was built at the University of Arizona. Today, this laboratory contains the world's largest accumulation of tree ring specimens from both living trees and ancient timbers. Built under the football stadium at the University of Arizona, the Laboratory of Tree Ring Research (LTRR) preserves almost 1 million wood specimens used to study important scientific issues, such as climate change and global warming.

Through Douglass's efforts, scientists have been able to map climate history in the United States hundreds of years before European settlement. A true pioneer, Douglass also believed that dendrochronology was a valuable tool for measuring future climate.

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