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Dendrochronology (also called tree-ring dating) is a technique of dating past climatic changes through a study of tree ring growth. Each year a tree adds a layer of wood (xylem) to its trunk and branches thus creating the annual rings we see when viewing a cross section. Wide rings are produced during wet years and, narrow rings during dry seasons. These changes in tissue formation can be used to calculate age of the tree itself or surrounding archaeological artifacts.
Clark Wissler of the American Museum of Natural History can be credited as one of the first individual scientists to realize the importance of such naturally occuring rings in trees can be used to tell how old it is. Wissler worked with A. E. Douglass now considered the father of dendrochronology at the University of Arizona to further studies and solidification of such dating as a worthwhile scientific endeavor.  
Xylem is a plant tissue that can be found within trees, being responsible for transporting water and mineral nutritients, from the root of vascular plants, distributing it to the necessary parts.  Pith is found at the center of the tree stem, then rings can bee seen followed by a layer that insulates between what is called vascular cambium (the source of xylem and phloem) and the outer appearance of the tree, or its bark.
Each year a new layer of xylem is produced, thus giving us the tree rings we all can observe and count. These tree-rings are wood cells formed within a year or growing season, thin walled cells formed in the early growing season (earlywood) and thicker walled cells produced later in the growing season (latewood).
Consistent yearly tree-rings are produced within a temperate environment.
Apparent old ages
By using dendrochronology scientists have dated certain living trees or their root systems in certain cases as having ages in excess of what is expected given the Biblical chronology of the global flood.
National Geographic reported in April of 2008  that scientists led by Leif Kullman, professor at Umea University's department of ecology and environmental science in Sweden found the worlds oldest growing tree root system.
|“||Trees much older than 9,550 years would be impossible in Sweden, because ice sheets covered the country until the end of the last Ice Age around 11,000 years ago, Kullman noted.||”|
|“||"Prior to our studies the general conception was that spruce migrated to this area about 2,000 years ago, so now you will have to rewrite the textbooks," Kullman said.||”|
These types of findings are used by evolution proponents to aid their debate in silencing anything related to Biblical chronology. The relevant dates include the global flood which was around 2300 BC, and the seven day creation week with the inclusion of trees that happened around 4004 BC. The global flood was so complete that no living thing was left, and logically what would follow is the lack of possibility for there to be a tree older than about 2000 years.
The science being practiced that is causing these apparent old ages for trees that couldn't possibly of existed so long, Kullman's findings present a case that is disputed by Gerald Aardsma who found that trees can produce multiple rings in a wet year. As well as one of the founders of the modern creation science movement, Walter Lammerts, who documented in the lab that trees can also display extra rings in short drought periods.
Principles of Dendrochronology
There are seven dendrochronological principles that contain certain assumptions, that are used as a framework or theory to interpret tree-ring data. The most prominent of such assumptions, uniformitarianism is also an assumption that is quite prevalent within evolutionary science.
The uniformitarian principle is the assumption that present day physical and chemical processes responsible for the environmental change that producet tree-ring formations is the same today as it has been in the past.
The limiting factor principle is one which states that rings can only form as fast as the limiting factor. For example, if rainfall is the limiting factor than tree rings, or essentially tree growth is reliant upon the factor of rainfall.
Aggregate Tree Growth
The aggregate tree growth principle states that many conditions, both natural and human induced are responsible for any one tree-ring series growth pattern. Many factors of a tree growth patterns have to be taken into account, however in some cases one factor may be left out as potential data due to isolation of a different factor that requires study.
The four factors taken into account are:
- Tree growth related to normal physiological aging processes within the tree itself.
- Climate or weather induced factors.
- Outside of the forest factors such as disease, or insect problems.
- Random natural or human caused processes that effect tree growth.
The principle of ecological amplitude recognizes the ability for different species of trees to grow in many locations or specialized habitats. This is important to consider because useful trees for dendrochronology are often found near zones of their natural range.
The site selection principle recognizes that certain species of trees propagate and survive better in certain conditions than others do. A tree-ring series found in certain species of trees that have high survivability near desert like conditions would mean that a dendrocrhonologists could deliberately sample those trees to glean past drought conditions.
Essentially what is the most practiced principle is comparing dates of tree-ring series from one tree to many other trees from a different location. Once tree rings from one general area are compared with corresponding patterns observed in another area it is compiled as a set of data and crossdating has been achieved. Through many over-lapping ring chronologies, what is called a master dendrochronology can be defined and used to interpret past conditions more precisely. 
The principle of replication states the need of more than one stem radius per tree and more than one tree per site. Essentially samples will range up to many trees per site or even many sites of trees. This is an attempt to identify and minimize noise or environmental factors of little interest.  
Woody vascular plant stem cross-section
- ↑ Dating Techniques; Dendochronology Minnesota State Universiy
- ↑ "dendrochronology." Encyclopædia Britannica. 2008. Encyclopædia Britannica Online. 8 Jan. 2008 
- ↑ Xylem by Wikpedia
- ↑ Oldest Living Tree Found in Sweden by James Owen for National Geographic News, April 14, 2008
- ↑ Tree Rings and Biblical Chronology ICR IMPACT No. 252 June 1994 by Frank Lorey, M.A.
- ↑ Crossdating - The Basic Principle of Dendrochronology by Lori Martinez, Laboratory of Tree-Ring Research, and The University of Arizona.
- ↑ Principles of Dendrochronology Henri D. Grissino-Mayer's Ultimate Tree-Ring Web Pages
- ↑ Principles of Dendocrhonology by Henri Grissino-Mayer of the Laboratory of Tree-Ring Research at the University of Arizona, Tucson
- Tree ring dating (dendrochronology) by Don Batten, Ph.D.
- Are tree-ring chronologies reliable? by Mark Aardsma. May 2001
- Compatibility of Biblical Chronology with C-14 Age by R. H. Brown. Origins 21(2):66-79 (1994)
- Can Tree Rings Be Used To Calibrate Radiocarbon Dates? by R. H. Brown, Origins 22(1):47-52 (1995).
- More Creationist Research, PART II: BIOLOGICAL RESEARCH, Variable Production of Growth Rings in Bristlecone Pines by Duane T. Gish, Ph.D., Creation Research Society Quarterly 26(1):5 June, 1989
- Tree-Rings Dating and Multiple Growth Ring Per Year Aardsma, Dr. Gerald E., Creation Research Society Quarterly, volume 29, March 1993, pp. 184-189.
- Dendochronology by Rachel Neff, Erin Woike, Laura Stevens, Eric Claus, Eric Weaver.
- Time and Cycles Lesson: "Logs of Straw: Dendrochronology" by U.S. Department of the Interior, U.S. Geological Survey.