Dendrochronology is based on one of many physical, chemical, and biological processes

which produce layered event evidence preservation. In trees, the generative cambium

tissue lies near the surface where it replaces the nutrient carrying phloem between it

and the bark, provides new growth initiatives, and replaces itself while leaving a layer

of strengthening xylem on the inside. The xylem, which is not replaced but is continually

added to as conditions permit, provides the layered event evidence.

   Colorful annual rings are produced in the xylem of trees grown in latitudes where

extremes in temperature occur. Spring and summer growths can often be differentiated.

The age of the tree can be determined by counting the rings, and if it is still alive

or the year of death is known, each ring can be assigned its year.

   Variations in ring size occur according to weather and other effects. These variations

are similar among like species in similar conditions. Though never exactly the same,

the variations produce a strong enough signal to assign years to rings of trees that

grew partly inside and partly outside a known sequence. In this way, the basic signal

is lengthened further and further back in time, till today, dendrochronologists can

date rings grown in certain European oak and pine to nearly 12000 years ago. Floating

sequences, though not exactly dated, exist from much earlier, and can be placed

within certain dated parameters going back as far as the Cretaceous Period. Thousands

of local dendrochronological databases worldwide of many different species provide

sequence comparisons which cover the last hundreds, sometimes thousands, of years.

   The ability to determine dated ring sequences in wood has direct applications to

climatology, ecology, cosmology, archaeology, paleontology, geomorphology,

geochronology, and forensics.

   Though some are hardier than others, all plant species are genetically restricted to

grow within certain climatic boundaries. Besides required nutrients and symbiotic

associations, available sunlight, moisture, and temperature constraints determine

survival and affect growth rates. Cyclic patterns occur over tens, hundreds, and

thousands of years, in these climatic conditions, and they produce comparative

patterns in tree rings. Reconstructions of past cycles predict future cycles.

In many areas, certain factors are dominant and limit the patterns to reconstructions

of rainfall amounts and drought or to temperature variations, heat waves, and cold

spells. Hard winters are often dendrochronological markers. Climatic conditions

affect the ecosystem as a whole, including human populations, and produce isotopic

changes within the rings.

   Because ecosystems change with climate, reconstructions of prior ecosystems predict

interspecies relationships and ecosystem health in the future. Pollution of air,

soil, or water, are reflected in tree ring chemical and isotopic changes, as are

volcanic and meteoric impact events.

   Forest histories are developed from events recorded in tree rings. These include

forest movement, population changes, management, exploitation, and land use

histories. Fire scars, clearing events and other changes in the rings show fire

frequency, intensity, spread, and watershed responses. Pest and mold outbreaks,

infestations, population dynamics, and defoliations leave evidence in the rings.

   Ecosystem water supply and watershed futures are drawn from ring enhanced drought,

snowpack, flood, streamflow, lake and groundwater level reconstructions.

   Solar sunspot activity and its effect on global temperatures is seen in ring

studies, as are cosmogenic atmospheric radiocarbon level variations. Nuclear

power plant and bomb created radiocarbon is commonly measured in tree rings.

Anthropogenic and natural hemispheric variations in atmospheric radiocarbon are

also indicated.

   Archaeological artifacts such as houses, cliff dwellings, roads, bridges, boats,

coffins, fences, furniture, and panel paintings have been directly dated using

tree rings, and all artifacts found with them can be dated by association.

Carpentry tool usage and other technological changes, architectural style, and

timber transport, processing, and construction or fuel usage are also indicated.

This and other evidence lead to broader sociological patterns involving migration,

population, land use, cultural interchange, and economic issues.

   Because each ring retains the radiocarbon put into it during growth, minus that

reverted to nitrogen, tree ring analysis is ideal for the calibration of

radiocarbon dating methods. Cross correlation occurs with all Holocene dating

techniques, particularly tephrochronology, pollen analysis, ice core analysis,

and varve analysis.

   Geomorphological changes such as volcanic eruptions, earthquakes, avalanches,

glacial movement, meteoric impacts, and landslides have been studied using

dendrochronology. And occasionally, forensic applications surface, such as proving

a valuable archaeological or historic artifact a fraud.

   The number and dispersal of samples and types of trees sampled, is determined by

the study parameters. Cored or sliced samples need to be perpendicular to the rings

or the angle distorts the ring sizes. Samples are dried, fit into holders, sanded to

a high finish, and sometimes stained to better express the ring features.

   Sample rings are optically measured, often with computer aided vision and motion

equipment, and then archived. Intratree samples are compared to eliminate tree

specific variations. Measures are often averaged and changed to percentages above or

below the norm in order to facilitate comparisons. Skeleton plots of large variations

or indexed moving averages of multiple adjacent rings provide the signal used to

compare with the available chronological databases. When an unknown signal is matched

to a known, a reliability value is determined by established methods measuring

closeness of match.