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 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.
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
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.