Dendrochronology, or tree-ring dating, is the scientific method of dating based
on the analysis of patterns of tree rings, also known as growth rings.
Dendrochronology can date the time at which tree rings were formed, in many
types of wood, to the exact calendar year. This has three main areas of
application: paleoecology, where it is used to determine certain aspects of
past ecologies; archaeology and the history of art and architecture, where
it is used to date old panel paintings on wood, buildings, etc.; and
radiocarbon dating, where it is used to calibrate radiocarbon ages. In some
areas of the world, it is possible to date wood back a few thousand years, or
even many thousands. As of 2013, fully anchored chronologies in the northern
hemisphere extend back 13,900 years. History
The Greek botanist Theophrastus first mentioned that the wood of trees has
rings. In his Trattato della Pittura, Leonardo da Vinci was the first person
to mention that trees form rings annually and that their thickness is
determined by the conditions under which they grew. In 1737, French investigators
Henri-Louis Duhamel du Monceau and Georges-Louis Leclerc de Buffon examined
the effect of growing conditions on the shape of tree rings. They found that in
1709, a severe winter produced a distinctly dark tree ring, which served
as a reference for subsequent European naturalists. In the U.S., Alexander
Catlin Twining suggested in 1833 that patterns among tree rings could be used
to synchronize the dendrochronologies of various trees and thereby to reconstruct
past climates across entire regions. The English polymath Charles Babbage
proposed using dendrochronology to date the remains of trees in peat bogs or
even in geological strata. During the latter half of the nineteenth
century, the scientific study of tree rings and the application of
dendrochronology began. In 1859, the German-American Jacob Kuechler used
crossdating to examine oaks in order to study the record of climate in western
Texas. In 1866, the German botanist, entomologist, and forester Julius
Ratzeburg observed the effects on tree rings of defoliation caused by insect
infestations. By 1882, this observation was already appearing in forestry
textbooks. In the 1870s, the Dutch astronomer Jacobus C. Kapteyn was using
crossdating to reconstruct the climates of Holland and Germany. In 1881, the
Swiss-Austrian forester Arthur von Seckendorff-Gudent was using
crossdating. From 1869 to 1901, Robert Hartig, a German professor of forest
pathology, wrote a series of papers on the anatomy and ecology of tree rings.
In 1892, the Russian physicist Fedor Nikiforovich Shvedov wrote that he had
used patterns found in tree rings to predict droughts in 1882 and 1891.
During the first half of the 20th century, the astronomer A. E. Douglass
founded the Laboratory of Tree-Ring Research at the University of Arizona.
Douglass sought to better understand cycles of sunspot activity and reasoned
that changes in solar activity would affect climate patterns on earth, which
would subsequently be recorded by tree-ring growth patterns.
Growth rings Growth rings, also referred to as tree
rings or annual rings, can be seen in a horizontal cross section cut through the
trunk of a tree. Growth rings are the result of new growth in the vascular
cambium, a layer of cells near the bark that is classified as a lateral
meristem; this growth in diameter is known as secondary growth. Visible rings
result from the change in growth speed through the seasons of the year; thus,
critical for the title method, one ring generally marks the passage of one year
in the life of the tree. The rings are more visible in temperate
zones, where the seasons differ more markedly. The inner portion of a growth
ring is formed early in the growing season, when growth is comparatively
rapid and is known as "early wood"; the outer portion is the "late wood" and is
denser. Many trees in temperate zones make one
growth ring each year, with the newest adjacent to the bark. Hence, for these,
for the entire period of a tree's life, a year-by-year record or ring pattern is
formed that reflects the age of the tree and the climatic conditions in which the
tree grew. Adequate moisture and a long growing season result in a wide ring,
while a drought year may result in a very narrow one.
Direct reading of tree ring chronologies is a learned science, for several
reasons. First, contrary to the single ring per year paradigm, alternating poor
and favorable conditions, such as mid-summer droughts, can result in
several rings forming in a given year. In addition, particular tree species may
present "missing rings", and this influences the selection of trees for
study of long time spans. For instance, missing rings are rare in oak and elm
trees. Critical to the science, trees from the
same region tend to develop the same patterns of ring widths for a given
period of historical study. These patterns can be compared and matched
ring for ring with trees growing at the same time, in the same geographical
zone. When these tree-ring patterns are carried back, from tree to tree in the
same locale, in overlapping fashion, chronologies can be built up—both for
entire geographical regions and sub-regions. Moreover, wood from ancient
structures with known chronologies can be matched to the tree ring data, and
the age of the wood can thereby be determined precisely. Cross-dating was
originally done by visual inspection; computers have been harnessed to do the
task, applying statistical techniques to assess the matching.
To eliminate individual variations in tree-ring growth, dendrochronologists
take the smoothed average of the tree-ring widths of multiple tree
samples to build up a ring history, a process termed replication. A tree-ring
history whose beginning and end dates are not known is called a floating
chronology. It can be anchored by cross-matching a section against another
chronology whose dates are known. Fully anchored chronologies extending back
more than 11,000 years exist for river oak trees from South Germany and for
pine from Northern Ireland. The consistency of these two independent
dendrochronological sequences has been supported through comparison of their
radiocarbon and dendrochronological ages. Another fully anchored chronology
that extends back 8500 years exists for the bristlecone pine in the Southwest
US. Sampling and dating
Dendrochronology makes available specimens of once-living material
accurately dated to a specific year. Dates are often represented as estimated
calendar years B.P., for before present, where "present" refers to 1 January
1950. Timber core samples are sampled and used
to measure the width of annual growth rings; by taking samples from different
sites within a particular region, researchers can build a comprehensive
historical sequence. The techniques of dendrochronology are more consistent in
areas where trees grew in marginal conditions such as aridity or
semi-aridity where the ring growth is more sensitive to the environment,
rather than in humid areas where tree-ring growth is more uniform. In
addition, some genera of trees are more suitable than others for this type of
analysis. For instance, the bristlecone pine is exceptionally long-lived and
slow growing, and has been used extensively for chronologies;
still-living and dead specimens of this species provide tree-ring patterns going
back thousands of years, in some regions more than 10,000 years. Currently, the
maximum span for fully anchored chronology is a little over 11,000 years
B.P. In 2004 a new calibration curve,
INTCAL04, was internationally ratified to provide calibrated dates back to
26,000 B.P.. The part of the new calibration curves that relies on
tree-ring evidence spans 12,410 calendar B.P., and a further 14,700 calendar
years prior to that. Dendrochronology practice faces many
obstacles, including the existence of species of ants that inhabit trees and
extend their galleries into the wood, thus destroying ring structure.
Reference sequences European chronologies derived from
wooden structures initially found it difficult to bridge the gap in the 14th
century when there was a building hiatus, which coincided with the Black
Death, however there do exist unbroken chronologies dating back to prehistoric
times, for example the Danish chronology dating back to 352 BC.
Given a sample of wood, the variation of the tree-ring growths provides not only
a match by year, it can also match location because the climate across a
continent is not consistent. This makes it possible to determine the source of
ships as well as smaller artifacts made from wood but which were transported
long distances, such as panels for paintings and ship timbers.
Applications = Radiocarbon dating calibration =
Dates from dendrochronology can be used as a calibration and check of
radiocarbon dating = Climatology =
In areas where the climate is reasonably predictable, trees develop annual rings
of different properties depending on weather, rain, temperature, soil pH,
plant nutrition, CO2 concentration, etc. in different years. These variations are
used in dendroclimatology to infer past climate variations.
= Art history = Dendrochronology has become important to
art historians in the dating of panel paintings. However, unlike analysis of
samples from buildings, which are typically sent to a laboratory, wooden
supports for paintings usually have to be measured in a museum conservation
department, which places limitations on the techniques that can be used.
In addition to dating, dendrochronology can also provide information as to the
source of the panel. Many Early Netherlandish paintings have turned out
to be painted on panels of "Baltic oak" shipped from the Vistula region via
ports of the Hanseatic League. Oak panels were used in a number of northern
countries such as England, France and Germany. Wooden supports other than oak
were rarely used by Netherlandish painters.
Since panels of seasoned wood were used, an uncertain number of years has to be
allowed for seasoning when estimating dates. Panels were trimmed of the outer
rings, and often each panel only uses a small part of the radius of the trunk.
Consequently, dating studies usually result in a "terminus post quem" date,
and a tentative date for the actual arrival of a seasoned raw panel using
assumptions as to these factors. As a result of establishing numerous
sequences, it was possible to date 85–90% of the 250 paintings from the
14th to 17th century analysed between 1971 and 1982; by now a much greater
number have been analysed. A portrait of Mary, Queen of Scots in
the National Portrait Gallery, London was believed to be an 18th-century copy.
However, dendrochronology revealed that the wood dated from the second half of
the 16th century. It is now regarded as an original 16th century painting by an
unknown artist. On the other hand, dendrochronology was
applied to four paintings depicting the same subject, that of Christ expelling
the money-lenders from the Temple. The results showed that the age of the wood
was too late for any of them to have been painted by Hieronymus Bosch.
While dendrochronology has become an important tool for dating oak panels, it
is not effective in dating the poplar panels often used by Italian painters
because of the erratic growth rings in poplar.
The 16th century saw a gradual replacement of wooden panels by canvas
as the support for paintings, which means the technique is less often
applicable to later paintings. In addition, many panel paintings were
transferred onto canvas or other supports during the 19th and 20th
centuries. = Building history =
The dating of buildings with wooden structures and components has also been
done by using dendrochronology. While archaeologists can date wood and when it
was felled, it may be difficult to definitively determine the age of a
building or structure in which the wood was used; the wood could have been
reused from an older structure, may have been felled and left for many years
before use, or could have been used to replace a damaged piece of wood. The
dating of building via dendrochronology thus requires knowledge of the history
of building technology. Examples:
cliff dwellings of Native Americans in the arid U.S. Southwest.
The Fairbanks House in Dedham, Massachusetts. While the house had long
been claimed to have been built circa 1640, core samples of wood taken from a
summer beam confirmed the wood was from an oak tree felled in 1637–8. An
additional sample from another beam yielded a date of 1641, thus confirming
the house had been constructed starting in 1638 and finished sometime after 1641
as wood was not seasoned before use in building at that time in New England.
The burial chamber of Gorm the Old, who died c. 958, was constructed from wood
of timbers felled in 958. Related chronologies
Similar seasonal patterns also occur in ice cores and in varves. The deposition
pattern in the core will vary for a frozen-over lake versus an ice-free
lake, and with the fineness of the sediment.
Some columnar cactus also exhibit similar seasonal patterns in the
isotopes of carbon and oxygen in their spines. These are used for dating in a
manner similar to dendrochronology, and such techniques are used in combination
with dendrochronology, to plug gaps and to extend the range of the seasonal data
available to archaeologists and paleoclimatologists.
A related technique is used to analyse fish stocks through the analysis of
growth rings in the otolith bones of fish.
Further reading "Tree Ring Science", the academic site
of Prof. Henri D. Grissino-Mayer, Department of Geography, The University
of Tennessee, and the Laboratory of Tree-Ring Science, see [1]
See also Acanthochronology
Baumkuchen, cake that resembles growth rings
Dendroarchaeology Dendroclimatology
Dendrology Herbchronology
Paleoclimatology Post excavation
Sclerochronology Timeline of dendrochronology timestamp
events Varve
References External links
Nottingham Tree-Ring Dating Laboratory Video & commentary on Medullary Rays,
heart wood and tree rings. Video & commentary on Tree Rings -
Formation and Purpose Oxford Tree-Ring Laboratory
Bibliography of Dendrochronology Multilingual Glossary of
Dendrochronology Digital Collaboratory for Cultural
Dendrochronology International Tree-Ring Data Bank
Ultimate Tree-Ring Web Pages Laboratory of Tree-Ring Research
University of Arizona