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young; Archbishop Ussher’s calculations of a 6000-

year-old Earth were endorsed in the margins of com-

monly used Bibles. However, believing that the Book

of Nature and the Book of God must be telling the same

story, Christian geologists worked to find ways to in-

terpret the geological data in a framework that allowed

them to retain at least some of the Genesis story of

Creation, whilst accepting the empirical evidence of

their new science.

One accommodation was that of the English clergy-

man, Thomas Chalmers, who attempted to harmonize

geology with the Bible through the ‘gap theory’.Inhis

1815 book, Evidence and Authority of the Christian

Revelation, Chalmers argued that there was a temporal

gap between Chapters 1 and 2 of Genesis. This pre-

served a literal 6-day Creation, but placed it after a long

pre-Adamite creation. The evidence of the rocks that

the Earth was ancient was thus acknowledged without

the wholesale abandonment of Genesis. The gap or

‘ruin and restoration’ compromise also had the ad-

vantage of allowing for a relatively recent Creation,

which pleased religious conservatives. The gap theory

remained popular through the nineteenth century and

well into the twentieth.

Another compromise between the Bible and science

was offered by Scottish stonemason Hugh Miller. In

his popular 1847 book, Footprints of the Creator,he

proposed the ‘day-age theory’, in which the 6 days of

Genesis were not 24-h days, but long periods of time.

This compromise was even more scientifically flexible

than Chalmers’ gap theory: it allowed for the accept-

ance of virtually all the geological data by requiring

that Genesis be taken more figuratively.

Other religious views were more concerned with

Darwinian evolution than geology. ‘Progressive cre-

ationism’ accepted the sequence of fossils in the geo-

logical column: God was believed to have created

increasingly more advanced forms through time.

The doctrine of ‘theistic evolution’, in which God

was thought to use evolution and natural selection

to bring about the current variety of living things,

similarly had little impact on geology. During the

early- and mid-nineteenth century, Christian geolo-

gists worked to harmonize the ‘two books’ and,

working with professional clergy, eased the worries

of Christians that modifications in Ussher’s view of

a 6000-year-old Earth would create irreparable rents

in the fabric of Christianity.

Not all Christians agreed, however. Some felt that

these compromises were unacceptable because they

required the Bible to be ‘interpreted’ rather than being

read at face value. What came to be called ‘Scriptural

Geology’ took the position that, when the ‘two

books’ were in apparent conflict, the book of God’s

word was to be given priority. The proponents of

Scriptural Geology tended to be neither trained geo-

logists nor university-educated clergy; rather, they

were self-taught, educated laymen. Most of them

promoted a young Earth and a historical Flood. At

best Scriptural Geology was a rearguard movement

that had little effect on the views of professional

science. As will be discussed later, however, some of

the same challenges to evolution promoted by the

Scriptural Geologists reappeared in the twentieth

century in the guise of ‘creation science’.

By the end of the nineteenth century, practising

scientists and professional clergy in both the USA

and on the continent accepted an ancient age of the

Earth and rejected the Flood of Noah as a universal

historical event that shaped the planet’s landforms.

Virtually all scientists likewise accepted biological

evolution, although not necessarily Darwin’s mech-

anism of natural selection. In the USA, however, evo-

lution was about to be attacked with the emergence

of a religious position called ‘Fundamentalism’ and,

even more importantly, by the efforts of a dedicated

amateur geologist and, later, a hydraulic engineer.

Young Earth Creationism and

Flood Geology

The Fundamentalist movement in American Protest-

antism began with a series of small booklets, collect-

ively called The Fundamentals, published between

1910 and 1915. Fundamentalism was partly a reaction

to a theological movement called Modernism that

began in Germany in the 1880s. Modernism included

viewing the Bible in its cultural, historical, and even

literary contexts; biblical Creation and Flood stories,

for example, were shown by comparison of ancient

texts to have been influenced by similar stories from

earlier non-Hebrew religions. With such interpretations,

the Bible could be viewed as a product of human

agency – with all that that suggests for the possibilities

of error, misunderstanding, and contradiction – as well

as a product of divine inspiration. Fundamentalists

in response stressed divine inspiration and absolute

accuracy of scripture, including Biblical miracles such

as Noah’sFlood.

Millions of copies of The Fundamentals booklets

were printed and distributed. Most of the authors of

The Fundamentals were day-age creationists, allow-

ing for an old Earth, but insisting on a recent

appearance of humans. Although not all The Funda-

mentals booklets were anti-evolutionary, the Fun-

damentalist position hardened against evolution

fairly quickly. Fundamentalists were motivated by

religious sentiments – if evolution were true, then

what of the accuracy of the Bible? – and also a con-

cern that evolution was the source of many corrosive

CREATIONISM 383

social practices, such as child labour, ‘laissez-faire’

capitalism, and exploitation of workers and im-

migrants. The erroneous association of evolution

with World War I German militarism further tainted

evolution in the eyes of many.

Fundamentalism remained an almost exclusively

American religious movement, not attracting much

interest in Great Britain or Europe. However, the

Bible-based religious view proved to be a fertile ground

for anti-evolutionism, and science was pressed into ser-

vice by a number of Fundamentalist evangelists to sup-

port the cause. Harry Rimmer, Arthur I. Brown, and

others hammered evolution before large public audi-

ences, and anti-evolution tracts and books sold well.

The target audience of these evangelists was the general

public, not the scientific community; the latter remained

staunchly evolutionist, and professional scientists were

scarce indeed among the anti-evolutionists associated

with the growing Fundamentalist movement.

Into this environment came a Seventh-Day Adventist

andself-trainedgeologistnamedGeorgeMcCready

Price, who was to develop the first version of

the most influential creationist perspective of the

twentieth century. Seventh-Day Adventist prophetess,

Ellen G. White, had claimed visions of a 6-day Cre-

ation and a universal Flood; Price sought to scientific-

ally validate her views. He recognized that geology

was the key to disproving evolution because, if it could

be demonstrated that the Earth was young, then bio-

logical evolution would be impossible. He therefore

concentrated on attacking conventional interpretations

of the geological record. For decades, he vigorously

argued that fossils could not be used to determine the

order of geological strata, because there were instances

where fossils were ‘in the wrong order’.Heaccused

geologists of trying to ‘shore up their theory’ of evo-

lution by proposing ‘highly unlikely’ theories, such

as overthrusting. A favourite example was the Lewis

Overthrust. Extending from central Alberta to the

eastern edge of Glacier National Park in Montana,

this massive structure is composed of Precambrian

limestone overlying Cretaceous shales. Price argued

that a limestone slab of such immense size could

not have been positioned by natural forces. In general,

Price ascribed all geological features to the 6 days

of Creation, the period between Creation and the

Flood, or the Flood itself, and called his view Flood

Geology.

Price tried to distinguish his ‘new catastrophism’

from earlier catastrophic geological views, such as

those proposed by Cuvier. He worked tirelessly to

promote Flood Geology, and was given a boost when

William Jennings Bryan referred to him as a noted

scientist during the Scopes Trial. By the 1940s, Price’s

views had a wide influence on evangelical Christian

views of creation and evolution. His major rivals

within that community were the gap and day-age

theories, but some evangelicals also believed in

progressive creationism or theistic evolution. Price

rejected all of these as being insufficiently biblical,

and clung to the literalism of a young Earth and a

global Flood.

Seventh-Day Adventists were considered to be

theologically suspect by many conservative Chris-

tians. As a result, Price did not always receive the

credit his pioneering work deserved. By the 1950s,

Flood Geology had begun to slip out of favour with

evangelicals, especially with the 1954 publication of

Bernard Ramm’s The Christian View of Science

and Scripture, which promoted progressive creation-

ism rather than the 6-day Creation and a universal

Flood. The popularity of Ramm’s book generated

a backlash from more conservative evangelicals.

Theologian John C. Whitcomb and hydraulic engin-

eer Henry M. Morris presented the basics of Price’s

Flood Geology – without mentioning Price – in their

1961 book, The Genesis Flood, which proposed

a universal Flood, Flood Geology, biblical literalist

Genesis theology, and a young Earth as the only

acceptable scientific and religious position for a true

Christian.

In some respects, The Genesis Flood was even

more conservative in its presentation of a purportedly

scientific foundation for Genesis than were Price’s

publications. Whitcomb and Morris argued that the

entire universe was created between 5000 and 7000

years before the present, whereas Price argued that

only the solar system, the Earth, and living things

were created during the Adam and Eve creation.

Whitcomb and Morris revived the vapour canopy

theory, an earlier view that, on the second day of

creation, the Earth was shrouded in a canopy of water

vapour. This provided the source of the ‘waters above

the firmament’ that produced the forty days and nights

of rain at the beginning of the Flood, and also produced

a greenhouse effect enhancing the ‘garden’ of Eden. By

shielding the surface of the planet from ionizing radi-

ation, the vapour canopy also reduced cellular damage,

allowing Methuselah and his contemporaries to have

lifespans of hundreds of years. Consonant with cre-

ationists’ objections to radiometric dating, the vapour

canopy was also claimed to alter the ratios of

Cin

the atmosphere, making age estimations based on

C inaccurate. The Genesis Flood also presented the

familiar young Earth model with the Flood as the

source of sedimentary features around the planet, and

reprised Price’s idea of the fossil record being the result

of ‘hydrodynamic sorting’. Regular and streamlined

animals would be naturally sorted by the Flood waters

into lower strata; irregular (and more intelligent) fauna

384 CREATIONISM

would either temporarily escape to higher ground

or would float there to be interred at higher elevations.

The book seized the attention of the evangelical

community in a way that the Adventist Price’s books

did not. It sold tens of thousands of copies during

its first decade, and remains in print today. The

nineteenth century effort to support the literal 6-day

Genesis Creation and a young Earth through scien-

tific data and theory was finally institutionalized

when Morris and his colleagues organized the Cre-

ation Research Society in 1963 as an association

of conservative Christian scientists promoting young

Earth creationism. In 1972, Morris organized the

California-based Institute for Creation Research (ICR)

to promote ‘creation science’. Its staff and publications

expanded; within a decade Morris could claim an

ICR publication list of 55 books. The current book

list numbers in the hundreds, and these and other cre-

ationist books have been translated into Afrikaans,

Chinese, Czech, French, German, Hungarian, Italian,

Japanese, Polish, Portuguese, Romanian, Russian, and

Spanish. The ICR and other creation science min-

istries, such as Answers in Genesis, have been

very effective in shaping the misunderstanding – and

rejection – of evolution by millions of conservative

Christians. As a result, the word ‘creationism’ itself

is now usually understood to mean young Earth

creation science.

Creation Science and Geology

Proponents of creation science argue that there are

only two possible views: creationism and evolution;

thus arguments against evolution are arguments in

favour of creationism. Creation science literature

thus centres on alleged examples of ‘evidence against

evolution’, such as anomalies in scientific literature,

that are then used to argue that the evidence for

evolution is deficient. Echoing Price’s contention

that the geological column is not reliable, creation

science proponents deny that missing strata at para-

conformities ever existed: instead, they are evidence

for rapid deposition by the Flood. Proponents also

point to fossils that are out of order, such as an alleged

human footprint occurring on top of a trilobite,

or ‘Moab man’, a human skeleton found in the

Morrison Formation (Late Jurassic) in Utah. Despite

the report of the physical anthropologist who excav-

ated the Moab skeleton that it was an intrusive burial

and a

C date of 300 years, creationists continue

to promote the burial as evidence of a distorted and

misleading geological column.

The most famous ‘out of place’ fossils, however, are

alleged human footprints occurring with dinosaur foot-

prints, claimed to have been found in Pennsylvania,

New Mexico, and Texas, the last being the best

known. The Cretaceous limestones of the Paluxy

River near Glen Rose, Texas, present many well-

preserved tridactyl dinosaur footprints. Creationists

have claimed since the 1930s that human footprints

are found in the same strata, proving that dinosaurs

and humans existed at the same time. Analysis of

these footprints by palaeontologists and geologists has

shown that none of the claimed ‘mantracks’ is valid:

all can be explained as natural erosional features,

eroded dinosaur tracks, or carvings. Although the

ICR has retreated slightly from promoting the Paluxy

River ‘mantracks’, other creationists continue to do

so, as can be seen by even a cursory web search.

Like Price, modern creation science proponents

know that the age of the Earth is a critical issue for

evolution; without an old Earth, biological evolution

could not have occurred. A great deal of creationist

literature, therefore, focuses on efforts to demon-

strate that the Earth is young rather than ancient.

For example, when presented with evidence from ice

cores, varves, or even tree rings, creationists argue

that these annual indicators of age are flawed, because

more than one layer, varve, or ring can be formed in a

year, thus throwing off the counts.

Many of these young Earth arguments rely on ex-

trapolations of various rates of change. When a meas-

ured rate is extrapolated backwards (or forwards), a

result is produced that is incompatible with the

accepted age of the Earth. Engineer Walter T. Brown’s

book, In the Beginning, includes numbered ‘evidences’

against evolution, including this one on the decay of

the Earth’smagneticfield:

84. Over the past 140 years, direct measurements of

Earth’s magnetic field show its steady and rapid decline

in strength. This decay pattern is consistent with the

theoretical view that a decaying electrical current inside

Earth produces the magnetic field. If this is correct, then

just 20 000 years ago the electrical current would have

been so vast that Earth’s structure could not have sur-

vived the heat produced. This implies Earth could not be

older than 20 000 years.

Of course, the rate of decay of the Earth’s magnetic

field is not linear, but periodically reverses, and thus

the fear of a too-powerful field 20 000 years ago is

groundless. Other rate arguments similarly rely on

taking a known rate and extrapolating it in a linear

fashion, resulting in conclusions at variance with

modern geology:

80. The continents are eroding at a rate that would level

them in much less than 25 million years. However, evo-

lutionists believe fossils of animals and plants at high

elevations have somehow avoided this erosion for more

than 300 million years. Something is wrong.

CREATIONISM 385

83. Meteoritic dust is accumulating on Earth so fast that,

after four billion years, the equivalent of more than 16

feet of this dust should have accumulated. Because this

dust is high in nickel, Earth’s crust should have an abun-

dance of nickel. No such concentration has been found

on land or in the oceans. Therefore, Earth appears to be

young.

Creation science proponents argue that, after the

Flood, a strict uniformitarianism has held, but, during

the Flood, God used processes that are different from

those now in effect. This selective catastrophism

is necessary to avoid the implications of stand-

ard uniformitarian interpretations, which are over-

whelming evidence for a slowly changing planet and

thus an ancient Earth. The young Earth model de-

mands accelerated rates of change well beyond any

observed today. Coral reefs would have needed to

accumulate at 40 000 times their current observed

rate to form as quickly as a Flood Geology model

requires. The interior of the USA has sedimentary

deposits of great thickness, averaging a kilometre

in depth, some of which, such as calcareous muds,

are the results of animal activity; these too would

have needed to form at mind-boggling rates. In the

Grand Canyon, the Kaibab and Redwall limestones

together comprise about 300 m of sediment; to pro-

duce deposits of this thickness during the year of the

Flood would have required deposition of 80 cm per

day – clearly more than has ever been observed or

even imagined.

As part of the effort to attack the age of the Earth,

creation science proponents hold that radiometric

dating methods are unreliable. The Grand Canyon

Dating Project, a project of the ICR, recently attempted

to demonstrate this by showing alleged discrep-

ancies between dates of Grand Canyon lava flows col-

lected by ICR staff and by others. In the ICR study,

Pleistocene lava flows were dated earlier than the

Proterozoic Cardenas basalts, but other geologists

have pointed out that the technique used by the ICR

(Rb/Sr) may give false isochrons by reflecting the age

of the mantle source from which the lava flows

were derived, rather than the eruption time of the

lava itself.

A more recent ICR research project critiquing

radiometric dating is the Radioisotopes and the Age

of the Earth (RATE) project, begun in 1997. In 2000,

Vardiman et al. published a 675-page book describing

the project and its expectations for research. A final

volume of conclusions from the project is expected in

2005.

The fact that ICR scientists have undertaken re-

search distinguishes them from their nineteenth-

century Scriptural Geology predecessors. The amount

of research performed to date, however, is very small,

and its quality has been questioned by professional

geologists. Most of the literature consists of the select-

ive search of scientific data for anomalies cited to

disprove evolution, and the little actual research

that has been performed, such as efforts to

C date

dinosaur bones or the Grand Canyon Dating Project,

has not inspired confidence in the quality of creation-

ist research efforts. As a result, the professional

geological community has largely ignored creation

science, except as a social movement that has had

remarkable success in popularizing poor science to

the general public.

Further Reading

Cole JR, Godfrey LR, Hastings RJ, and Schafersman SD

(eds.) (1985) The Paluxy River Footprint Mystery –

Solved! Creation/Evolution, 5(1), pp 1–56.

Forrest B and Gross PR (2004) Creationism’s Trojan Horse,

The Wedge of Intelligent Design. Oxford: Oxford

University Press.

Larson EJ (1997) Summer for the Gods: The Scopes Trial

and America’s Continuing Debate Over Science and

Religion. New York: Basic Books.

Lindberg DL and Numbers RL (eds.) (1986) God and

Nature: Historical Essays on the Encounter Between

Christianity and Science. Berkeley, CA: University of

California Press.

Livingstone DN, Hart DG, and Noll MA (eds.) (1999)

Evangelicals and Science in Historical Perspective. New

York, NY: Oxford University Press.

Lynch JM (ed.) (2002) Creationism and Scriptural Geology,

1817–1857. London: Bristol Thoemmes Press.

Numbers R (1992) The Creationists. New York: Knopf.

Scott EC (2004) Creationism vs Evolution: An Introduc-

tion. Westport, CT: Greenwood Publishing Group.

Stassen C (2003) A Criticism of the ICR’s Grand

Canyon Dating Project. Retrieved December 29, 2003,

from http://www.talkorigins.org/faqs/icr-science.html.

Strahler A (1999) Science and Earth History, The Evolu-

tion/Creation Controversy (revised edn.). Buffalo, NY:

Prometheus Books.

Vardiman L, Snelling AA, and Chaffin EF (eds.) (2000)

Radioisotopes and the Age of the Earth: A Young-Earth

Creationist Research Initiative. El Cajon, CA, and St.

Joseph, MO: Institute for Creation Research and

Creation Research Society.

Wise DU (1998) Creationism’s geological time scale.

American Scientist 86: 160–173.

386 CREATIONISM

DELTAS

See SEDIMENTARY ENVIRONMENTS: Deltas

DENDROCHRONOLOGY

M Bridge, University College London, London, UK

Introduction

In recent decades tree-ring studies have opened up a

whole new area of climatic and other environmental

reconstructions at various geographical and temporal

scales. These generally go under the heading of den-

drochronology – although some people reserve this

term only for the use of tree rings in dating studies.

Dendrochronological dating relies on the fact that

whilst each tree-ring series reflects peculiarities of

the life history of the individual tree, trees of the

same species growing at the same time over wide

areas respond similarly to the weather experienced

both during an individual growth period, and perhaps

also to conditioning throughout previous seasons.

Not all species have clearly distinct ring boundar-

ies, and those without them are of little use to den-

drochronology. Some species not only do have clear

rings, but may have clear divisions within the rings,

such that the ring can be divided into the ‘earlywood’

which forms during the spring and early summer,

and the ‘latewood’ which forms through the summer

and early autumn. Whilst many studies make use of

the whole ring, in recent years many have focused on

part of the ring, usually the latewood, which is usually

more independent of the influences of previous growth

in its variations. Most dendrochronological work has

been done in temperate zones, although some tropical

trees have been used.

With historical timber, samples of wood from nat-

urally preserved timbers e.g. in river gravels, peat

bogs and similar environments, usually take the

form of cross-sections cut from trunk remains. His-

torical wood in situations such as standing buildings

is generally cored down the radius of the trunk.

Many of the studies outlined below, however, depend

on samples from living, or recently felled, trees.

Living trees can be sampled by extracting radial

cores, usually of the order of 5 mm in diameter,

taken using especially designed manually operated

borers. The tree ‘compartmentalizes’ the wound,

sealing it off from the rest of the living tissue, and

any damage to the tree is generally minimal.

During the late 1960s and the 1970s, the develop-

ment of radiocarbon dating and the realization that

the formation of radiocarbon in the atmosphere had

not been constant through time were the stimuli

for several laboratories to attempt to build long

(multimillennial) tree-ring chronologies. These pro-

vided wood samples of known calendar age with

which to investigate changes in radiocarbon levels

through time, and then to calibrate the radiocarbon

time-scale. This has made possible ‘wiggle matching’,

which in suitable circumstances can provide more

accurate radiocarbon dates.

As the chronologies were being constructed it was

noticed that some periods had scarce wood remains

which was thought to reflect environmental changes,

either in the vigour of tree growth, or in the condi-

tions that favour the preservation of the wood. Once

several long chronologies became available, it was

noticed that certain periods showed common growth

responses over wide geographical areas. For example,

a downturn in oak growth lasting about a decade

after 1628 bc was reported in Irish bog-oaks. This

was subsequently found to occur at the same time

in oaks growing in northern England and northern

Germany.

This decade showed the lowest growth rates for

many centuries in each locality, and must therefore

reflect a large climatic forcing agent. It was suggest-

ed that the most likely candidate for this particular

period of growth reduction was the Santorini (Thera)

volcanic eruption, often linked to changes in the

Minoan civilization. Accepting this as dating that par-

ticular event was controversial, but even if some were

unable to accept this hypothesis, some major factor

had to be responsible for the observed response.

Throughout the rapid growth of dendrochronology

in the late twentieth century, many studies focused

DENDROCHRONOLOGY 387

on dating discrete events such as volcanic eruptions,

earthquakes, landslides, avalanches, forest fires,

floods, severe frosts etc. through history, giving im-

portant background information about the frequency

and intensity of these phenomena, and in many in-

stances giving the first calendar dates for them. These

events may be represented in the tree-ring series by

sudden growth rate changes, scars, or abnormal cells.

Longer-term variations and rates of change for

such phenomena as river-flow changes, lake-level

changes, saltwater ingression, changes in ocean cur-

rents through time (mapped by dating driftwood),

glacial advance and retreat (reflected in subfossil

trunks from valley sides), erosion rates, human forest

clearance and the like have all been subjects for

dendrochronological study. Periods of building of

prehistoric lakeside settlements have been found to

be coincident over areas of central Europe, perhaps

reflecting migration patterns in response to climatic

changes.

Dendroclimatology

It was not long before people realized that well

replicated chronologies were themselves a proxy

dataset of the major climatic influences on

growth, and people looked for the best methods of

extracting this information at annual, decadal, cen-

tury and millennial time-scales. In some areas the

relationships between climatic factors and growth

are relatively simple, and the dominant limiting

factor to growth exhibits a clear correlation with the

width of the annual ring. In semi-arid areas for

example, the ring width is generally a reflection of

rainfall levels.

Some basic environmental information can be

gained simply by looking at long-term trends in the

ring width itself. For example, different centuries

show quite different growth rates for similarly bio-

logically aged oaks preserved in river gravels in

Germany, used to construct a multimillennial chron-

ology. Even without any complex calibration of the

climate–growth relationship, it is possible to draw

broad inferences about changing conditions at par-

ticular times, with narrow mean ring widths repre-

senting poorer growth conditions and wider mean

ring widths more favourable growth conditions.

The sensitivity of trees to external changes in their

environment changes with the site conditions. In

more extreme environments it is possible that rings

may be missing, or partially missing around the cir-

cumference, or if unfavourable conditions are experi-

enced during the normal growth period, a slowing

down and then resurgence of growth may produce

apparent ‘false’ rings. These problems are generally

detected when ‘cross-matching’ the samples during

chronology construction. Unlike many scientific stud-

ies, sampling for dendroclimatological reconstruction

relies on careful selection of the trees, not random

selection. The reasons for this become clear when one

considers the nature of the ring width itself. The ring

width in any given growth season is the result of a

number of factors: the biological age of the tree, envir-

onmental factors unique to that tree, stand-wide influ-

ences atypical of growth elsewhere, a large number

of other factors, and of course the regional climatic

signal that is of interest in these studies.

One can minimize the influence of non-climatic

factors and maximize the climatic signal by careful

site selection. Trees growing at the margins of the

population, whether that be altitudinally or latitudin-

ally are generally most sensitive to climatic factors.

By choosing the dominant trees with no obvious signs

of damage or disease, one enhances the relationship

between ring width and regional climatic influence.

By increasing the number of samples one reduces the

individual tree responses and enhances the ‘climatic

signal’. The ‘signal to noise ratio’ has been studied in

detail and will vary from location to location and

species to species, but as a general guide a minimum

of 15 to 20 trees seems to produce a representative

sample.

The biological age of the tree generally has an effect

on ring width, with a natural tendency to put on

narrower rings as the tree increases in girth, even

though overall productivity levels may be very simi-

lar. This so called ‘age trend’ can be readily removed

by fitting curves to the overall series and then taking

account of the difference of the observed ring width

from the theoretical mean value at a given time. This

process, known as ‘standardization’, results in the pro-

duction of a series of indices of growth, and makes

direct comparison between trees of different biological

age more readily achievable.

Just how best to derive this series of indices for

subsequent analysis has been the subject of a vast

literature within dendroclimatology, and each new

study really needs to justify the particular methods

employed to remove age-related trends. The problem

of course is that the more closely one fits curves to the

original data, the more climatic information one may

be subtracting, and whilst the great advantage of den-

droclimatology is that it has the potential to produce

annual resolution in the results, longer-term trends in

growth may also reflect climatic information, and may

be lost in the process of standardization. Even so, the

resulting series is often highly autoregressive, that is to

say that growth in any one year often reflects the

overall vigour of the tree for the few prior years, and

this autoregression needs to be removed to enhance

388 DENDROCHRONOLOGY

the year-to-year variation. Autoregressive Moving

Average (ARMA) models have been perhaps the

most frequently used standardizing models, though

the literature is full of alternative methodologies,

each of which may be more suitable in particular

conditions.

Having satisfactorily arrived at a well replicated

index series representing growth, the next stage is

generally to model how the climatic information in-

fluences the observed growth. One of the great prob-

lems here is the availability of reliable meteorological

records, and the length of those records for regions

where the most environmentally sensitive trees are

growing. The most useful tree-ring information is

often from remote areas, and even where there are

local records, these are generally only available over

recent decades. Not surprisingly, an early study of oak

growth in Britain showed that the variation in ring

width attributable to mean monthly rainfall and tem-

perature data varied according to distance from the

meteorological station used.

Nevertheless, a process of stepwise principal

component regressions generally allows a response

function to be derived in which individual monthly

rainfall and temperatures prior to, and during, the

growth season can be recognized in order of their

importance. For example, in a typical temperate

northern-hemisphere model, April, May and June

rainfall, and May, June and July temperature may

be found to have the greatest influence on growth.

A transfer function can then be derived so that

these major elements are reconstructed throughout

the length of well replicated tree-ring chronology.

Typically, in the more sensitive trees growing at the

margins of distribution, one factor can be derived as

the major influence on growth, and the reconstruc-

tions are concerned with aspects of either rainfall or

temperature.

Given that biological responses are often not linear,

and that relatively crude data in the form of monthly

means are the normal tools employed, it is surprising

how reproducible the results have been. In 1982 a

study of response functions for oaks growing at 16

sites in the British Isles was published. This showed

many similarities in responses, including the surpris-

ing result that temperature in the December prior to

growth was significantly negatively correlated with

ring width in the following growth season. This was

mathematically derived, without reference to the

biology of the trees, and a tentative explanation was

put forward. This suggested that a warm December

resulted in the trees metabolizing the food reserves

which were subsequently not available to the tree the

following spring, resulting in less growth. In a cold

December, the tree had ‘shut down’ allowing the

reserves to be utilized for growth when conditions

improved. This highlighted a need for better under-

standing of tree physiology which is still a require-

ment for better modelling of the climate–growth

relationship.

Some early reconstructions were tempted to use

very short periods of meteorological data to calibrate

the model – often this was all that was available,

although the ring series may go back several centuries.

There is also a need to verify the model, so the short

data series was divided into two parts, with one

short series being used to calibrate the model and the

other part to verify the reconstructed values over an

equally short period. Hundreds of years were then

reconstructed on this unsatisfactory model. Whilst

this is tempting, and no doubt may highlight very

different periods of climate within the tree-ring time-

series, it is clearly open to inappropriate interpre-

tation. Guidelines on methodology also suggest that

in order to remove any effects of changes in response

over time, the calibration and verification of the

model should be done both ways around, i.e. using

the outer years to calibrate and verify using the

inner years, and then reversing the periods used for

calibration and verification.

Critics of this response function approach to den-

droclimatology point out that not only are responses

seldom linear, and that dramatic changes can result

from crossing threshold values, but that many influ-

ences on tree growth can occur at widely varying

time-scales, perhaps in a matter of hours. For

example, a severe frost on a single night in the late

spring, when growth has not been going on for long

and leaves are tender, may damage a significant pro-

portion of the photosynthetic capacity of the tree and

have a profound effect on growth for the remainder

of that growth season. Such an event is unlikely to

show up in a crude measure such as mean monthly

temperature. Critics also point to the observable

fact that whilst generally quite representative values

are derived for much of the series, reconstructed

values rarely recreate the more extreme years well,

tending to underestimate the meteorological data (see

Figure 1).

A number of studies, particularly based in Europe,

have looked at growth in ‘pointer’ years – that is,

years where a large proportion of the trees show a

marked growth change in the same direction – and

have managed to look in greater detail, often at daily

weather records, to see what factors are responsible

for these growth changes.

These two approaches both yield very valuable

information, and are not mutually exclusive. It is by

combining several approaches that the maximum in-

formation is likely to be gained in the future, although

DENDROCHRONOLOGY 389

of course the latter approach is only possible where

the data are available.

Many dendroclimatic studies have concerned

themselves with reconstructing one particular factor

in a limited geographical area, for example summer

temperature in a particular country or state. Another

approach, which has been invaluable, is to produce a

network of chronologies over national, continental or

hemispherical ranges and to use these as the basis of

more wide-scale reconstructions. This approach en-

ables an assessment of the reconstruction values at the

intermediate points to be made, and can identify areas

within the network where further sampling is desir-

able. More ecologically based studies have shown

changes in tree response that are elevation dependent

(Figure 2).

Micro-Anatomical Variations

Whole ring, or earlywood/latewood width studies

are the commonest form of dendroclimatological

studies. In parallel with these studies however, there

is a long history of studies in the relationship of

intraring features with external growing conditions.

Cell size, cell differentiation, cell distribution and

other characteristics (e.g. wall thickness) have all

been studied.

Stable Isotope Studies

This discussion has so far concentrated on the use of

ring-width studies, but it is important to remember

that dendrochronological studies (or tree-ring studies

if one prefers) can and have used many other aspects

Figure 2 A 487-year (AD 1505 –1992) reconstruction of August–July annual precipitation. Drought years are indicated on the lower

portion of the graph (þ) and are defined as years when precipitation is more than one standard deviation less than the average of the

complete series. The heavy line represents an 11-year low-pass filter. (Reproduced with permission from Case RA and MacDonald GM

(1995) A dendroclimatic reconstruction of annual precipitation on the Western Canadian prairies since

AD 1505 from Pinus flexilis James.

Figure 1 A comparison of the tree-ring reconstructed and instrumentally recorded precipitation for the 1905–1987 calibration period.

(Reproduced with permission from Case RA and MacDonald GM (1995) A dendroclimatic reconstruction of annual precipitation on the

western Canadian prairies since

AD 1505 from Pinus flexilis James. Quaternary Research 44: 267–275, copyright Elsevier 1995.)

390 DENDROCHRONOLOGY

of the properties of the rings. Most notable amongst

these is the use of X-ray densitometry. In this, uniform

thickness cross-sections of the tree (usually from

cores) are pre-treated to remove resins and other

chemicals, and then subjected to an X-ray source,

with film below. The resulting photomicrograph can

be scanned to reveal not only ring-width information,

but also the density of the cellulose across the ring.

It has been found that the maximum latewood density

in conifers is generally more highly correlated with

meteorological factors than is simple ring width, and

another advantage is that there is much less auto-

correlation in density measurements. The disadvan-

tages of this method are that is it far more costly

and technologically demanding than pure ring-width

studies.

Another area of tree-ring studies with a long

history is the consideration of changing chemical

isotopes in the wood itself. Both temporal and spatial

variation can be found in the isotopic records of

trees. Studies on variations in the deuterium,

C and

O isotopes have been used to look at long-term

changes in temperature and precipitation.

Deuterium levels in tree rings contain information

about the isotopic composition of the water taken up

by the tree, and this is generally a reflection of the

isotopic composition of the precipitation in the area

where the tree grew. This isotope signal is closely

associated with condensation temperatures, and deu-

terium levels in tree rings have therefore been used as

records of palaeotemperature. Because of isotopic

fractionation processes within the tree, deuterium

levels have also been associated with humidity levels.

It has also been shown that as a result of the covaria-

tion between temperature and relative humidity, that

when this is taken into consideration, it is possible to

produce a more sensitive temperature record from

deuterium levels.

Carbon isotopes have been found to be linked with

temperature, relative humidity, light intensity and

water availability. Some of these associations, such

as that with temperature, can vary considerably in

different site conditions.

Stable carbon and hydrogen isotopes originate

from different sources, the air and groundwater re-

spectively, and hence a combination of isotope studies

is likely to give a broader picture of environmental

changes. Differences have been found between the

isotopic composition of earlywood and latewood,

and it is generally assumed that latewood values are

more representative of the conditions at the time of

wood formation, since there appears to be some

utilization of stored food reserves during the forma-

tion of earlywood. Differences have also been found

with the biological age of the tree, and therefore

growth trends need to be removed as in many

tree-ring studies.

In southern Germany, periods with high values in

both isotope records occur around 5270 bc, 2990 bc

and 2180 bc and are thought to represent times with

high temperatures and low water availability. Simi-

larly, low values around 6230 bc, 5600 bc and ad

390 probably represent unusually wet, cool periods.

The 6230 bc corresponds with an event reported in

North America in high-resolution palaeoclimate re-

cords, and it has been suggested that this may repre-

sent a time of rapid melting of the Laurentide ice-

shield into the North Atlantic.

Other periods of coincidence in the hydrogen

and carbon isotope records do not appear to be asso-

ciated with information from other palaeoclimate

records, and so some caution must be employed in

their interpretation. It is hoped that the development

of other proxy records – such as long-term varve

chronologies – may help resolve these issues.

The ratio of

Oto

O in the cell walls of plants

results from changes in groundwater and evapor-

ation. Levels vary with altitude and latitude, and

oxygen isotope levels have long been regarded as

records of palaeotemperature. It has been suggested

from a study in India that relative humidity levels may

be recorded in the tree rings, and it is likely that, as in

other isotope studies, site conditions play a large role

in determining the usefulness of particular studies.

Further research into tree physiology will undoubt-

edly unravel some of the more complex relationships

between external environmental conditions and the

incorporation of different stable isotopes into the

wood. These will enhance the use of this specialist,

and relatively expensive, area of tree-ring research.

Conclusions

Although very long chronologies have been con-

structed for many parts of the world, the replication

of tree-ring data needed limits the time-frame for

which these reconstructions are considered reliable,

and the majority of those so far produced cover little

more than five or six centuries, with a few notable

longer contributions. The tree-ring-derived tempe-

rature reconstructions generally show greater cen-

tury time-scale variability than is found from other

proxy datasets, and suggest a cooler period in the late

fifteenth and during the sixteenth centuries than is

suggested by non-tree-ring data.

In recent years it has been suggested that the cli-

mate–growth relationship has fundamentally chang-

ed in character, perhaps as a result of increasing

carbon dioxide levels or decreasing ozone levels.

There seems to be a decoupling of the relationships

DENDROCHRONOLOGY 391

observed prior to the 1950s. Whilst this does not

totally discredit the numerous reconstructions that

have been carried out in recent decades, it does mean

that their conclusions need to be used cautiously,

and it poses serious problems for the calibration

of future models used for climate reconstruction.

Wide-scale ecologically based studies are highlighting

variations in tree response at different site types in

recent decades. It is perhaps more useful to extract

climatic data from several different species growing in

a variety of sites, but for which there is a limited

historical dataset, rather than to attempt much longer

reconstructions based on relatively few trees, in order

to best understand the complexities of our changing

climate.

Elsevier Encyclopedia of Geology - vol I A-E

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