| Principles
of Dendrochronology
As
with any science, dendrochronology is governed by a set of principles or
"scientific rules." These principles have their roots as far
back as 1785 (the Principle of Uniformitarianism) and as recent as 1987
(the Principle of Aggregate Tree Growth). Some are specific to
dendrochronology while others, like the Principle of Replication, are
basic to many disciplines. All tree-ring research must adhere to these
principles, or else the research could be flawed. However, before one can
understand the principles, one needs to know basic definitions of terms
used in tree-ring research.
Definitions
The Uniformitarian Principle
The Principle of Limiting Factors
The
Principle of Aggregate Tree Growth
The Principle of Ecological Amplitude
The Principle of Site Selection
The Principle of Crossdating
The Principle of Replication
Definitions
dendrochronology: (dendron
= tree, chronos = time, logos = word = the science of): The
science that uses tree rings dated to their exact year of formation to
analyze temporal and spatial patterns of processes in the physical and
cultural sciences.
dendroarchaeology: The
science that uses tree rings to date when timber was felled, transported,
processed, or used for construction or wooden artifacts. Example:
dating the tree rings of a beam from a ruin in the American Southwest to
determine when it was built.
dendroclimatology: The
science that uses tree rings to study present climate and reconstruct past
climate. Example: analyzing ring widths of trees to determine how
much rainfall fell per year long before weather records were kept.
dendroecology: The
science that uses tree rings to study factors that affect the earth's
ecosystems. Example: analyzing the effects of air pollution on tree
growth by studying changes in ring widths over time.
dendrogeomorphology: The
science that uses tree rings to date earth surface processes that created,
altered, or shaped the landscape. Example: analyzing changes in
tree growth patterns via tree rings to date a series of landslide events.
dendroglaciology: The
science that uses tree rings to date and study past and present changes in
glaciers. Example: dating the inside rings of trees on moraines to
establish the approximate date of a glacial advance.
dendrohydrology: The
science that uses tree rings to study changes in river flow, surface
runoff, and lake levels. Example: dating when trees were inundated
to determine the sequence of lake level changes over time.
dendropyrochronology: The
science that uses tree rings to date and study past and present changes in
wildfires. Example: dating the fire scars left in tree rings to
determine how often fires occurred in the past.
dendroentomology: The
science that uses tree rings to date and study the past dynamics of insect
populations. Example: dating the growth suppressions left in tree
rings from western spruce budworm outbreaks in the past.
tree ring: A
layer of wood cells produced by a tree or shrub in one year, usually
consisting of thin-walled cells formed early in the growing season (called
earlywood) and thicker-walled cells produced later in the growing
season (called latewood). The beginning of earlywood formation and
the end of the latewood formation form one annual ring, which usually
extends around the entire circumference of the tree.
tree-ring chronology: A
series of measured tree-ring properties, such as tree-ring width or
maximum latewood density, that has been converted to dimensionless indices
through the process of standardization. A tree-ring chronology
therefore represents departures of growth for any one year compared to
average growth. For example, an index of 0.75 (or 75) for a given year
indicates growth below normal (indicated by 1.00, or 100).
standardization: The
process that removes undesirable long-term variations from a time series
of measured tree-ring properties by dividing the actual measurements by
those predicted from a statistically derived equation that relates tree
growth over time to tree age. Usually this process tries to remove the
growth trends due to normal physiological aging processes and changes in
the surrounding forest community.
increment borer: An
auger-like instrument with a hollow shaft that is screwed into the trunk
of a tree, and from which an increment core (or tree core) is extracted
using an extractor (a long spoon inserted into the shaft that pulls out
the tree core). These instruments are quite expensive, normally ranging
from $200 to $500.
Principles
of Dendrochronology
The Uniformitarian
Principle
(Click picture to
enlarge.) This principle states that physical and biological processes
that link current environmental processes with current patterns of tree
growth must have been in operation in the past. In other words, "the
present is the key to the past," originally stated by James Hutton in
1785. However, dendrochronology adds a new "twist" to this
principle: "the past is the key to the future." In other words,
by knowing environmental conditions that operated in the past (by
analyzing such conditions in tree rings), we can better predict and/or
manage such environmental conditions in the future. Hence, by knowing what
the climate-tree growth relationship is in the 20th century, we can
reconstruct climate from tree rings well before weather records were ever
kept!
For example, the graph
above shows a long-term precipitation reconstruction for northern New
Mexico based on tree rings (click on the image to see an enlarged version
of the graph). The reconstruction was developed by calibrating the widths
of tree rings from the 1900s with rainfall records from the 1900s. Because
we assume that conditions must have been similar in the past, we can then
use the widths of tree rings as a proxy (or substitute) for actual
rainfall amounts prior to the historical record.
The
Principle of Limiting Factors
(Click picture to
enlarge.) As used in dendrochronology, this principle states that
rates of plant processes are constrained by the primary environmental
variable that is most limiting. For example, precipitation is often the
most limiting factor to plant growth in arid and semiarid areas. In these
regions, tree growth cannot proceed faster than that allowed by the amount
of precipitation, causing the width of the rings (i.e., the volume
of wood produced) to be a function of precipitation. In some locations,
rainfall is not the most limiting factor. For example, in the higher
latitudes, temperature is often the most limiting factor that affects tree
growth rates. In addition, the factor that is most limiting is often acted
upon by other non-climatic factors. While precipitation may be limiting in
semiarid regions, the effects of the low precipitation amounts may be
compounded by well-drained (e.g. sandy) soils.
The
Principle of Aggregate Tree Growth
 |
This principle states
that any individual tree-growth series can be "decomposed" into
an aggregate of environmental factors, both human and natural, that
affected the patterns of tree growth over time. For example, tree-ring
growth (R) in any one year (indicated by a small "t",
where t could be "1" for year 1, and "2" for
year 2, etc.) is a function of an aggregate of factors:
1. the age related growth trend (A) due to normal physiological aging
processes
2. the climate (C) that occurred during that year
3. the occurrence of disturbance factors within the forest stand
(for example, a blow down of trees), indicated by D1,
4. the occurrence of disturbance factors from outside the forest
stand (for example, an insect outbreak that defoliates the trees, causing
growth reduction), indicated by D2, and
5. random (error) processes (E) not accounted for by these other
processes.
(The Greek letter in front of D1 and D2 indicates either a "0"
for absence or "1" for presence of the disturbance signal.)
Therefore, to maximize the desired environmental signal being studied, the
other factors should be minimized. For example, to maximize the climate
signal, the age related trend should be removed, and trees and sites
selected to minimize the possibility of internal and external ecological
processes affecting tree growth.
The
Principle of Ecological Amplitude
 |
(Click picture to
enlarge.) This principle states that a tree species "may grow and
reproduce over a certain range of habitats, referred to as its ecological
amplitude" (Fritts, 1976). For example, ponderosa pine (Pinus
ponderosa) is the most widely distributed of all pine species in North
America, growing in a diverse range of habitats. Therefore, ponderosa pine
has a wide ecological amplitude. Conversely, giant sequoia trees (Sequoiadendron
giganteum) grow in restricted areas on the western slopes of the
Sierra Nevada of California. Therefore, this species has a narrow
ecological amplitude. This principle is important because individual trees
that are most useful to dendrochronology are often found near the margins
of their natural range, latitudinally, longitudinally, and elevationally.
The diagram above shows the different forest types as one increases
elevation along a mountainside. To maximize the climate information
available in ponderosa pine tree rings, we would likely sample trees at
their lower elevational limit around 7000 feet (2130 meters).
The
Principle of Site Selection
(Click picture to
enlarge.) This principle states that sites useful to dendrochronology
can be identified and selected based on criteria that will produce
tree-ring series sensitive to the environmental variable being examined.
For example, trees that are especially responsive to drought conditions
can usually be found where rainfall is limiting, such as rocky outcrops,
or on ridgecrests of mountains. Therefore, a dendrochronologist interested
in past drought conditions would purposely sample trees growing in
locations known to be water-limited. Sampling trees growing in
low-elevation, mesic (wet) sites would not produce tree-ring series
especially sensitive to rainfall deficits. The dendrochronologist must
select sites that will maximize the environmental signal being
investigated. In the figure below, the tree on the left is growing in an
environment that produced a complacent series of tree rings.
The
Principle of Crossdating
(Click picture to
enlarge.) This principle states that matching patterns in ring widths
or other ring characteristics (such as ring density patterns) among
several tree-ring series allow the identification of the exact year in
which each tree ring was formed. For example, one can date the
construction of a building, such as a barn or Indian pueblo, by matching
the tree-ring patterns of wood taken from the buildings with tree-ring
patterns from living trees. Crossdating is considered the fundamental
principle of dendrochronology - without the precision given by
crossdating, the dating of tree rings would be nothing more than simple
ring counting!
The
Principle of Replication
(Click
picture to enlarge.) This principle states that the environmental
signal being investigated can be maximized, and the amount of
"noise" minimized, by sampling more than one stem radius per
tree, and more than one tree per site. Obtaining more than one increment
core per tree reduces the amount of "intra-tree variability", in
other words, the amount of non-desirable environmental signal peculiar to
only tree. Obtaining numerous trees from one site, and perhaps several
sites in a region, ensures that the amount of "noise"
(environmental factors not being studied, such as air pollution) is
minimized. |
