Principles of Dendrochronology
As with any science, dendrochronology is governed by a set of principles or "scientific rules." A principle can be defined as “A basic generalization that is accepted as true and that can be used as a basis for reasoning or conduct.” Some are specific to dendrochronology while others, such as 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.
The Principle of Crossdating
The Concept of Cognitive Classification
The Principle of Trees as Dynamic Entities
The Principle of Plurality
The Concept of Parsimony
The Principle of Aggregate Tree Growth
The Principle of Limiting Factors
The Concept of Sensitivity
The Principle of Spatiotemporal Replication
The Principle of Site Selection
The Concept of Tree Selection
(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.
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.
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.
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.
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.
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.
The science that uses tree rings to study changes in river flow, surface runoff, and lake levels. Example: dating when trees were inundated by water to determine the sequence of lake level changes over time.
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.
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.
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.
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).
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.
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
Henri D. Grissino-Mayer, 2015. The Long, Steady Decline of Uniformitarianism in Dendrochronology: What if the Present is No Longer the Key to the Past? Paper presented at the Annual Meeting, Association of American Geographers, 21–25 April 2015, Chicago, Illinois.
Henri D. Grissino-Mayer 2017. The Time is Right: Redefining the Principles in Dendrochronology. Paper presented at the Annual Meeting, American Association of Geographers, 5–9 April 2017, Boston, Massachusetts.
Revised Online, 27 April 2017 and 7 November 2017
NOTE TO READERS
You may notice that the principles below represent a major change in the way we approach dendrochronology. This is because, as a scientific discipline evolves, so too must the principles to which it adheres. Over the past three years, I have been working on revising these principles and I've presented numerous talks across the U.S. about possible changes. I kept adding new principles while simultaneously revising or even deleting long familiar principles. Eventually, I settled on the principles below as being representative of those to which we actually should adhere and have been adhering. I've sought input from individuals on these revisions and to all of them I'm especially grateful.
I'm sure, over time, we may find that one or more principles below are not really needed or that new principles need to be introduced. Such is the nature of dendrochronology. Such is the nature of science. One glaring change is that you will no longer notice the inclusion of the Principle of Uniformitarianism (or Uniformity). This principle was most applied to reconstructions of past climate, assuming that the climate response seen in trees during modern times was the same as the climate response in trees during previous times. Study after study has now shown this not to be the case. In fact, the introduction of "divergence" into the dendro-lexicon in the late 1990s epitomized the changing response of trees to climate over time, first scrutinized in detail in the mid-1980s. It is time we retire uniformitarisnism. Perhaps it actually was retired decades ago...
"Paradoxically, in suggesting that this term now be dropped from use, we pay a most fitting tribute to its vital role in the history of geology." -- Stephen Jay Gould, 1965, "Is uniformitarianism necessary"? American Journal of Science, 263, 223-228.
The Principle of Crossdating
This principle states that matching patterns in ring widths or other ring characteristics (such as ring density patterns) among several tree-ring series allows the identification of the exact calendar (or relative) year in which each tree ring was formed. 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! For example, one can date the construction of a building, such as a barn, pueblo, or log cabin, by matching the tree-ring patterns of wood taken from the buildings with tree-ring patterns from living trees. Michael G.L. Baillie said it best in his 1982 book Tree-Ring Dating and Archaeology (page 263): "A tree-ring sample either dates or it does not. No amount of pressure will make a tree-ring pattern match if it does not." If you cannot confidently assign exact years (calendar or relative) to your tree-ring samples, then you cannot report any "possible" dates for that sample! We DO NOT give "possible" dates or dates with a plus or minus factor!
The Concept of Cognitive Classification
While crossdating is the central principle in dendrochronology, cognitive classification is the central concept of (successful) crossdating. This concept recognizes that, first and foremost, dendrochronology is a highly cognitive science. Tree-ring scientists rely heavily on visual inspection and microscopic examination of wood specimens so that we can ensure that we have assigned the correct calendar (or relative) year to the tree ring. This careful inspection helps us classify and record the narrow and extremely narrow rings, the very wide or average width tree rings, false rings, micro-rings, traumatic resin ducts, past injuries to the xylem from a vartiety of disturbance processes, changes in growth rates, even a location where a ring should be but is not. Basically, this concept reinforces a central concept taught to all fledgling dendrochronologists: "Know your wood!" Dendrochronologists absolutely must spend considerable time getting to know the wood samples being analyzed to become better informed about the tree's life history.
As used in dendrochronology, this principle states that rates of plant processes are constrained by the primary environmental variable(s) that is most limiting. Notice this is Liebig’s Law of the Minimum! Tree growth “is controlled not by the total amount of resources available, but by the scarcest resource.” 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 monthly, seasonal, or annual total precipitation, causing the width of the rings (i.e., the volume of wood produced) to be a function of precipitation. In higher latitudes and elevations, temperature is often the most limiting factor. For many forest trees, especially those growing in temperate and/or closed canopy conditions, climatic factors may not be most limiting. Instead, processes related to stand dynamics (especially competition for nutrients and light) may be most limiting to tree growth. 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.
Complacent tree-ring series Sensitive tree-ring series
If trees within a site have plentiful nutrients, sunlight, enough water, minimal competition, and a favorable temperature regime, tree growth will be the same from year to year, producing a “complacent” series of tree rings with little to no discernible signal. Not only must some environmental factor limit tree growth, but this limitation ideally should vary from year to year, producing a “sensitive” series of tree growth. This sensitive series will have physical tree-ring characteristics that contain a record of whichever environmental variable (or variables) is most liming to tree growth.
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.
Hal Fritts said it best in his 1976 book Tree Rings and Climate: "Sites are deliberately selected to draw observations from that population of tree-ring data that contain the desired information." 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, steep slopes, 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 above, for example, note that the most sensitive tree-ring data for white oak are actually found in the northwestern portion of its range, not along the edges (John Sakulich Ph.D. dissertation, 2011). Could more continental interior locations (as opposed to near-maritime locations) foster more sensitive tree-ring series for species with wide ecological amplitudes???
Dendrochronologists often engage in targeted or purposive sampling of trees as well as sites. Specific trees that grow in specific sites yield a high level of sensitivity to fluctuating climatic conditions, including trees that grow at high elevations; along steep slopes; in thin soils; that are mature (young trees are less sensitive); do not display serious injuries (from fire, insects, windthrow, etc.); with cylindrical boles; have heavy lower limbs; sparse foliage; atypical growth form (indicating a stressed tree); exposed roots; extreme/compressed spiral grain; a dead spike-top (crown exhibiting internally-induced dieback); a trunk in stripbark form; a trunk of short stature that resembles an inverted “V” (or carrot); and trees that grow in isolation from other trees.
This principle recognizes that the environmental signal being investigated (e.g. climate or wildfire) can be maximized if we increase both the temporal and spatial resolutions of the samples we collect. For example, signal is maximized and noise is minimized when we collect more than one core per tree, more than one tree per site, and more than one site in the region. This gives dendrochronologists the ability to make inferences on ever larger spatial scales. At the same time, the more samples collected, the more the desired signal is further maximized for any portion of the chronology. More samples collected increases the chances of increasing the sample depth further back in time so that we can ensure stronger interpretations of the desired signal.
Ever since Hal Fritts's seminal 1976 book Tree Rings and Climate, dendrochronologists have taught, embraced, and adhered to the Principle of Uniformitarianism (or Uniformity): "The present is the key to the past." This principle was most applied to reconstructions of past climate, assuming that the climate response seen in trees during modern times was the same as the climate response in trees during previous times. In recent decades, however, study after study has shown that the present may not necessarily be the key to the past. In fact, the introduction of "divergence" into the dendro-lexicon in the late 1990s epitomized the changing response of trees to climate over time, first scrutinized in detail in the mid-1980s. Because uniformity may not hold and may be untrue in specific locations and in certain tree species, uniformity is no longer a "generalization that is accepted as true."
Instead, we must recognize the inherent biological nature of trees as dynamic entities, able to adapt, change, and alter their behavior due to internal physiological mechanisms as well as external driving forces. The reality is that trees can change their responses to climate over time, making reconstructions of climate particularly vexing. If the climate-tree growth relationship is not stable over the 20th centruy, we cannot assume it was stable in previous centuries. A tree is essentially a "dynamical system," i.e. trees must be recognized for their long-term evolving behavior. Trees evolve. Trees change. Once we recognize and accept that uniformity is not a requirement and accept that trees evolve over time, then we can more accurately evaluate and interpret past environments recorded in the tree-ring record.
This principle embodies practically every pedagogical facet, aspect, and technique of dendrochronology, both in teaching dendrochronology and in research using dendrochronology. Plurality embraces adherance to the concept of "Multiple Working Hypotheses" (MWH) as first presented by Thomas Chamberlin in 1890. Scientists must consider all possible explanations for a perceived phenomenon and test and examine the evidence, eventually selecting the one hypothesis that remains after all others have been rejected.
Simultaneously, parsimony embodies the need to choose and apply the simplest explanation when multiple hypotheses are presented. This concept is essentially "Occam's Razor," i.e. "Given multiple explanations, the simplest one is usually the correct one."
By adhering to the two concepts in this principle, dendrochronologists can avoid the danger of favoring a "pet hypothesis" while ensuring the best explanation is chosen. Since its inception, dendrochronology has been hammered by those with such pet hypotheses. Dendrochronologists must let the trees speak for themselves and guide the acceptance or rejection of the many hypotheses proferred. For example, a dendrochronologist observes a scar at the base of a tree, obvious from the bark being removed. But before the dendrochronologist can accept one hypothesis ("this scar was caused by a wildfire"), the dendrochronologist must acknowledge and test other prossible factors that could have caused that scar, including: lightning; rockfall and other mass movements; adjacent tree fall; bark removal by animals; bark removal by native groups; and mechanical scars caused by humans (e.g. logging operations)