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Eastern Hemlock

Latin name:
Tsuga canadensis

Extracted from a waterfront pier near Wilmington, Delaware, the tree used to make this portion of the pier
actually came from a forest located in central Pennsylvania. The outermost tree ring dates to the late 1830s.

Giant Sequoia

Latin name:
Sequoiadendron giganteum

A close up of numerous fire scars on a giant sequoia cross section from Sequoia National Park in California, dating back well prior to A.D. 1000. Look closely! Can you find the sad bearded face cradled by his hands, as if he was crying?

Douglas-fir

Latin name:
Pseudotsuga menziesii

This photo shows the tree rings from a beam extracted many years ago from a pueblo in northeastern Arizona. The section shows many false rings and many micro-rings, suggesting this tree may have been growing in a marginal environment.

Ponderosa Pine

Latin name:
Pinus ponderosa

Close up of tree rings of a ponderosa pine collected at El Malpais National Monument in New Mexico, USA, showing tree rings centered around A.D. 1400. Notice the variability in ring widths indicative of sensitivity to year-to-year variation in precipitation.

Douglas-fir

Latin name:
Pseudotsuga menziesii

Perhaps my most requested image of tree rings, obtained from a small Douglas-fir growing in the Zuni Mountains of west-central New Mexico by my colleagues Rex Adams and Chris Baisan. Not very old, but has some of the most beautiful rings of all my displays!

White Oak

Latin name:
Quercus alba

Oak cores from the Hoskins House in Greensboro, North Carolina, site of a famous battle during the Revolutionary War. The house was built from trees cut in 1811 to 1813, not cut and built in the 1780s as the historical agency had hoped.

Ponderosa Pine

Latin name:
Pinus ponderosa

This ponderosa pine once grew at El Morro National Monument in New Mexico, USA, and was cut many years ago. Once you get up close to the stump, you can see a very old scar from a fire many hundreds of years ago that scarred the tree when it only about 12 years old!

Bahamian Pine

Latin name:
Pinus caribaea var. bahamensis

We collected many cross sections of Bahamian pines that had been cut for an industrial park on the island of Abaco, but the rings are very difficult to date! Many false rings, and the pine appears to terminate tree growth during the dry season.

Longleaf Pine

Latin name:
Pinus palustris

This cross section was one of many that came from an old crib dam across a creek that was exposed after a modern dam broke in Hope Mills, North Carolina in 2003. Such sections from old-growth longleaf pines are very rare and provide information on climate back to AD 1500!

White Oak

Latin name:
Quercus alba

Sometimes you don't have to look far to find beauty in wood, and sometimes it may not be a living tree! After an oak tree was cut a year or two before this section was obtained, decay fungi had already set in, beginning to break the wood down to its basic elements.

Southwestern White Pine

Latin name:
Pinus strobiformis

I collected this fire-scarred pine on Mt. Graham in southern Arizona in fall 1991, and it remains one of the best examples of how we can determine the season of fire by looking at the position of the scar within the ring.

Bristlecone Pine

Latin name:
Pinus longaeva

Bristlecone pines have become one of the best proxy records for those who study the history of volcanic eruptions because the cool temperatures caused by these eruptions create "frost rings" that form when the cells implode from the cold.

Eastern Redcedar

Latin name:
Juniperus virginiana

Many well-preserved eastern redcedar sections have been recovered from prehistoric sites in eastern Tennessee, and they have more than enough rings to date, but we don't have a long enough living-tree reference chronology to overlap with them!

Red Oak

Latin name:
Quercus rubra

Oak is by far the most common genus we find in the many historic structures we date using tree rings in the Southeastern U.S. The genus has good ring variability and rarely has problem rings. This section came from a historic tavern in Lexington, Virginia.

Sugar Maple

Latin name:
Acer saccharum

Maple, birch, beech, and basswood are all examples of hardwood species that form diffuse porous wood, meaning that the ring contains many small-diameter vessels all through the ring. Identifying the ring boundary on this wood type is a challenge to tree-ring scientists.

Live Oak

Latin name:
Quercus virginiana

Live oak is an example of an evergreen oak, which is not common within this genus. As such, the wood is semi-ring porous and the rings are very difficult to see and date. Ring growth is also very erratic, not forming the concentric around the tree that we require.

Douglas-fir

Latin name:
Pseudotsuga menziesii

These cores were collected on Mt. Graham in southern Arizona and show a major suppression event beginning in 1685 when missing rings became evident, followed by many micro-rings. This suppression was caused by a major wildfire in 1685!

Ponderosa Pine

Latin name:
Pinus ponderosa

I find it amazing what trees can record in their tree rings! Here we see a cross section of a pine that was damaged by a major flood in the year 1945 in the Chiricahua Mountains of southern Arizona. Notice the reaction wood that formed afterward.

Pignut Hickory

Latin name:
Carya glabra

Sometimes gray-scale imagery helps define tree rings when measuring. Although classified as "ring porous" species, the rather ill-defined tree rings in hickory tree species form large earlywood vessels and smaller latewoood vessels.

Subalpine Fir

Latin name:
Abies lasiocarpa

Decay has set in on the tree rings of this dead and downed subalpine fir that once grew on Apex Mountain in British Columbia, Canada, but the tree rings can still be measured and crossdated despite this!

White Fir

Latin name:
Abies concolor

We found a beautiful fire scar on this white fir that was used to build a cabin in the Valles Caldera of New Mexico. Thought to have been built in the early 1900s, we instead found the cabin was built form white fir and Douglas-fir trees cut in 1941.

Overcup Oak

Latin name:
Quercus lyrata

These oak cores were collected in northeastern Arkansas to investigate a change in the hydrologic regime of a wildlife refuge beginning in the 1990s. We found that trees at this site experienced a major disturbance event in the 1960s.

Western Juniper

Latin name:
Juniperus occidentalis

Near Frederick Butte in central Oregon, we discovered an unusual stand of western junipers that had the most unusual lobate growth forms we had ever seen. This site yielded a drought-sensitive chronology dating back to the AD 800s!

West Indies Pine

Latin name:
Pinus occidentalis

Above 3000 meters on the highest peak in the Carribean, we found an entire forest of these pines, many with fire scars, living on a steep rocky slope. The forest looked more like the dry ponderosa pine forests of the western U.S.

Whitebark Pine

Latin name:
Pinus albicaulis

Whitebark pines growing in the northern Rockies of the western U.S. can grow to be over 1,000 years old, but the species is slowly being decimated by the introduced white pine blister rust. Many of these ancient trees are now dead with ghostly white trunks.

Shagbark Hickory

Latin name:
Carya ovata

Curiously, tree-ring scientists rarely analyze some of the more common hardwood species in the eastern U.S., such as this hickory, perhaps because such forest interior trees may contain a weak climate signal necessary for crossdating.

Virginia Pine

Latin name:
Pinus virginiana

Blue stain found in many sections of dead pines (both in the western and eastern U.S.) is caused by a fungus carried by a pine beetle. The fungus spreads into the phloem and sapwood of living and dead pines, sometimes creating stunning patterns!

Pinyon Pine

Latin name:
Pinus edulis

Burned sections of pinyon pine are commonly found in archaeological sites in the southwestern U.S. These sections can be carefully broken or surfaced with a razor to reveal the ring structure inside to assist in dating the years of construction of the site.

Red Spruce

Latin name:
Picea rubens

Conifers in the highest elevations of the Appalachians of the eastern U.S., such as this red spruce, don't experience wildfires very often, but when fires do occur, they can create numerous fire scars even in this fire-intolerant species. Notice the growth release!

White Spruce

Latin name:
Picea glauca

This tree was located in the Canadian Rockies on the toe slope of an active avalanche path. The scar was created by a debris flow or snow avalanche which struck the tree, killing a section of the living tissue. The avalanche can therefore be dated to its exact year!

Engelmann Spruce

Latin name:
Picea engelmannii

I worked considerably in the spruce-fir forests of southern Arizona in my earliest years in dendrochronology, and learned that trees with limited sensitivity can provide a vast amount of information on the history of these forests.

Ponderosa Pine

Latin name:
Pinus ponderosa

The lava flows of El Malpais National Monument in New Mexico contain vast amounts of remnant wood, mostly ponderosa pines such as this sample, and the tree rings on these samples go back nearly 2000 years! Notice the year AD 1400 on this section.

Chestnut Oak

Latin name:
Quercus montana

In the southeastern U.S., hardwood species are often scarred by wildfire. Most often, this also will cause considerable decay in the sample, but this oak had several well preserved fire scars, suggesting fire was common in these drier, lower elevation sites.

Ponderosa Pine

Latin name:
Pinus ponderosa

I originally sampled this stump in 1991 for its fire scars, located in El Malpais National Monument of New Mexico. I found it again 20 years later and was happy you could still see the tree rings and fire scars clearly! It had originally been logged in the 1930s!

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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

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. 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

As used in dendrochronology, this principle states that rates of plant processes are constrained by the primary environmental variable(s) 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 (for example, 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.

 


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

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

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

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

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.

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