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Dendrochronology

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What is dendrochronology?

Dendrochronology is the science of dating events and environmental variations by the comparative study of growth rings in trees and aged wood.

“dendros” = the Greek word for tree
“chronos” = the Greek word for time

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Common Knowledge

What is the age of this tree?

If this cross section were collected in 2000, what was probably the wettest year this tree experienced? What was the driest?

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Tree Rings

Characteristics necessary for patterns to be used for dating:

Only one ring added per growing season (annual ring in the Cross Timbers).

6 rings = 6 years

1

2

3

4

5

6

Pith

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Tree Rings

Environmental limits on tree growth:

Only one environmental factor must dominate in limiting growth.

In the

Cross Timbers, growth

is primarily affected by precipitation.

2000

Wide

Narrow

“Dry”

1999

1998

“Wet”

1997

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Tree Rings

Tree rings are easy to count because each one is divided into two parts, earlywood and latewood.

Earlywood

Latewood

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Tree Rings

Earlywood - made up of cells that have large lumen (openings). It typically develops as a result of vigorous growth during the growing season (spring and summer).

Earlywood

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Tree Rings

Latewood - made up of cells that have a more compact lumen (“flattened”) resulting in a darker appearance. It forms at the end of the growing season as the tree transitions to dormancy.

Latewood

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Tree Rings

The sharp contrast between the last-formed latewood cells of one growing season and the first-formed earlywood cells of the following season is what delineates the boundary of an annual ring.

Annual

Ring

Boundary

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Tree-Ring Sampling

Data Sources

Cross sections
Increment cores

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Tree-Ring Sampling

Cross sections (1)

Complete section

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Tree-Ring Sampling

Cross sections (2)

Partial section

From Arno & Sneck, 1973

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Tree-Ring Sampling

Increment cores

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Trees & Fire

Stand-Replacing Fires

High-severity fires (“crown fires”)
High tree mortality

Stand-replacing fires kill most of the trees on a site, creating conditions that favor the establishment of a new forest where most of the trees are about the same age – a new cohort. Click Increment cores taken from the new or “replacement” stand of trees can provide some information about when a stand-replacing fire may have occurred. Further evidence is often required to support the occurrence of a stand-replacing fire because other conditions could result in widespread mortality, such as droughts, pest outbreaks, changes in community competition, and changes in climate. Click

Cohort: Organisms that form a group of approximately the same age. As a result, such organisms are subject to an environment which may differ from that to which organisms from an earlier or later time are exposed. A group of individuals of the same age, recruited into a population at the same time.

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Trees & Fire

Surface Fires

Low-severity fires
High tree survival; trees may be scarred (“catface”)

Fire Scar

Surface fires are those that burn over the ground surface consuming duff and fine fuels. Surface fires are typically of lower severity or intensity. However, some could be severe enough to be stand-replacing. Low-intensity fires kill some trees (especially younger and older ones of species with thin bark) and burn around others. Fire scars may be produced on some of the surviving trees. Once a tree is initially scarred, future fires are likely to scar the damaged area again. Repeated fire scars create a triangular scar, called a catface, at the base of the tree. By collecting partial cross sections from surviving trees and complete cross sections from dead trees, fire scars can be used to determine the historic frequency of low-intensity fires on a study site. Click

The fire scar video may be shown.

Duff: Partially decomposed organic matter lying beneath the litter layer and above the mineral soil. Includes the fermentation and humus layers of the forest floor.

Fine fuels: Dead fuels comprised primarily of herbaceous plants but which could also include woody plants less than 0.25 inch (6.4 mm) in diameter and litter found on the ground surface.

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Fire Scar Formation

Cambium

The yellow layer or band in this diagram depicts cambium tissue. Cambium that is severely damaged can no longer produce other tissues necessary for tree survival and growth. Click A low-intensity surface fire passes by the tree, but enough heat and physical damage is produced to cause cambium death in one area. Click We will assume that there has not been enough cambium damage to kill the tree. Click As time passes, Click the surrounding cambium is able to continue with tree growth. Click The scarred area begins to be overgrown and could eventually close. Click Click This partial cross section is from a tree that has survived at least three fires. Click Click Click On cross sections with multiple fire scars, the number of annual rings between scars indicates how much time has elapsed between fires. Click

Cambium: Tissue within the stems or roots of trees and plants which produces the secondary tissues xylem and phloem. Cambium is located in the interior of a tree a short distance beneath the bark.

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Tree-Ring Sampling

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Trees & Fire

With a basic understanding of dendrochronology and the effect of fire/heat on trees, historic fire frequency and fire intensity can be determined.

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Activity Resources

Deschutes National Forest (Oregon)

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Dendrochronology

Part B: Stand-Replacing Fire Timeline
Pact C: Fire-Scar History Chart
Part D: Fire Management in the Cross Timbers