Dendrochronology - Link Sharable

Dendrochronology

Paleoclimate Lab: Section 2

How to Examine a Tree Core

Introduction to Dendrochronology

Tools to Study Tree Rings

Introduction to Dendrochronology

Dendrochronology, also known as tree ring dating, is a type of dating technique using growth rings in order to determine the age of a living or dead tree. A tree ring is formed from a single year of growth. The thin, typically dark line is typically fall / winter growth, when rainfall is low and growth is slower. The lighter ‘rings’ are spring / summer growth, when rain is more plentiful and warm weather allows for increased growth. Climate data can be determined by looking at the rings (the light color part between the darker rings). Thicker rings indicate wetter years and thinner rings indicate dryer years. Since trees within a region experience the same climate patterns, they will have ring patterns that match. Cores taken from dead trees can be compared to ones that are still living in order to date when that tree died by lining up matching patterns. By doing this across many trees of increasingly older age a climate timeline can be built.

Tools of Dendrochronology

Forestry workers and scientists will use an instrument called an increment borer to obtain core samples. It creates a ½ inch hole and removes a core sample that is 1/8 inch in diameter from the tree. The wound is then plugged to prevent infection in a living tree. Information about the tree and its environment is obtained by examining the xylem rings. The height of the tree is determined using an abney level. The abney level provides the angle for a from a particular standing distance. Using that angle, the observers eye height, and the observers distance the tree height can be solved without climbing. Diameter tape is used to measure the main trunk or branches of a tree. Age can be estimated using the diameter tape if previous cores had been taken.

Note: These tools will be demoed in class. This is here for you to review term and for future reference

Tree Cores

Start by examining how to select the correct type of tree. Then look at the step-by-step on counting tree rings.

+ Info on Selecting the right type of tree

Please note: There are two potential errors with this method of aging. The first is due to the early growth of the tree. Since the early growth can’t always be assessed accurately most age counts are approximate and accurate within two years of the number counted (can be more then that depending on the species of tree). Given that, the more accurate way of saying the age for the example diagram is that the tree is between eight and ten years old. The second potential error stems from climates being different around the world. It is important to be aware of the climate patterns of the region. If a region has two, or more, wet seasons with dry periods in between this may create false rings; where the tree produces two or more rings in a year. For this lab, assume that the trees are from an environment that has one wet season (spring and summer) and one dry season (fall and winter).

Selecting Trees that Have Rings

Different types of trees are suitable for examining tree rings. Gymnosperms, cone-bearing plants (such as pine trees) often form rings with their annual growth. Among angiosperms (flowering plants), dicots can be used for dendrochronology. Monocots cannot. Tree rings are formed from the vascular tissue, which in bundles in plant stems and trunks. In dicots the vascular bundles are arranged in rings and form a new ring with each year of growth. Older rings die and form structural support for the plant. This is what we see as rings. Monocots have vascular bundles that are randomly arranged. These plants lack distinctive growth rings.

+info

Every tree core or cutting has several parts (hover over eyes to the left). The outermost ring is the bark. The bark is a protective layer and is not considered as part of the tree rings. Just inside of the bark is the last annual ring, in other words, the most recent ring. This layer is the year the tree sample was taken (if tree was living) or the last year the tree was alive (if tree was dead). Often this ring is not useful for climate data, since the tree was cut down part way through its growth cycle. Beyond the first annual rings are the annual rings from previous years. The further from the bark the older the ring is. These rings are useful for giving climate data and for comparing to other tree cores. The first annual ring is the growth of the tree during its first year or couple of years. When a tree is young it grows faster and its rings may less distinctive or be part of the pith. On the end, in some trees, is the pith. The pith is a soft material that is composed of early vascular cells and nutrient storage cells.