Student Handout

Dendrochronolog Sheet

Name: _______________________
Tree Species: __________________
Date the tree was sampled (cut): ___________________

Method: Start with the year the tree sample was taken and count the years down from there. Mark the smallest rings with a long line (these represent the poorest growth years).

Teacher Handout

Dendrochronology: Investigating the Recent Past
By Chris Marion; February 1998

Introduction to Wood Anatomy and Tree Ring Formation

A tree ring is made up of 2 distinct bands of cells. The earlywood, the light-coloured band, is laid down in the spring and early summer, when water availability is the highest. Latewood is produced later on in the summer and in the early fall. Latewood cells are somewhat smaller than earlywood cells, have a much thicker cell wall and much smaller lumen, and therefore the colour is much darker. At the end of the growing season, wood production shuts down until the following spring, when the light coloured earlywood gives a sharp contrast with the previous year’s dark, tight latewood.

Water availability and warmth of the growing season are the two main factors affecting ring width. A wet, warm growing season will lead to the formation of wide, light-coloured bands in most trees, with ring width increasing with the length of the favourable growing season. Dry and cold summers will result in narrower rings; a particularly long but relatively clement late summer and fall will be recorded as a wider latewood band. Deciduous species are more difficult to work with, as the latewood is not much different in colour from the earlywood.

Many events will happen in the life of a tree that may be recorded in its wood either at the ring level or as more obvious scars or deformations in the tree itself. Frost, insect epidemics, and droughts are a few examples of events that will be recorded in the rings at the precise year in which they occurred. Fire will stop the production of wood where the cells were damaged, leaving a scar which might eventually close with time. This scar can also be dated. Wide rings point to an increase in growth rate due to positive events; for example, an opening in the forest canopy following the death of a tree will allow the understory saplings to grow better due to an increase in the availability of light and rain.

Sampling, Processing, and Dating of Wood Material

Sampling of the trees can be done in two ways, using an increment borer for live trees or by cutting tree disks from dead or felled trees. The wood samples need to be sanded carefully to make dating easier. Counting back from the outside ring towards the inside of the disk will give an approximate age for the tree. Using pencil marks at every 10th ring helps keep track of the number of rings counted.

Marker Years

While most tree rings look more or less the same, some rings known as marker rings may be noticeably different from their neighbours. Such rings are useful for cross-dating; some of them even speak of localized or widespread disturbance events that are of interest to ecologists.

Frost Rings

Frost rings are caused by late-spring or early-fall frost events. The cambium is affected by the cold, and a few abnormal layers of cells are produced before the cambium resumes its organized cell divisions. Frost rings appear as a band of darker, disorganized cells within a regular ring.
False Rings

False rings happen when for a short period during the growing season, growing conditions resemble that of the end of the season. For example when drier and colder conditions prevail for a few days, a few layers of thick-walled cells are formed, and they have the appearance of latewood. A close inspection of the suspicious ring will reveal that, although cell walls have thickened, cell size has not really decreased and the return to thinner-walled cells is progressive, not sharp as it would be if spring had just returned.
Light Rings

Light rings are produced when wet summers are followed by a very short fall. Trees do not have the time to shut down properly and latewood formation is minimal. A light ring may be hard to detect, as the dark band of latewood can be very thin, very light, or even non-existent.
Narrow Rings

Narrow rings are the most useful of all marker rings. They appear faithfully in almost all of the trees of an area since they are usually influenced by climate felt at a regional scale. Depending on the tree species in which they are recorded, they speak of drier/wetter or warmer/colder conditions than those preferred by the species.

A series of narrow rings may indicate a few years of unfavourable growing seasons.

Dating with wood: some applications of Dendrochronology