Most of us have heard about the concept of counting tree rings to estimate the age of a tree. But did you know that these rings in timber and tree trunks do not merely demonstrate its annual growth but also show the events of climatic and environmental changes a tree has experienced? This field study is called “Dendrochronology”.
Dendrochronology originates from the Greek words: “Dendron” (tree) and chronos (time). As climate changes are now proceeding at a very unpredictable rate, this procedure can be used to collect the records which were already taken in the tree trunks of a specific area. Hence, we can understand the climate response in trees, keep track of the seasonal events and gain perception about anthropogenic impacts on the environment. Additionally, dendrochronology is at the core of climatology, ecology, and archeology and helps us link climate history to forest history and human history (Mees and Seys, 2023).
The tree-ring samples can be obtained by plugging an increment borer, which is a simple metal tube of small diameter, into a tree to get a core extending from bark to the center without cutting down a tree. (Internet 1) This method is used to see the distinct patterns of the wood from trees growing in different climates (Speer, 2010).
A tree ring illustrates one year with two layers in different colors showing the seasons (Figure 1):
A thicker light colored layer which forms in the spring and early summer.
A thinner dark colored layer which forms in late summer and fall. (Internet 2)
Figure 1: (Internet 3)
From the evidence of the difference in the growth of tree-rings, it can be considered that environmental factors, which are climate and temperature, are the primary factors affecting its growth and recorded in the tree-ring sequence (Fritts, 1976).
High temperature in vegetative season makes the tree-ring width wider, but the lower temperature will narrow it down. Precipitation can also be a limiting factor that has the same observations, lower precipitation means thinner tree rings and vice versa. However, precipitation is considered a less effective factor and is sometimes negligible (Fritts, 1976).
Moreover, the color of the tree’s rings near its center tend to be darker. When a tree gets older, the woody tissue of the trunk and limbs divides into two zones: heartwood and sapwood. Heartwood cells are located closest to the centre of the tree, which usually contain tannins or other substances that make them darker and sometimes more aromatic than sapwood, which occurs near the outer part of the stem (Internet 4, Figure 2).
Figure 2: (Internet 5)
But how can you know the exact period that a tree lived in, knowing its lifespan?
A method which can be used to deduce such information is called “Radiocarbon dating”. Carbon has three isotopes: 12, 13, 14, of Carbon 14 (14C) is radioactive. Radiocarbon dating is a method of age determination that depends upon the decay of radiocarbon (carbon-14) to nitrogen (Figure 3).
Figure 3: (Internet 6)
Carbon-14 is an atom that is continuously formed in nature by the interaction of neutrons with nitrogen-14 in the Earth’s atmosphere; the neutrons required for this reaction are produced by cosmic rays interacting with the atmosphere. (Internet 7)
However, carbon-14 is unstable and gradually decays after an organism dies, as it is no longer replenished through respiration.
Carbon-14 has a half-life of 5,730 ± 40 years—i.e., half the amount of the radioisotope present at any given time will undergo spontaneous disintegration during the succeeding 5,730 years into nitrogen.
In contrast, the amount of carbon-12 it has will stay the same. Thus, we can measure the ratio of carbon-14 to carbon-12 and conclude how many millennia have passed since an organism died. (Internet 8)
Figure 4: The decrease of Carbon-14 of an organism with a compatible number of half lives (Internet 6)
Dendrochronology and radiocarbon dating are two methods used to estimate the age of trees. Dendrochronology studies the rings of trees and the patterns they exhibit, which can provide insight into climate changes and human history. Radiocarbon dating is used to determine the age of trees by measuring the ratio of carbon-14 to carbon-12, which changes over time due to radioactive decay. These methods are useful for understanding the environmental changes that have occurred over time and can help us make informed decisions about the future.
Sources:
Fritts, H.C. (1976). Tree rings and climate. Academic Press.
Mees, J. and Seys, J. (2023). Book of abstracts – VLIZ Marine Science Day. VLIZ Special Publication 90, 112.
Speer, J. H. (2010). Fundamentals of Tree-Ring Research. Geoarchaeology, 453–455.
Internet 1: Quora, URL: https://www.quora.com/How-do-scientists-get-a-sample-of-tree-rings-without-cutting-the-tree-down [accessed: 19.03.2023]
Internet 2: Center for Science Education, URL: https://scied.ucar.edu/learning-zone/how-climate-works/tree-rings-and-climate#:~:text=A%20tree%20ring%2C%20representing%20one,because%20the%20trees%20growth%20slows. [accessed: 19.03.2023]
Internet 3: Open Oregon, URL: https://openoregon.pressbooks.pub/forestmeasurements/chapter/4-7-increment-coring/ [accessed: 19.03.2023]
Internet 4: Easy Biology Class, URL: https://www.easybiologyclass.com/difference-between-heartwood-and-sapwood/ [accessed: 19.03.2023]
Internet 5: NASA, URL: https://climatekids.nasa.gov/tree-rings/ [accessed: 19.03.2023]
Internet 6: Global Monitoring Laboratory, URL: https://gml.noaa.gov/ccgg/isotopes/decay.html [accessed: 19.03.2023]
Internet 7: Britannica, URL: https://www.britannica.com/biography/Willard-Libby [accessed: 19.03.2023]
Internet 8: Youtube, URL: (https://www.youtube.com/watch?v=phZeE7Att_s), [accessed: 19.03.2023]