Tree Rings: why they form and what scientists can learn from them

You may have noticed the unique rings that appear on tree stumps – and you might even know that you can figure out a tree’s age by counting the rings! But did you know that scientists can use tree growth rings to learn about our past climate and even estimate the date of historic volcanic eruptions? In this post, we will explore the exciting fields of dendrochronology and dendroclimatology – that is, the study of tree-ring dating and the study of past climates using tree rings!

How Tree Rings Form:

Growth rings form as trees grow within temperate zones: trees grow most rapidly during the warm season when their leaves are on their branches, performing photosynthesis. But in the colder season, leaves fall from branches, causing photosynthesis and growth to stall. This recurring fluctuation in growth rate each year creates the annual ring pattern.

Because of this, thicker tree rings form in years with the longest and warmest or wettest growing seasons. Cool or dry years lead to less favorable growing conditions and consequently, a thinner tree growth ring. Scientists can use tree rings in very old trees to learn about our past climate and even estimate the dates of significant geologic events like volcanic eruptions!

Tree Rings and Climate Change

Tree rings can also provide scientists with a valuable glimpse into our planet’s past climate. The patterns of thicker and thinner rings let scientist know when regions were experiencing events like droughts or cold spells: these less favorable conditions cause thinner growth rings during those years. For example, scientists in Scotland are using tree rings to learn about the variations and past behaviors of the jet stream – narrow, fast air currents in our earth’s atmosphere, important to global weather patterns. Increased variations in tree rings in recent times lead scientists to suspect that human-caused climate change has started making the jet stream less predictable.

Additionally, historic forest fire regimes (the timing and frequency of forest fires) can be tracked through tree rings. If a tree survives a forest fire, its ring from that year will often be marked with distinct scaring from the flames. Scientists use the scars and rings to figure out the natural timing of the fire regime for a region before humans interfered.

You may wonder – why do we need trees to learn about past climates? Surely humans have written records of weather events. However, some trees can live to be several hundred to 1,000 years old, or more! Scientists have even found bristlecone pines, a tree species with remarkable longevity, that have been around for multiple millennia – well before humans were able to accurately measure temperature.

The tree rings can also be counted in petrified wood! When available, preserved and petrified wood give scientists an opportunity to see even further back into the past than the oldest trees in an area – just so long as they can figure out when the ancient tree fell or was chopped down.

Tree Rings and Dating Volcanic Eruptions

Severe volcanic eruptions lead to less than favorable growing conditions, due to volcanic debris in the atmosphere. Less sunlight reaching the earth and poor air quality may cause thin or irregular tree rings following an eruption. Looking at the tree rings of very old trees can be a great way for scientists to narrow down the year of a historic volcanic eruption.

One recent example of using tree rings in this way involves a volcano on the Greek island of Santorini. Based on records, scientists knew a significant eruption occurred sometime around 1,600 or 1,500 B.C.E. When looking at an event this ancient, scientists are somewhat limited by the age of trees in the surrounding area – few trees alive today are known to have been alive in 1,600 B.C.E. However, for this study, the ancient wood used to construct a tomb was used. So long as the life of the ancient tree partially overlaps with the life of a tree living today in that area, scientists can use overlaps in the ring patterns of the living and ancient tree to approximate the date it was cut down. Then, using tree rings from the ancient wood, scientists were able to narrow down the time frame and now estimate the eruption occurred close to the year 1,560 B.C.E. This was possible because scientists could detect a chemical anomaly (or sudden change from the normal chemical make-up) in the ancient tree’s growth rings around this time, caused by a sudden change in growing conditions following the eruption.

Scientists can also look at trees growing on top of volcanic debris deposits – the eruption that formed the deposits must be at least as old as the oldest tree growing on the deposit. Scientist must be careful using this method, however. Take for example one well known volcano – Mount Rainier in Washington state, USA. For a while, scientists believed that Mount Rainier had erupted sometime between 1820 and 1854, based on a study involving counting the number of rings of trees growing upon moraines (mounds of debris deposits) that were assumed to have formed from an eruption (Crandell, 1969). However, more recently, scientists have discredited the tree ring study by showing that these moraines formed from older volcanic deposits being moved around by non-volcanic avalanches (Sisson & Vallance, 2009).

How Scientists Observe Tree Rings

Scientists can learn a lot from looking at tree rings – but they do not necessarily need to chop down a tree to do this! The trees that give the scientists the opportunity to see the furthest back into history are the oldest trees – it certainly would not be worth chopping down these incredible, ancient living beings, just to get a look at their insides.

To look inside of a tree, scientists can use a thin, hand-operated drill to extract a thin “core” of wood. Imagine pushing a hollow straw through a layered Jello, to pull out a thin section – in that thin section you can see all the different stripes, representing each layer – or in the case of a tree, each ring! Then the stripes in the core can be counted to determine the number of rings, and the age of the tree. Trees can heal from these small puncture wounds – just like we heal after scraping a knee or getting a paper cut.

Activity: Observing Tree Rings and Tree Anatomy

As a plant anatomy learning activity, next time you see a cut log on the side of a trail, a stump, or some chopped firewood, try counting the number of rings to determine the tree’s age when it died. For additional learning fun, you can also try identifying the different parts of a tree cross section: the bark, phloem, cambium, xylem (sapwood and heartwood), and pith:

1. Bark is the rough, protective outer coating. Bark helps protect the tree from insect and fungi invaders.
2. The phloem underlies the bark. The phloem’s function is to transport sugars. Sugars form in the leaves during photosynthesis, and the phloem moves the sugars to other parts of the plant where they are needed for growth.
3. The cambium is a thin layer of dividing cells between the xylem and phloem. This is where secondary (outward) growth occurs.
4. The sapwood underlies the cambium. The sapwood is the living part of the xylem, which actively moves water upwards through the tree. Water is drawn up through the ground via the tree’s roots. Water leaves the tree through tiny pores called stomata in the leaves, in a process called transpiration.
5. The heartwood is old, inactive xylem tissue. Heartwood no longer has living cells and does not move water. Heartwood provides structural support. Both living and dead xylem parts (sapwood and heartwood) contain tree rings that formed each year as the tree grew inwards from the cambium.
6. The pith is at the very center of the tree trunk and plays a role in the movement of nutrients. In older trees the pith is often very tiny or diminished, relative to the size of the heartwood.

Teach your kids and students about tree anatomy and tree rings with the free learning resource for this article. Download, print, and color in your very own tree anatomy diagram!

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References and Further Reading

1. Crandell, D. R. (1969). Results of Recent Eruptions. In The geologic story of Mount Rainier [E-Reader Version]. (pp. 13 – 23). Retrieved from https://www.gutenberg.org/files/60234/60234-h/60234-h.htm
2. Driedger C. L. & Scott. W. E. U.S. Geological Survey. (August 28, 2008). Mount Rainier – Living safely with a volcano in your backyard. Retrieved from https://geology.com/usgs/rainier/
3. NASA Climate Kids. (n.d.) What can trees tell us about climate change? Available: https://climatekids.nasa.gov/tree-rings/
4. Pearson, C., Salzer, M., Wacker, L., Brewer, P., Sookdeo, A., & Kuniholm, P. (2020). Securing timelines in the ancient Mediterranean using multiproxy annual tree-ring data. Proceedings of the National Academy of Sciences, 117(15), 8410-8415.
5. Robbins, J. (April 30, 2019). Chronicles of the Rings: What Trees Tell Us. The New York Times. Available: https://www.nytimes.com/2019/04/30/science/tree-rings-climate.html
6. Sisson, T. W., & Vallance, J. W. (2009). Frequent eruptions of Mount Rainier over the last∼ 2,600 years. Bulletin of Volcanology, 71(6), 595-618.
7. Wu, K. J. (April 6, 2020). Ancient Volcanic Eruption Dated Through Rings of Dead Trees. Smithsonian Magazine. Available: https://www.smithsonianmag.com/smart-news/ancient-volcanic-eruption-dated-through-rings-dead-trees-180974603/

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