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How tree rings tell time and climate history

Most of us learned as children that the age of a tree could be found by counting its rings. Rings of trees growing in temperate climates can indeed tell their age through their annual rings and also help determine the age of wood used to construct buildings or wooden objects. The ages of wooden objects can be revealed by cross-dating, the process of matching ring patterns between wood samples of known and unknown ages.

Tree rings

Concentric rings of various widths mark the annual growth of trees. Photo by Peter Brown, Rocky Mountain Tree-Ring Research.

What do tree rings tell us

The underlying patterns of wide or narrow rings record the year-to-year fluctuations in the growth of trees. The patterns, therefore, often contain a weather history at the location the tree grew, in addition to its age. In dry environments, such as the Middle East or U.S. Southwest, tree rings typically record wet or dry years, and in cooler areas (high latitudes or high elevation), the ring widths are often a proxy for temperature.

Archeologists have used the ring patterns in building timbers to estimate construction dates for some of the world’s most famous buildings, including the cliff dwellings at Mesa Verde National Park (nearly 1,000 years old) and the Church of the Nativity in Bethlehem (nearly 1,500 years old).

Mesa Verde

The Cliff Palace ruins at Mesa Verde were built in the 1200s by the Ancestral Pueblo people, but were abandoned in the late part of the century. Photo by Ken Lund, used under a Creative Commons license.

What's in NOAA's tree ring data base? 

NOAA National Centers for Environmental Information (NCEI) houses the International Tree-Ring Data Bank (ITRDB), which contains ring width data from forests worldwide, plus ring width data from old buildings, and even from rare Stradivari violins. The ITRDB contains ring width data from trees at over 4,600 locations on six continents, providing tree growth histories from around the world. New additions from field scientists are added regularly.

International Tree-Ring Data Bank

The ring patterns of more than 6,000 trees (green triangles) have been archived in NOAA's International Tree-Ring Data Bank

Climate scientists compare the tree growth records to local weather records. For locations where a good statistical match exists between tree growth and temperature or precipitation during the period of overlap, the ring widths can be used to estimate past temperature or precipitation over the lifetime of the tree.

In many parts of the world, trees can provide a climate history for hundreds of years, with some extending back 1,000 years or more. The resulting climate histories enhance our knowledge of natural climate variability and also create a baseline against which human-induced climate change can be evaluated. NCEI archives these climate reconstructions in addition to the tree ring measurements. 

Glimpsing the past

Tree ring data have been used to reconstruct drought or temperature in North America and Europe over the past 2,000 years. For example, tree ring based drought reconstructions for the American Southwest indicate a period of prolonged drought in the late 1200’s.  Archeologists believe that the drought was a contributing factor in the Ancestral Pueblo People abandoning the famous cliff dwellings at Mesa Verde, never to return.

Southwest droughts since 1200s

Severe drought in the U.S. Southwest in the late 1200s likely contributed to the abandonment of Mesa Verde (marked with open circle) by the Ancestral Pueblo people. Drought maps for the years 1275-1290 reconstructed from tree ring records show that over the 16-year span from 1275-1290, only two wet years occurred.  Graphic by Climate.gov, based on summer drought reconstructions from the Living Blended Drought Atlas, courtesy of NOAA and Cook et al., 2010, Journal of Quaternary Science, 25(1), 48-61. doi: 10.1002/jqs.1303. 

Comments

12-6-18 It is my understanding that tree rings sometimes give incorrect temperature compared to actual recorded temperature and have been compromised by manipulation of data(see hockey stick)!David H. Jackson,MD

Tree growth is an imperfect recorder of past climate, because trees are also affected by non-climatic factors including wind and insect damage and competition from nearby trees. These effects are minimized by collecting samples from numerous trees and averaging them together. The resulting reconstructions are validated with rigorous statistical analysis, and not used unless statistically valid. There has never been any manipulation of data to skew results.

In reply to by david h. jackson,md

You forgot one very important variable- rain. A year might have heavier than average rain and be warmer than average but it might instead be colder than average. I see no way to distinguish those scenarios from centuries ago without a matching historical record - regardless of how many trees are sampled. Perhaps other temperature proxies might accurately indicate past temperatures but I doubt tree ring analysis can do that. Since climate change science needs accurate data, just how accurate do tree ring reserachers claim this data is? Joe Zorzin (forester with 47 years experience)

In reply to by Bruce Bauer

I'm not sure what you're asking here. The whole post is about the International Tree Ring Data Bank at NOAA NCEI Paleoclimate Data Center.

In reply to by Peter Rathbun MA

Tree rings have been used as indicators of either temperature or rainfall (actually precipitation, since it may also come in the form of snow), depending on location. In dry areas such as the US Southwest or the Middle East, tree ring widths can match 70% of the variability in measured precipitation for the period of overlap, which is the length of instrumental record (typically about 100 years). When the tree rings are a good match to the instrumental record, they can be used as indicators of past precipitation or drought for centuries before instrumental data are available. The maps above depict Palmer Drought Index, which is a combination of precipitation and temperature, as higher temperatures combined with lower rainfall produce water shortages (drought) which limits plant growth. In high elevation or high latitude locations, tree rings more typically respond to temperature. Statistical matching of ring widths to measured temperature or precipitation is always done, and only trees with strong correlations to the measured data are used in climate reconstructions.

Hello, I am trying to get in touch with Bruce Bauer to ask if he might be interested in speaking about Tree Rings for a webinar we are planning. Thank you.

I have found your tree ring data to be very valuable in double checking my own tree ring measurements from an oak tree palaeoclimate study and much appreciate the efforts you have made in publishing this material in an accessible format.

Hi & thanks for an interesting read. Is there an up to date graph of tree ring data per climate such as 2000-2020? Even something like 1980-2020. I can only find graphs that end 22 years ago. I'm in 2022 post COVID times & am interested to see what temperature data in North America looks like via tree rings for the past 20 or 22 years etc. I'd like to be able to compare 21st century data to older data. 

Thanks so much. 

Mark in Tennessee,

For specific questions like this, your best bet is to contact that the experts with the International Tree-Ring Data Bank (ITRDB), which is maintained by the NOAA National Centers for Environmental Information. Their contact email is located in the footer of the databank's homepage.

Fair warning, though, I think such a dataset could be hard to find because the whole point of tree-ring data is to create a high-frequency, continuous record of environmental variables like temperature or precipitation over very long periods of time, the longer the better, generally speaking. Just like any other dataset, the shorter the period, the greater the level of uncertainty, so scientists aren't going to have a lot of motivation to create such short records.

But...I am not an expert! Perhaps such records exist as part of efforts to correlate tree ring data to climate variables during the modern observation record or something. Good luck!

 

I have a recently harvested log from an ash tree planted in West Wales in 1997. The rings are extremely asymmetrical with the centre very close to one side. What does this tell us about the tree's experience?

Hi, Len. I will pass this question over to the scientist who wrote this blog. I'm assuming you don't know anything about the site location? My hunch would be this is related to a physical characteristic of the site as opposed to climate. For example, the tree grew up touching a rock formation like a cliff or a big boulder. But maybe something like that could happen with strong, persistent prevailing winds.

I have the diameter of a Sycamore tree taken in 2020 and also from a scaled photograph from 1914.  Is there a way to estimate its present age?

As you probably know, the most accurate way to age a tree would be to take a core sample and count the growth rings. But foresters/aborists have come up with ways to broadly estimate a tree's age based on its circumference/diameter and knowledge of the species' typical growth rates in natural forest conditions (where it faces competition). The treehugger website explains how to do this, but unfortunately, the page doesn't include the growth factors for Sycamore (Platanus genus). So you will need to do a web search for its ISA species growth factor. If you can't find it, you could probably contact your county extension agent or the forestry or botany department at your state university.

In reply to by Fraser Forrest

Thanks Rebecca.  The use of a growth factor implies that growth is linear with time.  Therefore with two measurements at different times the age can be estimated.  With my Sycamore, 48" in 2020 and 30" in 1914 the 'birth' calculates to about 1740.  Probably good to within +-50 years.

I am working on a paper, just for fun in the SCA, that explains why climate change in medieval Europe forced fashion to change.  And this data will help others to look up data in their area of medieval Europe.  Thanks for the links

The pines of the Arboretum Park in Seattle are suffering from pine needle fungus which kills the trees and appears to spread among them. 
 

For the past few years park management has been cutting them down as they brown out and die. Now I’m an engineer not a botanist but do find myself observing the stump rings of these cut down trees and what I find is they are all about 85 to 87 years old which matches a period of park development in the 1930’s. 
 

What I see is a clear delineation of growth ring width where they start out quite wide and continue that way for about 60 years then about 25 to 27 years ago there is a marked reduction in growth ring width, it is very easy to see. Every tree that has been cut down shows this, it varies by a couple years where trees higher on the hill show it about 27 years ago while trees toward the bottom of the hill show ir about 25 years ago. 
 

I haven’t found any info that says pine trees slow their growth after 40 years so I see that as an open question? Or am I looking at a change in climate? These are questions for experts to answer, but the change is consistent in its timing and grossly obvious.

 

RJH Seattle

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