How are dead or fallen trees dated without knowing the final date of the end growth ring?

Submitted by YugeChungus t3_11i03ur in askscience

Currently looking at a fire scar data set for trees throughout North America. The data set includes fire scars identified back to around 257 CE. The data set also includes the final growth year (e.g. 1810). I could understand how the dating occurred if there was recorded dates when the trees were cut down, but a lot of the data came from dead trees with no record of their final growth year.

What are some ways trees could be dated without knowing the final growth year?

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IOnlyHaveIceForYou t1_jaxrmgd wrote

Trees growing in the same region will share patterns in the sequence of growth rings, reflecting climatic fluctuations. If you have a succession of such trees with overlapping dates, you can date over extended periods.

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mysilvermachine t1_jaxrn3q wrote

Dendrochronology works by comparing the sequences of variation in growth rings. Not every growing season is the same everywhere and there is more growth some years than others.

Over time massive data sets have been developed that allow growth sequences from different times, and locations to be compared.

So the variation in rings is compared to find a match.

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JohanZgubicSie t1_jaztbk4 wrote

That is really interesting to know, thank you! are such data sets publicly available?

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mysilvermachine t1_jb00lzb wrote

Usually in academia these things work on a collaborative basis, so if you are part of an institution that cooperates and contributes then the data is available.

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Ok-Championship-2036 t1_jb083up wrote

The growth of trees record the level of rainfall they received. There wouldn't be a reason for the tree to continue growing without showing evidence. when the tree dies is when it stops growing, not before.

We can date wood from lumber, even when it's from an archaeological site hundreds of years old. This is because we actually have a databank or library of tree rings. We can compare the exact measurements of each ring to the local weather data and just count backwards or forwards. So basically, you don't need to count every single ring, you just need to find a reference point in your local rainfall and then count from there.

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