The Fascinating World of Tree Rings: Concentric Counting and Post-Mortem Intervals

You might think of counting tree rings as an offbeat pre-school pastime from your days of finger-painting and making clay ashtrays — but did you know that it’s considered one of the most telling botanical tools in forensic science (along with diatoms, of course)? The discipline is called dendrochronology, which literally refers to the in-built timeline that trees carry around in their trunks. Concealed from the world, each tree keeps it’s own record of the seasons and climatic changes nestled underneath the rind that surrounds them.


Bones and bark

It’s certainly a challenge to estimate the post-mortem interval, or the time since death, when you’ve only got bones to go by. There’s no soft tissue to examine, no organs to analyse and weigh, or abrasions and markings to determine the stage of decomposition at which a given cadaver is. Plant roots, like their above-ground counterparts, carry in them annual growth rings which can be used to date remains.

They’re frequently found alongside with exposed or shallowly buried bodies — and sometimes even grow straight through them (if enough time has passed to eliminate any surrounding structures, of course). If you’ve happened upon a scene where your expertise is necessary to determine the PMI, we’ve got just the thing for you: a three-step process, designed to help you examine these roots and glean any additional clues that might be waiting for you in these concentric rings.


1. Digging, damage and deduction.

A body buried in a shallow grave still requires a fair bit of moving earth. In the event that a grave has been has been dug, the roots involved might be damaged. Keep in mind, though, that roots can be cut but still continue growing.

There are a few warnings here though: if the meristematic zone is damaged (a lining of unspecialised, baby cells within the plant), no additional xylem (a particular type of plant tissue) can be produced. This leaves a sort of permanent scar or lesion on the plant root. The number of growth rings that appear after this scarring shows you the number of years since the damage occurred.


2. Examine the remains of the day.

Roots that have steadily grown into human remains can be removed and cross-sectioned in the laboratory (this also goes for any personal effects that have been buried with the individual). Sectioning at the point of contact means that the annual rings can be counted more easily, and establishes a minimum time frame for the PMI. It might not be as conclusive as the previous method, but it’s still a clue that is valuable to the chase, if there’s no other barometer is available.

Remember though: the contact must be penetrative, meaning that the roots must be growing through clothing, bones or personal items, in order for any interpretation you make to be considered valid.


3. Remember that it grows both ways.

Branches grow longitudinally and radially (meaning length-wise and width-wise). Naturally, these are two additional factors you ought to consider in determing the PMI. If you’re wondering how you’d go about evaluating growth: time-frame can be estimated by measuring the length of a root from its point of contact with the remains to the opposite end.

Whilst you might not find yourself consulting a dendrochonrology handbook anytime soon, its simple and clear forensic application is something quite remarkable to keep in mind the next time you’re out in that leafy park. Here’s hoping you won’t happen upon a cadaver to go with that scenario.


Your turn: We’re sure there are several additional structures of trees and roots that might assist in establishing PMI or contributing to some other aspect of a forensic investigation. If you know any more, please let us know in the comments.



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