Answer: Are tree rings nested cones or cylinders?

  Simple questions... 

P/C Sevenstorm (from Pexels.com)

... can begin from humble circumstances and can lead to deep insights. 

In this Challenge, I saw the stump of a tree wondered if the number of growth rings near the top of the tree would be the same as the number of rings near the bottom.  I counted the rings and found that it was around 25 years old.  But then I saw another cross section of the tree, but much narrower and taken from much higher up on the trunk, a bit like this: 

THAT made me pause for a moment and wonder--if I could somehow X-ray the tree, how would the rings be arranged on the inside?  

In other words, are number of rings the same at the bottom of the tree as at the top?   

1. Are the rings in a tree trunk arranged as a cone or as a collection of cylinders?  In other words, if you count the rings at the top of the tree, would you see the same number of rings as at the bottom?  (If they're stacked cones, you'd expect the number to be different--if they're cylinders, you'd expect them to be the same.)  How ARE the tree rings organized inside of the tree trunk? 

This turns out to be more complicated than it sounds.  (See what I mean?  Even simple questions about things you see everyday can lead to unanticipated depths.)  

I was thinking of the inside of tree like this: 

Dan's sketch of a tree

And, if you took a vertical cross-section of the tree, you'd see something like one of these two sketches of the very top of the tree:

Cross section of a tree.

The question I had in mind was this: Are the tree growth rings arranged like the one on the left? (Like nesting cones stacked on top of each other.) Or more like the one on the right?  (All of the growth rings flow from a single point on top of the tree.) 

In other words, are the growth rings conical or cylindrical?  

This matters because if you cut the tree at level 1 (see above), the left tree would show only 2 rings, while the right tree would show 3 rings.  So... which is it? 

I did various versions of searches like: 

     [ tree growth rings same at top and bottom ] 

     [ are the number of tree rings the same at the top and bottom ] 

     [ are tree rings arranged in a cone or a cylinder ] 

These searches all led to Quora pages or Reddit pages, most of which disagreed with each other.  Some say the number of rings are constant from bottom to top (like cylinders) while some say the number varies from bottom to top (like stacked cones).  

I asked Bard and ChatGPT this question: 

     [ do trees have the same number of growth rings at
        the top of the tree as at the bottom of the tree?  ] 

ChatGPT:  

Yes, trees have the same number of growth rings at the top of the tree as at the bottom. Growth rings, also known as annual rings, are layers of wood produced by the tree's cambium (the layer of growing tissue) each year during its growing season. These rings indicate the age of the tree and can provide information about the environmental conditions during each year of the tree's life.

The number of growth rings is consistent throughout the entire height of the tree because the rings represent the growth of the tree in a given year, regardless of the height at which you are examining the tree. Whether you're looking at a cross-section near the base of the tree or a cross-section higher up, you will count the same number of growth rings, assuming the sections are from the same tree and the tree is healthy and growing normally.

The uniformity of growth rings throughout the tree is a result of the tree's growth process, which occurs outward from the cambium layer, encircling the entire trunk and branches. This process ensures that each year's growth is recorded as a ring around the entire circumference of the tree.

Gemini/Bard: 

Yes, trees generally have the same number of growth rings at the top as at the bottom of the tree. This is because new growth occurs at the tips of the branches and the main stem, and this new growth adds a new ring to the entire tree, from the top to the bottom. 

Imagine the tree trunk as a long, thin cylinder. As the tree grows taller, the cylinder gets longer, but its overall width stays the same. The growth rings are like the layers of paint on the cylinder, and each new layer is added on top of the previous ones, all the way from the top to the bottom.

Aside from telling me that trees don't taper as they get taller (really, Gemini?), interestingly, and oddly, the Gen-AI answer feature in Google gave a different answer in the SERP than regular Gemini!  (See below. The SERP says "no," but Gemini says "yes."  What's up with that?) 

Okay... How am I going to get an authoritative answer?  

Rather than asking the LLMs, or believing the conflicting stories in Reddit and Quora, I went to Google Scholar and Google Books.  Both have pretty reliable content, most of which is peer-reviewed (or in the case of Books, at least carefully reviewed by an editor and publishing house).  

In Google Books I did: 

     [ tree ring growth same at top of tree as at the bottom ] 

and found several books of interest, but one in particular stood out.   Tree rings: timekeepers of the past. Phipps, R. L., & McGowan, J. (1993). US Department of the Interior, US Geological Survey.

In this book (published by the USGS, a reputable organization) you'll read: 

Looking down at a stump top, it may be difficult to visualize how tree rings go together to make a tree. Perhaps this is because a tree is a three-dimensional object and a stump top displays only two dimensions. The idea of three-dimensional growth can be clarified, however, by disregarding for the moment the leaves, branches, roots, and bark, and considering only the wood portion of the tree trunk. At the end of its first summer, after germination from a seed, a tree seedling is composed of one annual growth increment or growth layer. This somewhat cone-shaped increment, a fraction of a centimeter in diameter and only a few centimeters high, will not grow or change dimensions after the year in which it was formed, but will remain the same size and in the same position as long as the tree exists.

During the second year, another annual growth increment is formed around and above the first year's increment. At the end of the third year, an increment has been formed around that of the second year, and similar cones of growth are formed in each succeeding year. The growth increment of each year thus increases the diameter and height of the tree trunk.  

Most intriguing, it also has this diagram: 

Figure from page 7 of "Tree Rings" (1993) 

Which shows pretty clearly that the correct model of tree rings is that of stacked cones.  (And is pretty similar to my hand-drawn sketch above.)  

But... just to be sure, I wanted a verification check. So, I also did a search in Google Scholar for papers on this topic.  There, my search was much shorter and to the point: 

     [ cross-section tree height ] 

That led me pretty quickly to a technical paper "Two models for predicting the number of annual rings in cross-sections of tree stems" published in the Scandinavian Journal of Forest Research (2007).  

I won't bore you with the details (it's a fun paper to read, but you have to be willing to read through the math), but the key insight for me was that one of the key parameters of the model is "Number of annual rings (cambial age) in cross-section at height h", which strongly suggests that the number of tree rings varies by height.  (Looking at the data in detail shows that it does.)  More support for the idea of "stacked cones" rather than a simple cylinder.  

  

2. If tree rings are the fingerprints of a tree, I know that cloned trees can have very different ring patterns, but what about humans?  Do identical twin humans have identical fingerprints, or not?  

This was a much simpler Challenge.  My query was much like Ramon's: 

     [ identical twins fingerprints ] 

which quickly leads to two articles from reputable sources.  Medline Plus and  Healthline both of which tell us that: 

The finer details of the patterns of skin ridges are influenced by other factors during fetal development, including substances taken during pregnancy and the environment inside the womb. These developmental factors cause each person’s dermatoglyphs to be different from everyone else’s. Even identical twins, who have the same DNA, have different fingerprints.

And there you have it: reputable source and a verification from a second reputable source.  

SearchResearch Lessons

1. Even simple questions can be difficult to answer.  I guess that's often the case (standard example, "why is the sky blue?" is tricky), but you'd think searching out the answer would be straightforward. As you see, this isn't always the case.  Here we had to dig into the Books and Scholar literature to get a decent answer. Which reminds me... 

2. (Once again) Don't trust the LLMs for answers to tough questions.  It's bad enough that both ChatGPT and Gemini/Bard got the answer fundamentally wrong, but the fact that Gemini gives different answers between the SERP and the regular chat interface should give you a clue that something's really weird here.  

I'll say it again:  Always verify. Always get a second source (and asking another LLM is NOT validating.)  

Keep Searching!