Unfortunate I think that may be misleading. If the plant isn't grown in the same conditions the size of the rhizome could vary with age, variety, soil composition, sunlight, dormancy periods, watering, etc.
Trees are very different than VFTs. Think of tree growth like a bunch of cylinders inside of each other, VFTs are more like a line of blocks. Each time you add a block to one end, generally at the other end the old block dies off. So you can't really tell the VFT's age from the size alone as you can't count all the blocks. Trees maintain their inner cyclinders so you can determine their age. The best you could do is statistically guess at the age based off the size of the rhizome.
</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote </td></tr><tr><td id="QUOTE">True, but wouldn't the same variants cause a difference in the number of rings on a tree[/QUOTE]<span id='postcolor'>
I don't think it would affect the number of rings, just the size of each ring. Each ring represents a years growth, a bad year would result in a thin ring, and a good year a fat one.