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Thinking About Multi-trunk Trees

Posted: Sun Apr 07, 2019 8:42 pm
by JHarkness

I've recently developed more of an interest in floodplain forests and trees and that has got me thinking about the multi-stem coppices that are the norm in such sites, particularly silver maples. There are always specimens where multiple trees fused together, but many silver maples, at least to my eyes, are clear coppices of same-seed origin. I've seen some where the pith lines reach the ground without quite meeting each other, but in many cases all of the trunks present belong to the same root system, and plenty of others where they do meet but not quite at ground level, add the constantly changing conditions in a floodplain forests including changes in soil ground level, and who knows where the "real" midslope is for the sake of doing a pith test.

What are people's thoughts on measuring multi-trunk silver maples that are clearly from same-seed origin but may not necessarily have meeting pith lines? Would someone go about measuring such a tree simply at the narrowest point of its trunks as long as there is no empty space between trunks? I've seen plenty of (mis)measured silver maples that include massive amounts of empty space and are not impressive trees in their own, again because they are several trunks, but clearly all belong to the same tree, would it not be better to measure them all in some way to record more than just single trunk size?

I don't feel it is right to simply measure the largest trunk of a multi-stem tree when the tree has many other substantial trunks that do belong to it. That would be analogous to measuring the circumference only of the largest limb of a low-forking open grown sugar maple. Thoughts?


Re: Thinking About Multi-trunk Trees

Posted: Mon Apr 08, 2019 7:46 am
by dbhguru

This topic is what the concept of functional circumference is all about. It is designed to level the playing field between single and multiple stems. Here is a post from 2018.


I’m staying in Crestone, CO and am surrounded by pinyon pines of every conceivable shape. Great opportunity to practice with computing functional circumferences. Taking the circumference below the branching yielded 63 points. Functional circumference gave 53 for the four trunks. But functional circumference circumference could have been done in slightly different ways. One wsy is to set the vertical height above the base at 4.5 feet, pass a horizontal plane through the point and take the cross-sectional areas of the stems penetrating the plane. Area would be taken at 90 degrees to the pith lines. Another method is to follow each pith line individually for 4.5 feet to get to the point where the cross-sectional area is taken. The 4.5-foot path will not necessarily be straight. This is Don’s and my preferred way. The formula to compute functional circumference takes several forms. The simplest is:

C = SQRT(SUM(Ci^2))
. Here Ci is a subscripted variable with C1, C2, C3, etc. represent the circumferences of the individual trunks.

This formula is equivalent to taking the sum of the cross-sectional areas of the individual trunks at 4.5 feet above the base and then computing the circumference of a single trunk that would yield the same cross-sectional area.

Interestingly, you've taken a similar path to myself in considering the multi-trunk forms that develop naturally especially in wetlands such as coppiced silver maples. I have no doubt that many represent a combination of more than one seed and also coppicing. We originally attempted to judge these complex forms via pith testing to sort out some obvious cheaters in the national register, but that route, while still valuable, is too rigid when dealing with cottonwoods, silver maples, willows, etc. Members like Matio Vaden warned us of trying to be too pure in the past in terms of the singe versus multi-stem judgments. He was right, but in our defense, our position was partly thrust upon us by American Forests reluctance to address the problem head on. We once proposed two lists or at least asterisking the multi-stems. Today, we're no closer to a solution than wee were during the period of 2013-2017 when we were riding high with AF. However, we believe that the functional circumference method is currently the best compromise.

Jared Lockwood and I will meet on May 6th in Simsbury CT to model the Pinchot Sycamore. Will you be able to join us. We'll stay over that night at complete the job on May 7th. Dr. Susan Masino will put us up at the Gifford Pinchot Inn. The invitation extends to you. Three of us could do the job much better than two.


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