Crown Considerations  Bob Leverett

TOPIC: Crown considerations

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Date: Sat, Nov 17 2007 1:11 am
From: dbhguru


In a project of Lee Frelich, I am measuring tree height by both sine and tangent methods, I am choosing to measure the "apparent top "by both methods. I then search for and measure the highest point. The reason we are taking this tack is that the apparent top is what is measured by the users of clinometer and tape measure only. Very few clinometer only users take the extra time to cross-triangulate different tops in order to form the correct right triangle for each point they are measuring. But even for those who do, they are still measuring the height of what they have chosen as the top and the odds are they have simply chosen what appears to be the highest point from their chosen vantage point. However, while the actual top of a tree doesn't change, as we change our location, the apparent top does - at least if we move far enough.

Ents who do tree measuring understand what has been said above. But there is more to discover from measuring tree height than just the comparison of two measuring techniques. We can determine the facts about the crown architecture of different species as we measure the apparent and true tops of the chosen species. We will quantify what our eyes readily see. For example, the tops of young to mature white pines are fairly well centered over their bases. The tree maintains apical dominance well into adulthood. Consequently, using sine-based or tangent-based measuring will often produce close to the same result. However, a 160-foot old-growth white pine in Cook Forest with a flat crown and a pronounced curve in its trunk is a very different animal. At such great heights, a forward thrusting limb can easily be mistaken for the top. The concept of the apparent top can easily be demonstrated for the old growth form and there will almost certainly be a pronounced difference between t
he height measurement from our sine-based procedure and the common tangent-based one for either apparent or true top. We can trace the change of form and location of the top for the white pine as a function of time. The same can be said for hemlock and spruce. The latter species can usually be measured even in its old growth phase with the tangent method without significant loss of accuracy.

The idea of an apparent top makes more sense when you are far enough away from a tree to see multiple high points at the same apparent distance away. They aren't actually equdistant. They just seem to be because our binocular vision can't resolve a small difference in horizontal distance of a few feet or yards at distances of 200 or more feet. The farther away two objects are from us, the less we can distinguish which is the actual farther f the two. We judge by other criteria, e.g. our expectations. In terms of several apparent tops of a tree that look equidistant, the one that sticks up the highest can be declared the apparent top. Another candidate is the one closesst to the vertcal line extended upward from the trunk. Such a choice feeds our notion that the true top is close to the vertical trunk line. Our choices will be limited for conifers, but broad spreading hardwoods have ideal shapes for demonstrating a choice of potential tops, with the winner being at the highes
t angle.

Those of us who measure trees frequently routinely search for the highest twig, but we do it with a set of expectations again for each species. We get very efficient at quickly sorting through the contenders and locating the highest point. From the outset, what is likely to prove to be the highest point seems "apparent" to us. But that won't be the case for the less experienced. I have often looked into the crown and chosen a leader that others with me hardly noticed. So to choose what is likely to be seen as the top by others requires that we momentarily suspend our experience and just scan the points that stick up the highest; i.e. are at the highest angle and maybe judge whether they are a top or a forward thrusting limb in only the most obvious cases. Those cases will increase as we move closer to the tree, but then the points farther into the crown will recede from view. So, we will have to move farther back until they reappear. At some point, we see the big picture an
d make our choice.

In time, ENTS can accumulate sufficient data to describe the changing shapes of crowns for the species we study. Our data will add to our overall understanding of each species.