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Measuring progressing lean
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<Mark Goodwin>
posted
Has anyone had occasion to measure trunk lean over time? Perhaps something like that used in measuring geologic movement along fault lines? The image I have is of a survey transit used to measure movement of a target on the trunk. It seems to me that a leaning tree of great size might be monitored to see if it has a progressive loss of footing before failure. If The initial measurement was taken from a point 90 degrees from the perceived direction of lean, then couldn't future observations be made from that same point in order to detect change in trunk position? Of course normal movement by wind would have to be ruled out. Interpretation of observed change in lean would have to be considered along with other things, such as ground level changes. Any ideas on this?
 
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<Mark Hartley>
posted
Reply to post by Mark Goodwin, on January 23, 2000 at 23:32:15:

Mark'

A digital level is a useful toy. You need to watch out for the influence
of reaction wood on your reading whatever system you choose.

Mark
 
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<Scott Cullen>
posted
Reply to post by Mark Goodwin, on January 23, 2000 at 23:32:15:

Mark, I think the geometry would be pretty straight forward. You could look for pretty significant movements with a clinomoter but the level of precision and the possibilty of observer introduced variation are too significant for smaller movements. A transit would be much more precise. You would need to lay out a point (A) on the ground 90 deg. below the tree point you will observe and measure a fixed level distance to another ground point (B). Point B should be permanently and precisely marked. If there is slope between points A and B you'd have to correct for it in your calculations. You'd need to establish a clear, repeatable target on the leaning tree... I'd guess that a target on the side of the tree facing you would be easier to fix the instrument on than a point on the lower circumference... makes sure in any case that point A is 90 deg below the target. You'd have to set the transit 90 deg above point B using a plumb bob or optical plummet if the instrument is equipped with one. You'd have to be sure the instrument is a consistent ( or at least known) height above point B. If there is any variation in height above point B or slope between points A & B 90 or angle between point A and target 90 you can adjust the calculations but you'd have to know that the variations exist. I think the same requirements would apply to a digital reading level with or without laser targeter. An of course keep careful records. If Wayne or Russ are reading he can probably give you the equations.

On the practical side you'd have to rule out seasonal stem deflection from foiliage weight or annual defelction as overall weight increases. You'd proably want to look for stem symptoms of stress like cracks, or ribs or buckling and for uprooting symptoms like lifted soil/roots on the side away from the lean or depressed soil/roots on the side toward the lean.
 
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<Colin Bashford>
posted
Reply to post by Mark Hartley, on January 23, 2000 at 23:32:15:

Here in the UK we are at times somewhat backward in the use of scientific equipment and instruments. Maybe it that we are so hard up or just downright mean that we have to come up with simple solutions. They say that necessity is the mother of invention so here is what we do.
on the inside of the trunk lean, that is the acute angle side we mark off a selected point up the trunk from a set and marked point at ground level. From this we hang a plumb-bob of a recorded length so that the measuring can be repeated at other times using the same known fixed points. It is then a simple task to record the distance from the line of the plumb-bob, extended down to ground level, back to the base of the tree.(Distance X). It might not be precise, but it is easy and inexpensive. I anticipate comments about an increase in basal stem diameter but consider these are minor factors compared to the movement of a potentially dangerous tree.This method clearly shows any significant movement in the tree since the last measurement. Any increase in lean will increase the distance X.
Colin
 
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<Scott>
posted
Reply to post by Colin Bashford, on January 24, 2000 at 03:58:57:

A very simple and straightforward approach. Are you always able to select a point on the tree taht you can reach from the ground or do you use a ladder?
 
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<Russ Carlson>
posted
Reply to post by Colin Bashford, on January 24, 2000 at 03:58:57:

Colin, you beat me to this response. I have used it track movement of trees, and it works well, if not with high precision.

I start with a set attachment (nail) at a practical height in the tree. The higher the set point, the more accurate the precision, but you have to watch for other influences (wind, bending, etc.) Set the plumb bob, and install a marker below. I have used wood stakes, driven in to the ground line, with a small surveyor mark on top. More permanent markers could be used. Just be sure that the setup is repeatable, as to where the plumb is set.

Now, if there is movement, it can be measured by the offset of the bob from the marker point, and the difference in lean calculated. Simple, and like most things with trees, close enough for practical work.
 
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<Mark Goodwin>
posted
Reply to post by Colin Bashford, on January 24, 2000 at 03:58:57:

Colin, I like simple and easy. I think it would be important to mark the exact point on the base of the tree, maybe drive in a small nail there, so that it would always be the same place. Also, you hang the bob first, right? Then, mark the point below it and measure to the trunk? Also, do you drive a nail to hang the bob?
 
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<Russ Carlson>
posted
Reply to post by Scott Cullen, on January 23, 2000 at 23:32:15:

What is the required precision? Laser levels, setup time, etc., may be overkill for the application. If you have the equipment, and time and cost are not significant factors, that's a good method.

THere is some software being marketed to predict tree shadows, directed toward golf courses, but useable by anyone. It claims a very high degree of accuracy. My point is, how much accuracy do you really need? We know trees are not static objects, by quite dynamic in the environment. The environment itself is constantly changing. What's the point in predicting the sun's location within a few arcseconds, when the sun itself covers over 1/2 degree of arc? The shadow is not highly defined anyway. And until you have an exact 3D model of the tree's structure and canopy, there are other inaccuracies introduced.

So here's my marketing pitch- Why spend thousands of $$ when my Tree Shadows shareware will do just as well? Check out the Tree Tech Solutions CD link at the top of this page.
 
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<Colin Bashford>
posted
Reply to post by Scott, on January 24, 2000 at 15:22:29:

Dependant on the lean, realistically if the tree was so upright that a ladder was essential, then the lean might be so slight as to make measurements difficult.
 
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<Colin Bashford>
posted
Reply to post by Mark Goodwin, on January 24, 2000 at 15:22:29:

Mark, folowing the keep it simple ethic the answers are Yes and Yes, but see also Russ's comments.
 
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<Scott Cullen>
posted
Reply to post by Mark Goodwin, on January 23, 2000 at 23:32:15:

Re-reading this thread I think there's a slight confusion. It concerns whether you measure movement in the A) vertical (i.e. has the angle of the trunk moved away from the vertical) or B) horizontal (i.e. is the point on the trunk closer to the ground) plane.

If you measure A then the distance from the permanent ground point back to the trunk can be important... but you could also use Russ's method and just measure the distance (if any) that the plumb bob has moved away from the permanent ground point... that movement would be equal to the change in distance to trunk. As Russ and Colin both point out, the higher the point on the trunk is the more sensitive this method will be to movement. While you could do the math to employ the change in tree point to ground point distance (plumb bob length) it would be very small while the trunk angle with ground is above 45 degrees (i.e. closer to vertical than to horizontal). I'd guess this method is most suited to this situation.

If on the other hand trunk angle with ground is less than 45 degrees (i.e. closer to horizontal than vertical), the movement off vertical would be less aparent and the movement toward the ground more so. In this instance you might measure plumb bob length to (level) ground. Any decrease in this length would mean an increase in lean and the fixed ground point is really not necessary.
 
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<Julian Dunster>
posted
Reply to post by Mark Goodwin, on January 23, 2000 at 23:32:15:

One other factor when measuring is the influence of time. Out here in the wet coast it is common to see conifers on slopes with a swept trunk, usually an indication of a soil creep problem, but can be the weight of snow cover under gravity when the tree is young. If one was to measure trunk angle at an early enough point, it might be conjectured that the tree was in danger of toppling, but often, geotropism takes over and the tree regains a vertical angle higher up the stem after a number of years.

I also think that we should consider more than just the inner angle issue. I often see red alders with extensive lean, but over time they do not uproot, but rather, the trunk snaps as the end weight becomes too great. Measuring the lower trunk angle alone would potentially miss this other affect.

But then again, maybe you were not originally contemplating that sort of an issue anyway.

Julian
 
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<Mark Goodwin>
posted
Reply to post by Julian Dunster, on January 23, 2000 at 23:32:15:

Thankyou for these additional thoughts. I have seen in a book the very thing you mentioned about the top of some trees straightening out after some earlier effect made them lean. I have also seen it in some of the large Ponderosa pines around my house. I was thinking mainly of loss of footing, but it is good to be reminded that there are other potentials to consider, such as end weight breakage. The tree which got me thinking about all this is a California foothill pine (Digger). In the wild they often lean over, especially as they attain their full size. This one has a raised soil area on the outside of the lean. However, there has been no noticeable increase in this feature. At about 100 feet tall and several feet through the trunk, and dropping cones that weigh pounds and have wicked "claws", it is a formidable tree. It is also located as a specimen in a lawn on a populous campus, which is what gives me concern. These trees generally all lean as they grow, but they also usually inhabit fairly sloping ground too. I think it may be useful to get a better understanding of the species, especially to see what the typical pattern of failure is in large old trees, whether from the roots or from trunk breakage. I guess I'll have to take some hikes and have a look!
 
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<Ed Milhous>
posted
Reply to post by Mark Goodwin, on January 23, 2000 at 23:32:15:

Anytime I have been asked about a tree and its lean, the question was more urgent... along the lines of, "Betty Lu says this tree on its way down right now, but I don't think it has moved a bit." When someone does not want to wait to see if further movement occurs, an old photograph often will provide good information. Try to position yourself at the point the picture was taken from, and line up a vertical object that is in the photo (such as a fence or building wall). Then compare the angle of the tree's trunk in the photo with what it currently is. Any change at the ground/trunk intersection will show up more significantly toward the top of the tree. This will give you an immediate indication of whether the tree has moved. (You do have to watch out for wide angle or telephoto lens distortions, though.)
 
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