Hazard Tree evaluation

"I've just been trying to wrap my head around the whole ratio of strength loss due to defects vs remaining strength of live wood"

So are many other people, but there is no such ratio. At some point you have to just spec it branch-by-branch.

"what mitigation or strength guide lines do you go by"

15% reduction -> ~50% gain in stability is a General guideline. The overriding problem seems to be this keep-or-kill decision that is imposed by our risk assessment training. We manage tree risk, but the authors thought they could avoid liability by saying "O no, we just assess risk." The result is a lot of CYA removals.

If we get our heads out of the Recommendation paradigm and just list mitigation options, we can stay within our competence and avoid the stress of playing God.
 
guymayor said:
"I've just been trying to wrap my head around the whole ratio of strength loss due to defects vs remaining strength of live wood"

So are many other people, but there is no such ratio. At some point you have to just spec it branch-by-branch.

"what mitigation or strength guide lines do you go by"

15% reduction -> ~50% gain in stability is a General guideline. The overriding problem seems to be this keep-or-kill decision that is imposed by our risk assessment training. We manage tree risk, but the authors thought they could avoid liability by saying "O no, we just assess risk." The result is a lot of CYA removals.

If we get our heads out of the Recommendation paradigm and just list mitigation options, we can stay within our competence and avoid the stress of playing God.
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Guy
Just to be sure are when you say gain in stability that's the same as load reduction?

Where did you get the 15% reduction = 50% gain in stability? I've been reading different papers but I'm finding numbers that are more like doubling factors. So your 15% would reduce the load by more like 30%.
But I've been reading about load reduction, I know your are picky about your words, so I'm wondering if it's the same thing.
 
yeah that's a better way of saying it; 15% length reduction ~=50% load reduction. Thanks!
Look for papers by matt Follett, John goodfellow, et al.
With the variables of course hard numbers are hard to come by. "Take the whip out of it" is a spec I saw a lot of in Sweden. ;)
Even so, what kind of doubling factors are you finding?
 
guymayor said:
yeah that's a better way of saying it; 15% length reduction ~=50% load reduction. Thanks!
Look for papers by matt Follett, John goodfellow, et al.
With the variables of course hard numbers are hard to come by. "Take the whip out of it" is a spec I saw a lot of in Sweden. ;)
Even so, what kind of doubling factors are you finding?
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I figured it was one in the same.
What I meant by doubling factors is reducing 10% of the crown comes out to be a 20% load reduction. I've got the book 'how trees stand up and fall down' and a few other sources but I'll try looking into those names.
The whip rule is a good one.
 
So I'm putting this up to not throw off red's thread.

1. I can get behind when Guy says if you see a tree standing still, you don't need to see anything else the tree is good enough.

2. As Red puts out, if you reduce that tree at all even with 3' 1/4" branches it'll be structurally more sound because there is less than there was.

3. I deal with silver maples than have had large cuts in the past (year unknown) that have led to defects (hollows) know as guy says who cares not a big deal(my summary not his).
But these trees would need a regular pruning cycle to make sure they got no larger especially if you account for girth growth weight.
As well a few of the trees I have been able to visit more than once even in one cycle of 5 years the cavities have worsened(expanded deeper into the tree).
These trees are often township trees and they react on a complaint basis not a regular routine. So they may specify prune back from house or over road but not what the tree actually needs.

So how do you respond (ideally) to trees like that the trees grow bigger as the same time the cavities get worse. Whether it's due to squirrels chewing through the codit or whatever.
 
Is there any table or charts that guides you on this? I know you likely don't cause the rules should go out the window on a lot of this but how do you gauge if the tree is growing and the defect is worsening? The whole it's there now so it's fine is ok, but weight to structure is changing in the opposite way than we usually like to see that can make people eire.
 
Kevin,
It has somehow been assumed that because I often make cuts 1' to 3' long that it is what I suggest a full tree prescription. I do think that these small cuts are important. But the most important are the largest cuts in the prescription and where there made. I just had half a prescription written up and lost it. Damn phone. I'll try again
 
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So I'll do this in three posts to avoid losing it again
In a large silver has less than ideal structure but no decay the lengths removed might top out at 12'. Diameters at 2.5". But in a large silver with medium but contained decay AND less than ideal structure the lengths removed might top out at 18' and 4" in diameter. 'Contained' is not easily concluded on so monitoring like Kevin mentioned is important. Perhaps even a tomograph as a baseline then a few years later, after reduction, a tomograph to see the change in decay relative to wood growth added.
 
So assume 36" dbh silver with less ideal structure and medium decay. Tree has good vigor. Multi stem spreader. 40 foot crown radius to be reduced by an average of 12 feet. Retrentchment. Unnatural finish due to decay. Risk reduction is traded off for lesser aesthetics. Reduce with control before Mother Nature does with chaos. Pics of tree later.
First the big cuts. 1 length of 20 feet. 7 lengths at 14-18 feet and 3-4 inch diameter. Made at the ends of the dominant stems and limbs.
Then 15-25 lengths at 8-14 feet and 2-3 inch diameter. These Lengths are taken off the ends of the branches which take over the lead where the 3-4 inch cuts were made AND to reduce branches that reach the new crown edge.
Now the smaller cuts. This is what makes the prescription thorough and helps complete retrenchment, hopefully causing shoots in the inner crown. 25-50 lengths at 4-8 feet and 1/2 to 2 inch in diameter. 25-50 lengths of 1-4'. The smaller of these small cuts (1'-6') are only made assuming vitality is good.
If vitaliy is bad a sooner second app might be suggested and less of the small cuts are made or are made as 1' long tip cuts to invigorate. Even less of the 8-14' cuts might be made on the portion of the tree that has no target. I've rarely applied this as I usually deal with minimal decay and poor structure. I did apply this to an oak and slightly less to two other oaks all reds. This prescription also assumes the decay involves all stems (decayed main trunk) so one stem would start with maybe 2 cuts at 4" or one at 6". Now that I look at it this is meaningless without being in the tree and adjusting prescription accordingly. With time you get closer at cutting the diameter You've prescribed and the number of lengths. But if you send a tree worker up to follow a prescription, how is he going to know what changes to make? What if he goes slightly larger to climb out less? I don't know sweat buckets or get a bucket truck? Any way the tree I'm referring to is in a backyard. Spider?
A front Boulevard tree might have these quantities halfed or less due to less crown
 
Another way to look at it:
risk is determined to be high and decay is not just hollow, it also includes bark. The decay area also has lots of good wood to compensate. However the tree is heavily crowned and has proved the decayed stem holds up a large crown. So it will hold up a crown with half the leverage forces (say 1/4 off the top reduction) twice as easily. Or will handle much more intense storms. But for how long is it overbuilt? The answer lies in monitoring. But how to answer this question. At what point does the progression of decay exceed the progression of growth of the good wood? And furthermore, when does this catch up to the improvement achieved by the reduction? The reduction buys time in which we can monitor. tomogrAphy?
 
The concept of wood being added to the outside while being lost from the inside is central to the old forestry concept of "pathological rotation" and production of wood volume or biomass. The goal was to select for harvest those trees that were passing through that part of the curve where the tree is losing more than it is gaining.

But for arboriculture, I see more failures related to cracks than to simple reduction in width of sound wood around a cavity. I'm much more concerned about cracks than about hollows or voids. Or am I out of touch?

In reviewing the earlier posts on this thread, I see yet again that Guy advanced the above point before I did. Still, might bear repeating. I know we want prescriptions, but how precise or exact do we want to pretend to be?
 
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In Toronto, we have a growing demand to retain trees with decay. And this is a good thing. Mostly and as long as risk is mitigated. Often decay involving long cracks or vertical sections of exposed decay.
Once reduced, the risk is mitigated, not gone. And I never suggest any exact conclusion. I simply say I'm going to reduce risk by reducing the loads in storm events. But risk will come back in time. Maybe 100 years. Maybe 10 years. So how do we know when? After reduction the monitoring is key. And if the decay is low on the tree the client should be told to take regular observation. Not to replace arborists but to alert. These outcomes are not measurable. And the prescriptons are a guideline at best. And the ones I've suggested are particular to a certain circumstance. They are speculation if nothing else.
I do agree Kevin that hollow is not bad. And cracks are a good point. when accompanied by long vertical exposure, hollows and cracks get together, in a bad way. Still mitigatable risk but often a worsening structure. Most structure gets stronger, some gets weeker.
 
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here's a generalization. Probably not new.
Vertical Hollows are relatively strong, vertically short voids even stronger. Hollows might be weekened by holes to the outside air. Hollows are really weekened by long vertical openings.
Cracks with no decay are often a result of overload. If load is not reduced these cracks often lead to failure
Just brainstorming and experience.
As for how much to reduce in each case, is it a guess or an educated guess or an experienced guess? Or mostly all 3?
A prescription is not an answer but might help an experienced guy to instruct a newer guy. Ive used other guys prescriptons and more often their concepts. And it's been great food for thought.
 
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"I see more failures related to cracks than to simple reduction in width of sound wood around a cavity. I'm much more concerned about cracks than about hollows or voids. Or am I out of touch?"

As usual, you are very much in touch with the tree, my friend!

Vertical spread of hollowing is a positive, in terms of weight reduction, flexibility, and recycling waste material.
The toughness of woundwood is vastly underestimated. Usually, it's not factored in at all. I tried to get this topic on the agenda at conferences but have literally been laughed off the program. no science behind that rejection, from what I have seen.
Oh well, gotta keep plugging away!
 
guymayor said:
"I see more failures related to cracks than to simple reduction in width of sound wood around a cavity. I'm much more concerned about cracks than about hollows or voids. Or am I out of touch?"

As usual, you are very much in touch with the tree, my friend!

Vertical spread of hollowing is a positive, in terms of weight reduction, flexibility, and recycling waste material.
The toughness of woundwood is vastly underestimated. Usually, it's not factored in at all. I tried to get this topic on the agenda at conferences but have literally been laughed off the program. no science behind that rejection, from what I have seen.
Oh well, gotta keep plugging away!
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Than unofficially how do you take wound wood strength and factor it in? Pretend the trees in my yard and we're figuring it out over a beer liabilities etc aside.
 
I'm dealing with a similar tree. Large cottonwood, probably 40-45" DBH. The hollow extends from about 1.5 feet above the ground to 5 ft. The hollow is large enough for an adult to climb into. The clients would like to preserve the tree as long as possible. Failure at the hollow is unlikely to cause damage as the predominant lean is away from the building. As I remember from seasons past the crown is still quite vigorous. What Im trying to decide is the correct pruning dose. Should we go for significant reduction or smaller cuts? The tree is old enough that I doubt it's got a significant amount of time left, but then again, that is definitely a hold over from the risk averse, cut em down mentality I'm attempting to move past. I'm trying to promote preservation whenever possible, but I'm finding it hard with the limited experience I have (I've only been at this for 7 years) as the dogma that is taught is to remove if there is any concern.
 
wyoclimber said:
I'm dealing with a similar tree. Large cottonwood, probably 40-45" DBH. The hollow extends from about 1.5 feet above the ground to 5 ft. The hollow is large enough for an adult to climb into. The clients would like to preserve the tree as long as possible. Failure at the hollow is unlikely to cause damage as the predominant lean is away from the building. As I remember from seasons past the crown is still quite vigorous. What Im trying to decide is the correct pruning dose. Should we go for significant reduction or smaller cuts? The tree is old enough that I doubt it's got a significant amount of time left, but then again, that is definitely a hold over from the risk averse, cut em down mentality I'm attempting to move past. I'm trying to promote preservation whenever possible, but I'm finding it hard with the limited experience I have (I've only been at this for 7 years) as the dogma that is taught is to remove if there is any concern.
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Trees like that I find depends whether or not they are wanting a tree care program or a one time fix
 
They are well established clients and definitely willing to be in it for the long haul. I'm finding that most of the knowledge about maintaining and preserving mature trees like that seems to reside in individuals memory banks (gained through personal experience) and a smattering of articles. As has been mentioned, the science really isn't there at this point, much of it is personal and anecdotal. This thread has definitely been invaluable. I'm all for chucking the rules of thumb, but the problem I see is that without some guidelines to teach to, our younger Arborists do more damage than good. At about 5 years in this industry, I finally had enough knowledge to understand how little I actually know.
 
If they're ok with routine visits (again hard to say without seeing the tree) but I go for reducing medium size branches that are 'out of structure/shape'. Think of that perfect scaffold structure take out the big extras, then in a few years do it again but with the lighter handed approach of smaller branches, a few years after that hopefully suckers have come in so you can 'groom' the favoured ones and entrench the crown after that monitor and clean out the dead wood and it should be ok you hope depending where you are that could be a 6-10 year plan.
 
I would make a special effort to observe the tree during a major wind event.
If that can't be done than you should set lines with the weight and against the tree wt and make observations. Listen for fracturing, look at the deflection and record how much pulling you are doing to get it to sway, the type of sway/in one or two planes and then prune accordingly. I always prune more in most cases to bring the cycles to failure to very low. Have the tree owners present more observations the better and helps to get people confident in the decision and the prescription. Always prune based on observations. No point cutting something up or removing something solely based on speculation.
 
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