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<Wayne Cahilly>
posted
Over this past weekend we have had two enormous limbs drop from Liriodendron in irrigated, public areas. Just curious, does anyone know of a list of species that are particularly suseptable to summer limb drop in the northeast us? I am aware that ash and lindens are frequent contributors but what else? What about irrigation management as a tool for reducing the problem, or mismanagement increasing the problem?

Lots to think about this morning as the guys are bucking limbs and clearing stubs.

Wayne
 
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<Peter Torres>
posted
Reply to post by Wayne Cahilly, on July 26, 1999 at 08:49:28:

Add poplars to your list. I believe the real problem is lack of rays in the cellulose/lignin/hemicellulose structure.
Fruit trees will do summer break-up due to weight of fruit. Some pine trees, to weight of cones and foliage. Whether there is a correlation with "true summer branch drop" and irrigation, I have not read it, although I see your point. It assumes that weight of water overcomes the lack of rays.
 
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<Scott Cullen>
posted
Reply to post by Wayne Cahilly, on July 26, 1999 at 08:49:28:

1) I'd be interested in comments about branch configuration (diameter, length, angle of attachment, combination or proportion of them) in addition to species.

2) Mitigation? If a species/branch profile and target possibility, as in a public garden, suggest a problem is cabling a good option? A) functionally? B) Some managers seem to feel cabling of public trees is not acceptable and tree ro brance removal is the only option (concern about liability if the cable fails to prevent damage). I personally feel that the manager may be able to mitigate more hazards on a given budget and maintain more canopy with prudently selected and properly installed cables. There's also the issue of future decay at the site of removal of a large branch. What do others feel?

3) With regard to Peter's response: Is irrigation suggested in the literature as a contributory cause of this problem or as a possible managment strategy. I think Shigo has suggested that branch wood (all wood but we're talking about branches) looses elasticity as it looses moisture and that lack of elasticity can lead to fracture or delamination and then fracture. Stress moves to strain moves to failure. (Sorry I can't cite particular references). I've intuitively associated "Summer Branch Drop" with this mechanism. What's the real story?

Is the "fruit weight failure," which might well be associated with abundant seasonal moisture distinguished from "Summer Branch Drop?"
 
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<Mark Goodwin>
posted
Reply to post by Wayne Cahilly, on July 26, 1999 at 08:49:28:

http://treefail.ucdavis.edu/plscripts/treefail/tfrp.pp?xqt=index.pp&browselvl=3

You might find useful information through this site, although based in California.

As for species I have seen exhibit what I think is sudden limb drop, Liquidambar, sycamore, American elm top my list.

By the way, I observe a branch failure in Italian stone pine in numerous trees every year. The branches twist and then split open along the top a few feet from the trunk.
 
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<Wayne>
posted
Reply to post by Mark Goodwin, on July 26, 1999 at 08:49:28:

Sycamore!! as it Platanus? Well that damages the medulary ray theory. Sycamore has more rays per sq.cm. than anything I know of.

I have seen limbs on Liquidambar drop under these conditions. We have several very wide spreading L. styriciflua that seem to drop something nearly every time the wind blows, but they have shed limbs under the current condtions as well.

The list increases!
 
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<Mark Goodwin>
posted
Reply to post by Wayne, on July 28, 1999 at 15:36:01:

Wayne, I may have confused the issue re: the sycamores. The trees I have seen drop limbs every year have been topped in the past, and it is possible that some of the limbs falling are large epicormics. It seems to be particular specimens, not sycamores in general.
 
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<Ken Six>
posted
Reply to post by Wayne Cahilly, on July 26, 1999 at 08:49:28:

I thought summer limb drop had to do with the exspansion of wetwood during (hotter) summer months. Not to be confused with limb or branch failure. Ken
 
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<Wayne Cahilly>
posted
Reply to post by Ken Six, on July 26, 1999 at 08:49:28:

Ken,

By my figuring, and given the fragmentary info in the literature, I have the Summer Limb Drop scenario kinda like this:

Summer limb drop seems to be most likely when preceeded by a dry spell of no less then seven days, in the mid to late growing season. Soil is dry, the tree is still actively growing, the tree has all the leaves it will have all year (therefore it also has the potential to be the heaviest it will be all year if fully hydrated) and then you get a rain, followed by a temperature spike exceeding 90 degrees.

Given this set of conditions, the tree begins to tank up on the new moisture in the soil, temps reach the 90's perhaps by noon or one in the afternoon and the tree begins to close down stomates to conserve moisture, the result is the curve defined by water-weight suddenly begins to climb, and sometime between 2:00 and 4:00 bango, a limb drops. Simplified to be sure, but it fits the observations that summer limb drop is an event that occurs between 2 and 4 on hot, still, mid to late spring days, after a rain that breaks a short drought.

What do you all think? This is why I asked about irrigation management to mitigate or exacerbate the situation.

Wayne
 
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<Russ Carlson>
posted
Reply to post by Wayne Cahilly, on August 10, 1999 at 20:52:43:

I agree with the basic scenario, Wayne. Although I would extend that time frame- I've seen cases where the limbs fell in early evening hours, as late as 6 or 7 pm. The key is the rain after a dry period.

In almost all cases I've looked at, there has been some sort of 'defect' in the limb, often old pruning wounds or stubs, and usually closed over for some time. These cause fractures to develop (see Body Language of Trees, Mattheck) that then propogate as microfissures. The mini-drought you described shrinks the wood, causing fibers to pop, accompanied by loss of strength. Now comes the rain, moisture increases over a day or two in the tree, then the whole combination focuses to create failure.

Well, its a theory.... [Smile]
 
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<Scott>
posted
Reply to post by Russ Carlson, on August 16, 1999 at 08:40:16:

Interesting hypotheses guys. Are your observations associating failure with internal moisture alone or is there still external water weight on the foliage? Russ, if the mini-drought causes fibers to actually fail would you anticipate later limb failures from various loading events (wind, rain some greater time later, snow in some following winter)? In other words short drought events would then lead to very common failure... we'd have forests of short stubby trees. Is it likely that the torn fibers are repaired, or maybe they don't experience physical failure, just loss of eslaticity a la Shigo. Maybe the torn fibers are just on the wosrt of the lot.
 
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<Wayne Cahilly>
posted
Reply to post by Scott, on August 16, 1999 at 20:59:34:

I have always figured water weight on foliage could compound the problem, but I have seen failures occur with dry foliage but moist soil (say the rain fall breaking the mini-drought had been late the day before or the evening before). Its the "still sunny day" failures that are most alarming cause that is when people are out and about, picnicing under trees.

I have a great chunk of wood in the back of my car right now Russ, summer limb drop on an Acer pseudoplatanus and the failure occurred at a target canker on the upper side of the limb, about 8 feet out from the trunk. Nice still day, 94 degrees, rain the evening before, the works!!

Wayne
 
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<Wulkowicz>
posted
Reply to post by Peter Torres, on July 26, 1999 at 08:49:28:


Since I talk so much, I might have mentioned this before and not remember saying it, I had two personal encounters with sudden limb drop, which has no no permutated into summer limb drop and further in summer sudden limb drop. (?)

In the summer of 1988, when I knew nothing about trees except not to drive into them, I was standing in a park at the start of the drought when a large limb fell next to me. There were two surprises for me in the incident, one, the sudden appearance of a new neighbor, and two, the fact that the limb weighed almost nothing. As I remember, the leaves were not particularly wilted, but the whole piece seemed to be made of balsa wood. I threw it in the back of a pickup I had borrowed from the 9th Street yard, intending to look at it the next day. Naturally, it was gone, lumped into the few tons of tree waste that always filled that yard, so my first attempt at arboreal forensics winked out of existance.

A few months ago, a green ash in my side yard dropped a large limb--which was very heavy--at a time I would have not associated with a drought. Of note with the ash, is that about 5 feet beyond the drop point, the limb and its subordinate branches have been dead for at least 12 years (yes, I watch everything I can, and try not to intervene) and the bark had only begun to loosen last year.

So, my experiences are on both sides of the common assumptions and perhaps limb drops occur for different reasons and we are incorrect in trying to assume a single cause.


tubs o' limbs
 
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<Mark Goodwin>
posted
Reply to post by Wulkowicz, on July 26, 1999 at 22:48:35:

I agree that causes may be many and varied, regarding sudden limb drop/summer limb drop. I believe we would all like to have a better understanding that might lead to prediction/prevention, if possible. I'm not sure how much that would help, though, since (other than cutting down trees) it is very hard to get people to stay out from under trees, and hard to get institutional types to agree to put up barriers to people under trees. Perhaps we may have to be satisfied with trying to manage water stress and look diligently for signs of branch damage. People tend to be unaware of the potential for danger under trees, especially on windless days. There are risks when one walks into a forest, where trees fail and fall. People don't think of city trees as a forest, it seems to me, even when we call it an urban forest. And people in tree care professions are called upon to lessen or eliminate the danger.
I would like to see more data on size, species, circumstances, as well as "autopsies", to find any predictive patterns, that might lead to better tree management.
 
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<Scott Cullen>
posted
Reply to post by Mark Goodwin, on August 17, 1999 at 16:36:52:

Some good points here mark.

1. I'm hearing two possible tracks with regard to water: a) that moisture stress - what we traditionally think of as drought or shortage - creates a propensity for failure, either through internal or "micro" failure or though a perhaps reversible loss of elasticity.... that we can manage this through supplemental irrigation. b) that excess moisture or an increase in moisture after stress leads to failure.... that supplemental irigation may be a caause as well as a cure.

2. Are most of the failures associated with identifiable defects? E.G. cankers, wounds, old pruning cuts etc. If so these seem to present the opportunities to abate through removal or mitigate through cabling/bracing (see my earlier query in this thread).

3. The very reason we like trees, that they are desireable in an urban environment is that we like to be under them, to enjoy them. Of course there is resistance to putting them out of bounds. Society sets the bounds for acceptable risk. Arborists, urban foresters and grounds managers should not shoulder the burden of avoiding all failures and all damage or injury as a subset of those failures. Society wants trees. Our job is to maintain them in a reasonable fashion. If there are identifiable defects that lead to "summer limb drop" and we can abate or mitigate risk associated with such specific conditions, maybe that is reasonable. Maybe in certain specific instances if society wants to preserve a tree or limb with a propensity for failure it should be fencned off to mitigate risk of injury. But should we be responsible for modifying microclimates to somehow try to control naturally occurring patterns of interaction between tres and environment? We CANNOT allow lawyers and damage awards to make us responsible for risks that society at large wants to accept. The more we accept that responsibility the more awards will be made and the snowball will just keep growing. Or alternatively we'll just cut them all down and pave everything over. And then we'll be responsible for everybody getting skin cancer from the sun and respiratory disease from the air pollution.
 
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<Wulkowicz>
posted
Reply to post by Scott Cullen, on August 17, 1999 at 23:34:43:


In both my experiences, there was a splintered stub, about 8 to 10 inches long, left on the parent limb, and both were angled upward in their original orientation. Certainly, I can't divine the universe from two encounters, but I can surmise that the circumstances of each might be sufficient to consider that limb drops can occur for many seemingly unconnected reasons.

The biggest problem we have is that the drops are infrequent and rarely observed, and in a context where most everything is picked up and thrown away as garbage. My saddest observation about urban forestry is that there is no time allowed to learn. Boom, off the tree and in the chipper! Anyone who stops to look at a piece that interests them get yelled at. So, I don't expect a lot of new knowledge from this venue. We had the ALB's for at least 3 years in Chicago and it took a park guy from the suburbs discovering them in a load of wood recieved from a friend in Chicago. Why didn't the city toops see them--because it wasn't their jobs and the bureaucracy, by definition, fears initiative.

When I find my photos, I'm going to send them off to Leigh Stone. This issue is one of his projects and I respect him greatly as a thinker.


Bob

Certainly, I'll put the on the web so others can have them as well.
 
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<Russ Carlson>
posted
Reply to post by Scott, on August 16, 1999 at 20:59:34:

Well, I said it was a theory [Smile]

I'm not sure if the fibers are breaking, or what, but both Shigo and Mattheck have mentioned loss of combined fiber strength as part of the problem. Obviously we still have much to learn on this-

In most cases I've looked at, I could find some old injuries that resulted in internal defects, even just cracking or cell separation. The problem of course is finding the defects at the point of failure, since the wood is usually spintered.

I don't know about the 'short stubby forest' theory. These are limbs that were probably ready to go anyway, if not by SLD, then probably the next good wind. As Wayne points out, they draw attention more when they fall on a calm sunny day, than when the same limb fails during a storm. In fact, it may have been that last storm that predisposed the limb to break on the calm day.
 
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<Scott>
posted
Reply to post by Wayne Cahilly, on July 26, 1999 at 08:49:28:

This topic is running concurrently at the UKTC List site. I think it's one of the links Russ provides.

One post there suggests that Mattheck specifically describes SLD as "unpredictable." This would be an important citation to have in a libility defense.
 
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<Nelda Matheny>
posted
Reply to post by Wayne Cahilly, on July 26, 1999 at 08:49:28:

Harris discusses summer limb drop in his book Arboriculture (p. 464 in 2nd ed., p. 493 in 3rd. ed.) I didn't add much info to the 3rd edition, because we don't seem to know much more about it today than we did in 1991.

The discussion includes species reported to experience summer limb drop. In England, failures have been reported on oak, sweet chestnut, beech ash, poplar, willow and horse chestnut.

Elsewhere, species include oak, elm, eucalyptus, ash, plane, pine, cedar, tree-of-heaven, Erythrina caffra, Ficus microcarpa, olive, Grevillea robusta, Sequoiadendron giganteum, Sophora japonica, silver mape, sweetgum, poplar.

I think we tend to call any limb failure that occurs on calm, hot days summer limb drop. I'm not sure all observers look thoroughly for possible contributing defects (perhaps we aren't sure what to look for!). More failures may be attributed to summer limb drop than should. I suspect as we become better investigators, we will find some branch characteristics that are common to many summer limb drop failures.

The question for us remains: How do we predict them? Since we have yet to identify disgnostic external characteristics common to those failures, the simple answer is, we can't! Cerainly we should be very careful of trees that have experienced summer limb drops in the past, because they are likely to have others in the future. Long, heavy, horizontal branches seem to be more likely to fail. I think we need to look closely at the shape of the limb, too. A diamond-shaped branch (in crossection) or rib indicates an internal crack (See Jim and my Photo Hazard Guide, p. 12). An aerial inspection may be required to observe this.

Jim and I are working on the 3rd edition of the Hazard Guide. We are very interested in people's experiences with tree failures, including summer limb drop. If you have any information, experiences and photos you wish to share, please contact me or Jim at nelda@hortscience.com and jim@hortscience.com
 
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<Russ Carlson>
posted
Reply to post by Nelda Matheny, on July 26, 1999 at 08:49:28:

Conspicuously absent in the species list is Robinia (black locust). In the mid Atlantic region we see quite a few failues. Typical of many of the other species, sometimes there are defects that can be observed (at least after the failure) and some with no apparent defect, either external or internal.
 
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<Mark Hartley>
posted
Reply to post by Wayne Cahilly, on July 26, 1999 at 08:49:28:

Just some thoughts on the issue.

If microdroughts caused this problem Australia would be feet high in fallen branches.

If rainfall occurs the day before is there enough time for new absorptive roots to develop
and take up water (the assumption ... drought leads to a decline in the number of absorbing roots)

It seems that tere must be a lot more questions asked before we start producin a model.

mark
 
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<Scott>
posted
Reply to post by Wayne Cahilly, on July 26, 1999 at 08:49:28:

OK Wayne, it's 8/26 and you no doubt know about the heavy rains in Metro NY today, and the preceding drought conditions. So I'm working in the office today and I hear something hit the roof.

Large Quercus borealis (or rubrum, whatever we've decided to call red oak). About 60 feet up (in a 90'-110' tree) a 3-4" side branch off a 10-12" branch broke about 12-18" from the crotch. About 20' long. It was caught by branches below it at about 45 degrees and so did not fracture completely, it's still hanging on.

So, if I can get a good ladder (I'm too old for this throw line to a 40' crotch business) and get it out of the tree I'll save the failed section and send it to you or Russ if either of you want to disscet it.
 
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<Bob W>
posted
Reply to post by Scott, on July 26, 1999 at 08:49:28:


How do we plit it three ways?

Tubs
 
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<Bob W>
posted
Reply to post by Scott, on July 26, 1999 at 08:49:28:


Damn! Did I mean to say split or plith?

Tubs
 
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<Scott>
posted
Reply to post by bob w, on August 26, 1999 at 20:10:31:

Well on-line of course. I still have to get up there and get it. Went out to pick up the paper this morning, looked, and it is UP there. And it's still raining and other projects are backing up.

But then, Wayne is closer but Russ seems to enjoy scanning in photos and diagrams. So let's organize this thing guys. Any body want to start by suggesting a dissection protocol?
 
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<Russ Carlson>
posted
Reply to post by Scott, on August 27, 1999 at 00:57:54:

Would love a look at it.

Protocol: We want to look for internal defects on both a macro and micro scale. The initial sample should include probably 2 feet or more on either side of the break. Probably want a few cross sections as well as some longitudinal cuts. Cuts should be with a sharp bandsaw, then carefull planed or sanded.

We'll also want a full description of the condition of the rest of the limb: Any past pruning, wounds, injuries, decay, etc., on both distal and proximal sections. Also need accurate measurements of the limb- length, diameter at attachment, at break, etc.

Will also need to know who gets the bill for this!? [Smile]
 
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