Today I looked at a co-dominant Red Oak With a split through the included bark crotch. The main crotch is at 10' height, DBH is 73.5". The split is on one side starting at 2.5' high up to the 10' crotch. The crack is 2'4" deep at 3' high, 3'deep at 5' high, and 6" wide at the top. The opposit side does not show signs of cracking or included bark. Cabling heights are less than 2/3's of the height from crotch to top.

The tree is just a few points from being the state champion Red Oak. The property owner would like to keep the tree if possible. Targets a very rural road, power lines, and potentially the corner of the house.

Would you please give your thoughts on this situation? Thank you, Eric

Reply to post by Eric Engstrom, on October 05, 2000 at 22:14:14:

Sounds like a tough decision, Eric. Worse scenarios have been dealt with, so this remains a possibility. Keep in mind when you consider any response here that we can only imagine the situation from your words. Nothing beats being there.

If the crack is running down one side, it sounds likely that the problem is torsion, a twisting of the trunk. The fact that one side is intact maens there may still be a lot of strength left on that side. The problem is to find a way to stabilize the motion of the limbs where they converge into the main trunk.

Cables are part of the picture, to limit stress on the crotch and trunk. If they cannot be placed at the recommended levels, you might install them lower, but may need heavier hardware to be effective. Pruning of the crown, done properly, can reduce the sail effect (wind) as well as weight (gravity).

Bolts in the main trunk will add to the stability of the trunk, and reduce the risk of further cracking. See the recent thread on bolts and hardware in the Tools section of this forum.

One thing you need to consider is the extent of any decay that may be present. The trunk should be carefull inspected, perhaps drilled or otherwise tested for decay. In a tree that large, there is probably a lot of decay and loss of strength.

Finally, the owner has to consider the risks. The ONLY way to eliminate the risk is to remove the tree. Everything else is compromise, and only buys time.

Reply to post by Russ Carlson, on October 05, 2000 at 22:14:14:

Russ wrote: "Pruning of the crown, done properly, can reduce the sail effect (wind)."

As part of the project that prompted the recent bolting thread I've been gathering all sorts of literature on wind and trees. There is quite a body of evidence in the forestry and the civil engineering literature that foliage has a "damping" effect on the frequency and duration of tree sway. Tree sway... the back and forth... contributes as much or more to tree failure (both uprooting and stem failure) as constant motion in one direction.

This is intuitively supported. Ever top out a tree with a lot of foliage below you? The top just falls away. Do the same thing with a tree completely limbed out below you.... you're going for a ride, better be well tied in.

This seems to go beyond the more conventional wisdom about total sail area and the more recent understanding about avoiding "lion's tailing."

So this is another area where our understanding is incomplete, making it difficult to know just what "properly pruned" means. Maybe it would be better to leave sail intact and use hardware suitable for the entire load.

Reply to post by Scott Cullen, on October 06, 2000 at 00:42:54:

Would like to know your sources, Scott. This has been an interest of mine, but the literature is not deep on this topic. Been trying to find a copy of Coutts (1995).

Pruning reduces sail effect, or more precisely, drag coefficient of the limb, by reducing cross sectional area. The problem to date is we aren't yet sure exactly how to measure the area in wind.

I agree the foliage and length of limbs has a damping effect. But that must be countered with the increased drag coefficient. Less drag = less pressure = less movement. The damping effect may slow the movement, but not necessarily reduce the amplitude of movement, and that is probably the critical issue for a split crotch.

I don't think we have enough information yet to know all the answers. And as you fully understand from your research on the guying of large roadside trees, the local factors of exposure and topography have tremendous influence.

Reply to post by Russ Carlson, on October 06, 2000 at 17:46:21:

"Would like to know your sources, Scott." Right now they are scattered all over the office on post it notes, down load printouts and scribbled observations. I'll try to get them pulled together into some sort of coherent list.

"...the literature is not deep on this topic." Actually there is quite a bit, but all pretty arcane. Most in forestry litearture and directed at stand characteristics. You have to try to pull bits and pieces out of it all.

"Been trying to find a copy of Coutts (1995)." It's composed of chapters each with extensive bibliography, so if you can get a copy that's the best place to start creating a list. You can order it on-line directly or indirectly from Cambridge University Press. Or try a forestry library, maybe your alma mater. I have a copy on loan from a colleague at Yale Forestry so I can't send you "mine."

"Pruning reduces sail effect, or more precisely, drag coefficient of the limb, by reducing cross sectional area. The problem to date is we aren't yet sure exactly how to measure the area in wind." Well, maybe not exactly. I think we have to distinguish sail area and sail effect. And there also seems to be a difference between effective area and drag co-efficient. If you have a two structures with x outline "normal to the wind" they have the same overall area. If a is a plane surface it's all catching wind. If b and c are lattice towers, say, they may have the same y density (ratio of solid area to open area) and much smaller effective wind area than a. But if b is made of angle iron and c of round tubes they have different drag co-efficients as well.

Now, a sail is intended to transmit as much energy as possible to a hull to drive it through the water or to an axle to spin the windmill or whatever the device is. More area equals more drive or transmitted energy. You design your system to be light so you're not wasting energy moving it.... the sail is a membrane. While trees are also efficient they must manufacture their components with sufficent strength to be self supporting. The leaves are like little "sails" but twigs, branches, leaders and trunks are not membranes... they have mass in addition to area. Ship rigging is designed to be relatively rigid so energy is trandmitted from the efficient sail to the desired movement. But the tree is attempting to resist breaking or overturning moments (movements). So from leaves to twigs and on through the system the various members move and twist and ABSORB energy rather than transmitting it to the points of failure.

So while crown reduction reduces "sail" area it may not reduce energy transmitted to points of failure. Stated another way there may not be a direct relationship between wind area and failure inducing movement.

"I agree the foliage and length of limbs has a damping effect. But that must be countered with the increased drag coefficient. Less drag = less pressure = less movement. The damping effect may slow the movement, but not necessarily reduce the amplitude of movement, and that is probably the critical issue for a split crotch."

I'm not sure that's true. It depends on the arrangement and nature of all elements not just surface area. More mass may reduce movement. I have to go back to the empirical example of the topped tree. I experienced that many times as a young climber and it has always stuck with me. Now in my old age I find it quite consistent with what I'm finding in the literature. While we intuitively think in terms of wind area, and the engineering calculations tend to start there I've found more reference to mass density of crown than to area of crown.

Another analogy might be an airplane propeller. It needs mass as well as shape. Remember the F4U Corsair with the gull shaped wing? The engine was so powerful that the prop had to have enough mass not to be torn apart by the energy and to keep the engine from tearing itself apart. The calculations called for a 13 foot diameter 3 bladed prop. They had to drop the wings in that gull shape to push the airframe high enough off the ground that the prop would clear.

I'm winging it a little here, but pressure I think increases as the square of wind velocity. Load increases directly with area. But mass increases geometrically.... the square of area the cube of length or width (of course you get anto variable aspect ratios and tapers but you get the general idea).

"I don't think we have enough information yet to know all the answers. And as you fully understand from your research on the guying of large roadside trees, the local factors of exposure and topography have tremendous influence."

There really are three big picture factors. LOAD which is most simply calculated as area x pressure. PRESSURE which is a function of wind speed which involves exposure, topography and other factors. This is also the "design" element. What are you designing for? 50 year wind, 100 year wind, 500 year wind. All of these things are variables in standard "design load" formulae. LOAD CAPACITY which is what the tree can handle on its own. The spraed or difference between load and capacity indicate the risk of failure. This includes the whole range of defect analysis, wood strength, soil, etc. etc. The particular point in this discussion is how much of capacity actually comes from mass.

It's related more to windthrow than to trunk splitting which is the point of this discussion, but as an aside capacity is also relate to weight... both dead weight and the increase of soil tensile strength as it is compressed by that weight.

Reply to post by Russ Carlson, on October 06, 2000 at 17:46:21:

The more I read and re-read this literature and correspond with others (you may be following the concurent thread on the ASCA list) the more I realize how incomplete our understanding is. When I say "our" I mean we arborists who are not engineers, the engineers who are not arborists and all of the above who have little snippets of data and resulting conclusions but not a coherent whole.

The various studies are incredibly focused on a particular facet of the problem or are so generalized as to offer no more than rules of thumb. Some of the conclusions seem contradictory, counter-intuitive or both. They may actually be, or maybe I need more engineering education to understand them.

I think I've made myself as familiar with the literature as most any arborist I know. So I'll be bold and speak for all of us. There's just way too much we don't know. Seems like we need some practical application stuff that fills in some of the gaps, but not quite sure how we get it.

Reply to post by Scott, on October 09, 2000 at 00:11:16:


Scott,

Thank you for the many tangential insights. I would tend to agree that we haven't begun to really understand many of the forces acting upon trees--or the tree's responses to those influences. Part of the problems include our proceeding from the perspectives of mechanical, structural, and civil engineers who are deadly afraid of dynamics and design like crazy to suppress or eliminate them. Where those forces can't be avoided, their designs are double or triple beefed-up according to some previously agreed-to safety factors.

It is unlikely that the tree's evolutional designs will give us many clues as to their issues and solutions because we are trapped in our thinking of planes and surfaces that are built as structures to meet our needs. The closest we get in a sense are the lattice towers usually built of steel that include many engineering tricks which can never be duplicated by living creatures because they must build their structures out of cells--not the crucibles of furnaces and rolling mills.

A sail maker who builds his sails like trees would starve to death. A tree who builds its sails to survive and stay productive has a dramatically different need and solution. The safety factor available to a tree needs only to be staying alive long enough to pass its characteristics on to the next generations.

I agree that crown reduction may very well have little to do with reducing wind loading, and I will go a step further in stating that I am not capable of looking at a tree, understanding its dynamics, and knowing where to prune to "reduce sail effect. " I suspect no one else does either. Of course, I can lurch back to the generalization of "cutting anything must reduce loading" and then confidently proceed to fix that creature, but no one really knows.

I watched a leafless red oak fling itself around during a storm here in Chicago in a violence that frightened me. The next year, a transient tree trimmer convinced the owner to cut it back severely and then sent his attendant monkeys into the tree where they cut away at everything they could reach.

Did he help? Beats me.

I doubt he ever comes back to check his work to see what were the effects. He hires minimum wage youngsters and sends them into dangerous situations. He scares homeowners, he scares his troops, and likely some innocent bystanders as well when his solutions later come crashing down.

This is all done under the standard of crown reduction and lightening limbs to provide a safer tree. It sounds good, it sells jobs, but it may be more demogogic than practical. The closer I can get a tree to a telephone pole, the safer it is likely to be under wind loads. Is that my best and most prudent standard?

_____________


"I'm winging it a little here, but pressure I think increases as the square of wind velocity. Load increases directly with area. But mass increases geometrically.... the square of area the cube of length or width (of course you get into variable aspect ratios and tapers but you get the general idea).

I agree, and the subtleties go well beyond the additional consideration of mass. Eddys, boundary layers, resonance, etc. are likely synergistic--and antagonistic--and contradictory in the general sense of chaos theory--and they all beg to be better understood.

Nature will not reduce the complexity in her designs because they hurt our little heads in thinking about them. She does what she must to keep her creatures alive, whether or not we understand. Our job is to move our capabilities forward, not stay fossilized in some constipated and oversimplified engineering fragments.

Your instincts are quite correct. Validating them is painful, but poking a stick at new truths is one of the few things I still find enjoyable as the offset in my old age. Joust at a windmill, lance a cliche, stick a thumb in a pompous pedantic eye...ah, now you're cooking. Keep up the good work.


Bob Wulkowicz (still in the States)

Reply to post by Wulkowicz, on October 09, 2000 at 08:29:36:

I've read a little more. The assumption seems to be that pressure does increase as the square of velocity for a SOLID. It has been hypothesized that since a tree crown is non solid pressure might increase more linearly. (In other words use an exponent on 1 rather than 2). Dr. Chris Baker at the University of Birmingham tested the hypothesis and concluded there was no conclusive evidence for either square or linear for the case tested. There was a huge difference in load however in in-leaf vs out of leaf cases, which is of course highly intuitive.

The issue of mass is not so clearly addressed. On it's face the analyses suggest that foliage area is mass and more mass catches more wind and results in more initial movement as Russ suggests. But it is also clear that amplitude, frequency and duration of movement are markedly reduced by damping effect of mass. I've not found (or understood) anything that supports my experience and intuition that crown mass while catching more wind may absorb or dissapate more than the increment it catches resulting in less net energy transmitted to points of failure below.

There are dynamic analyses in the literature. They are very complex. They have to make certain fixed assumptions. While they may be stepping stones to develop more complete long term understanding, simple anlyses may help us more in the short term.

I'm finding the engineers more paralyzed by missing simple pieces, like drag coefficents or density factors, than by complex analyses. There is indeed a fear (I suspect a legal rather than analytical) of moving into any area which is not completeley known and understood. "Give me a standard and I'll design to it, but don't ask me to design from scratch." Kind of flies in the face of what i thought I understood engineering to be.

So I've posed questions like, "what if wasn't a TREE and it was a man made model of a tree at Disneyland or it was a lattice tower, and your assigment is to make it stand up, what would you do?" We'll see.

The differences between trees and steel angles or tubes may not be as big an obstacle as you suggest Bob. The trees themselves are giving us data inputs. They've self engineered for the conditions they've evolved and grown over. We know they withstand those forces. So if we are faced with situations where those structures are somehow compromised... say a root cut or trunk cracked... we don't have to "engineer" the entire system, we have to try to understand the missing piece.

It's scary. I'm rambling in a tubsian fashion.

Reply to post by Scott, on October 11, 2000 at 02:49:54:

Hi Scott, I have been reading this thread with great interest and have understood most of the points raised to a limited extent. We are bantering around terms such as, "mass" and "crown density" and "sail area", which all exist in every tree. However, introduce into that discussion "dynamics" and "wind optimisation" and you are left with the proverbial "egg whisk" in your ear. I once was involved in making a video of an oak tree in a major windstorm, after the storm we replayed the video in slow motion, and from that slow replay it was amazing to watch how the tree was optimising its sail area to reduce the "sail effect". Being totally dynamic and changing from one micro second to the next, leaves me to believe that quantifying stresses and loads, under such conditions, to be impossible. No two trees are alike and no two windstorms are alike, the best we can get are general trends.
Incidentally, I also learned a lot about Ivy in Oaks from that video,

James.

Reply to post by James Causton, on October 11, 2000 at 09:08:55:

Difficult? Yes. Precise? Maybe. Field applicable? It depends. Impossible? No.

There are academics doing some pretty complex dynamic modeling. The alogrithms have been built into some pretty representative computer simulations. With digital imaging I imagine that individual specimens could be evaluated.

I think we need to and can move toward some generalized protocols that result in usable estimates of load.

Sorry if I'm repeating (I've had a number of correspondence in the last two days) but you've identified the difference between sail "area" and sail "effect..." the net load transmitted to failure points. And the dynamic response is a big part of that.

This is not completely unique to natural systems. Flexibility is built into skyscapers and bridges. Engineers actually have to back some of it out because humans aren't comfortable in a building that sways noticeably!