One of the interesting side discussions at the recent ASCA Academy in Newport had to do with windthrow. I went digging and relocated an interesting reference:

Coutts, M.P. and J. Groel, eds., 1995. Wind and Trees. Cambridge, New York and Melbourne: Cambridge University Press. 485 pp.
A compendium of 27 papers orginally presented at a 1993 symposium in Edinborough. Mostly on forest stands, but a few on individual trees. 2 by Mattheck et al. One on BC, are you out there Julian?

Also there was an inquiry by Ken Knox and a number of responses in Arboricultural Consultant, unfortuneately I can't find the issue.

Reply to post by Scott, on March 26, 1999 at 16:01:39:

Yes boss, I am here. I reviewed the book when it came out for a journal that I cannot even recall now. It's a very technical book with a lot od empirical data based on wind tunnel tests etc. Good libraray book to ask for but I would not buy it as a stabndard reference withouit looking at it 1st.

On the same subject, I am looking into windthrow windspeed effects on urban forests for a client to determine if there are any windspeed thresholds that might trigger windthrow. Does anyone have any references or contacts on this? I would like to hear from you if so.

Cheers

Julian Dunster

Reply to post by Julian Dunster, on March 26, 1999 at 16:01:39:

I would GUESS that the threshhold would vary based on lots of variables, though those variables might be clustered into susceptibilty classes (species, soil types, rooting area, root damage, etc.). You might be able to develop some conclusions based on histories of failure events / recorded windspeeds.

Reply to post by Julian Dunster, on March 26, 1999 at 16:01:39:

Just in, today's mail.

Hayes, Ed, 1999. "Patterns of Tree Failure." TREE CARE INDUSTRY, April, 1999; X(4):34-42.

Quite a section on windthrow, heavily dependent on Mattheck.

Reply to post by Scott, on April 06, 1999 at 19:54:53:

There was one bit in the article that surprised me. The part about spruces over a 14" diameter shedding their crowns; illustrates a difference between conditions experienced here in the Inland Pacific Northwest and this tornado event in Minnesota. I can't recall ever seeing a shed crown on any size of a spruce, other than trees that were physically sheltered from wind by a cliff or large building. In that handfull of cases, the location of the stem failure was directly comparable with the location of the edge of the buffering object. If spruces fail, in my experience, it is usually windthrow. I am now wondering what this experience in Minn. means for applications here in IPNW.

A couple of things come to mind which may have an influence on the types of failures experienced.

1) It seems to me that by the description in the article, an air pressure difference could have existed between ground and canopy during this storm event, which would lead to more failures in the crown. A higher pressure on the ground of captured air would tend to buffer the ground itself in a wind that is moving horizontally.

2) The wind speed itself, 158-206 MPH is in a wind class event by itself. While there would be some gusting, I (without having experienced such a storm) would guess that a 150 MPH wind that gusts to 175 MPH wouldn't be much of an appreciable fluctuation regarding how a tree branch or trunk is impacted. The gusting 20 - 60 MPH winds we typically experience here, rock branches and stems back and forth with a whipping action that leads to failure. Tornados have to work very hard to stay touched down on the ground, they tend to skip around; unless they have reached a magnitude that grounds so well it just plows a trench wherever it goes.

3) Twisting winds create vacuums and pressure areas along their fringes, complete with updrafts and downdrafts. How does one measure the failures caused by ups and downs without standing there in the storm and watching.

No real answers here, just more questions. In our efforts to understand trees and how they are impacted; it seems to me that we need to better understand the elements they face.

Best regards,
KWK

Reply to post by Kerry, on April 07, 1999 at 13:05:42:

My experience with spruces (mostly Norways and here in SW CT & SE NY) include failures of a) codominant stems with included bark, b) trunk failures where there is internal decay (biggest was a 4'+ dia trunk, tree 70'+ high, with no outer indication of the decay), c) large branch fracture under heavy ice or snow weight and d) large branch shear failure or delamination. Of these a & b are associated with wind. Spruce tops tend to be very limber. We also see windthrow but nor more frequently than the other types.

Reply to post by Scott, on April 10, 1999 at 14:00:01:

Spruces ae quite susceptible to windthrow in Delaware and SE Pennsylvania. The clay soils prohibit deep rooting, so the root plate is quite shallow. When the soil is wet and winds high, we can expect spruces to topple.

Reply to post by Kerry, on April 07, 1999 at 13:05:42:

I do not think it would be wise to apply any data from a tornado class storm to more normal windspeeds, such as up to 60 or 90 mph.
Wind is stochastic enough without applying data from extreme events. There asre so many variables, including soil type, soil saturation, tree species, tree individual, wind fetch, site variables.
One interesting thing about wind is that trees often fall in a direction counter to prevailing wind direction.

Reply to post by Russ Carlson, on April 11, 1999 at 21:05:37:

I guess shallow rooting and wet soil are the classic windthrow combination. We see that. But we also have a lot of upland sites where soil depth is pretty good and there are rocks. Big rocks, little rocks, all kindsa rocks. Remember "Mr. Blandings Builds His Dream House" with Cary Grant and Myrna Loy? "How long will this excavation take? Coupla daysa maybe, depends if we hit rock. Well it's been 4 days did you hit rock? Hit a letch. A letch? Yea, a letch. What is a letch? It's like a rock only bigger!" We've got lots of ledge too. The glacier left a lot behind with lots of nooks and crannies. So more often than not the roots are wrapped around good solid anchors and wedged into crevices.

Reply to post by Scott, on April 11, 1999 at 22:31:27:

>>Remember "Mr. Blandings Builds His Dream House" with Cary Grant and Myrna Loy?

Carefull, Scott. You're dating yourself.....

>>The glacier left a lot behind with lots of nooks and crannies. So more often than not the roots are wrapped around good solid anchors and wedged into crevices.

This makes for good rooting, in many cases. Lots of neat things to wrap around.

Spruces seem to be a greater problem. Even Mattheck separated them from other species. (Mattheck, 1994, Body Language of Trees; Figure 51, p84)

Reply to post by Kerry, on April 07, 1999 at 13:05:42:

Some interesting things about tornados and the way the wind moves. Tornadoes can move laterally very slowly, staying in one place a relatively long time, or they can race across the landscape at 50 mph or more. The wind speed is not a uniform slope to the highest speed, but usually a very narrow shear 'face' of wind associated with the edge of the tornado shaft. This means that a tree can experience winds of 30 mph one second, and 90 or more the next. So while there may not be much 'gusting' within the tornadic force wind, there sure is a gust front involved.

The other factor to consider is that the winds move in opposite directions on the opposing sides of the shaft. Trees in the direct path will experience winds of high speed in one direction, then within seconds, equally high winds from the opposite direction.

Re: TCI article- I felt there was insufficeint discussion of the trees that were blown down and removed shortly after the storm. We don't have any information about those trees, and it could add greatly to an understanding of what went on that day.

We also do not have any information of placement of the trees that were examined. Were they in the direct path, or lateral to it? How far? What were actual wind speeds where those surviving trees were located? All this would have made the article more informative.

Reply to post by Russ Carlson, on April 10, 1999 at 14:00:01:

That's a design characteristic of a TCI article. No offense intended, NAA just knows what its average audience needs and wants. Maybe the author could provide a more data enriched article for JofA or arboricultural Consultant.

Reply to post by Russ Carlson, on April 12, 1999 at 08:30:31:

Cable TV Russ. (Though I do remember first hand what the little loop and spring are for below the blade on a pole saw head).

Reply to post by Scott, on April 12, 1999 at 23:15:27:

Scott must be a lot older than me. I had to ask what the spring was for this winter when I bought a new head for the pole saw. We always used pruners I guess, no saws.
Bob

Reply to post by Scott, on April 12, 1999 at 23:32:53:

I recognize that the trade mags are not scientific journals, and can't present the whole picture. But the readers should be cautioned against over-analyzing these cursory results.

Reply to post by Russ Carlson, on April 13, 1999 at 06:46:31:

Great points about the wind sheer types Russ. I sure don't pretend to know a lot about tornados. The thing the article did for me was to bring forward some things about wind that don't ordinarily enter my thought process. Will I alter my practices much as a result of reading the article? Probably not. Has my consideration of the impacts of wind been broadened? Yeah, I guess it has.

While I have to agree with some of the critisms presented, I also have to respect that bit I gained from Ed's perspective. I'm not sure a trade magazine would be the place I would expect to see everything I ever wanted to know about impacts of tornados on trees. Such a work would most likely involve multiple authors whose backgrounds cover a broad array of agricultural, mechanical, structural, horticultural, meteorolical.... subjects.

I would have liked to see how structures or cliffs affected how the trees held up. Were they better off, worse off, or did it make any difference. Would it have made a difference how the trees were managed, or were they at the mercy of the double edged whammy Russ mentioned?

Best regards,
KWK

Reply to post by Kerry, on April 13, 1999 at 20:03:23:

To carry one step further, did one tree have an effect on survivability of another nearby? Any barrier to wind has an effect up to 7 times the height of the barrier, going downwind (for normal, horizontal surface winds). I would expect some effect from buildings that remian intact, but the nature of tornadic winds, coming from various directions, may negate those effects. An intersting idea!

Reply to post by Russ Carlson, on April 14, 1999 at 00:16:22:

Yes it is interesting to think about this. In the interim since I posted thoughts about the potential for a higher pressure existing at ground level, I have had to face the question of exactly what you mention - barriers. Now I must ask the question, "Is there a pressure change, or are barriers being effective - even against such tremendously strong winds"? Are the barriers creating or involved in a pressure difference?

Best regards,
KWK

Reply to post by Kerry, on April 14, 1999 at 11:26:34:

The barriers are creating pressure differences, albeit small ones in some cases. I don't remember all the physics involved, but many of the principles of fluid mechanics apply.

Reply to post by Russ Carlson, on April 14, 1999 at 22:57:45:

Boy, have I missed a lot of good discussion while I have been WORKING OUT IN THE FIELD!
I wanted to add something about the straight line storm, now known as a "derecho", I am told. In 1995, one came ashore in Northern NY around Watertown, turned SE and cut a swath across the Adirondack Park, downing literally millions of trees and killing 6 people.
Last September, another one arose within a matter of hours in this region. A single cell collapsed west of Toronto, the winds cut across the west end of Lake Ontario in a "straight line" (=derecho), then cut across the top of western NY from Buffalo to Syracuse, with speeds of 80-120 mph. A mere $21 million and two dead people make this a small and almost unseen storm!
During the latter, an interesting difference appears to have shown up. When the storm went through the southern portion of Rochester, with a well managed forest under the direction of Andy Pleninger since 1990, the heaviest damage occured in the sectors that had not received maintenance pruning for 4 years (5-year cycle in the city). Even there, the damage levels were low.
The same storm went through Syracuse one hour later. That city has had essentially no management for a couple of decades, I am told, and was clobbered. Very heavy damage everywhere. Although many factors may confound this comparison, it seems to me that there is a message there that we all need to get out.