pruning macrocarpa's

fosheezy

g'day, just wondering what your thoughts are on pruning cupressus macrocarpa's?

i took out a few hangers from a big old one a while back and we got called back the next day after some average wind to take out about 3 more break outs. it seems they'd be the impossible tree to manage...

i saw a video of ken james' lecture at the recent brisbane conferance that im sure a bunch of you blokes went to, that triggered my brain to thinking about wind loadings and stuff. is it possible to deadwood a mac without having crap break out in the next wind? or lift the crown, or whatever...

cheers,
anton.

TrevMcRev

Crown & Weight Reduction Pruning certainly works on these. The big laterals that develop in them over the years have massive leveraged loading on them.



Shorten the length of those a bit, removing some volume from the ends. It'll have the combined effect of taking load out of the smaller secondary branches as well as the major scaffold branches and usual multiple stems.



Ya gotta get right out there though!

Unfortunatly most cyp macs have had several major lower branches/stems cut out of them earlier on. People tend to keep just raising & raising them without actually "pruning" the parts higher up in the canopy. I believe this exagerates the problem and increases chance of failure of those parts.



Deadwooding alone doesnt fix much.

We cleaned out the interior of this avenue and weight reduced the exterior.

Amongst a heap of other cypress at the same site! (See my pics thread)

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treeseer

trev is totally right; gotta reduce the remaining branches. after all, breakage was due to heavy ends, right? to do it get a good polesaw; see june arborist newsl whoops i forgot where you are. i'll post a polesaw article tomorrow fyi.

fosheezy

that avenue looks wicked, would it have been so dense that you couldn't see through it before you started? also do you know of any break outs after the prune?

weight reducing does seem to be the critical thing when exposing any part of the canopy... if you lifted say 2m of foliage - would you recommend weight reducing the next 2 or more metres up or just the lowest branches?

Ive got a small row of them that need pruning that are pretty exposed to wind and im very hesitant to do any work on them. Your pics do give me a bit more confidence in giving them a go though. so with those trees down in portsea did you tackle every single limb on its merits and work the entire tree over that way? or did you do more work on the sea-exposed side?

sorry to pummell so many questions in but im a curious little arborist!

treeseer

Merits of the branch are judged in content of exposure and other site factors.

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Pole Tool final.doc (44.5 KB, 252 views)

TrevMcRev

Exactly as treeseer said. You take every branch on its merits (or lack of) and deal with as you see best.

When it comes to weighing up the need well... Its a combination of physics, engineering, science and.... instinct.

The physics and engineering are universal principles that have nothing to do with trees per se, but can be applied to them.

The science part is what you learn about arboriculture theory.

The instinct part comes from experience & intelligence.

Think about the principle of leverage for starters. Length or force X load or weight, depending on what you are applying the principle to.

10m long branch with 100kg of weight on the outer extremities.
10 X 100kg = 1000kg load back at point of attachment.
This will increase with forces applied by wind & rain etc.
This also increases/decreases dependant on angle of branch.
Dead on horizontal, the theory stands true. The more vertical it becomes, the equation reduces by a %'tage.
So then weigh that up against branch diameter, taper, defects, attatchment etc.

Mmmm, i wonder why some branches break.....

Understanding all that and matching it up against your experience is next.
Knowing the species characteristics, strengths/weaknesses, history, exposure, previous failures you've investigated etc.

Yes there are diehards out there that will tell you trees are self-optomising blah blah, well only up to a point, otherwise there wouldn't be any explainable failures, only random acts of god...

Most species naturally occur in a forest like situation of varying density.
They compete for light, racing each other to the top, wasting little resources on lower branches or canopy spread.

We pluck it out of the forest and decide to plant a single specimen in the middle of the back yard on its own.

The competition has changed, its now competing against itself. Lateral branches extend to capture light from under those above. Over the decades they will often start to overstep their optimum structral design to stay efficient in their job of farming light.

Eventually many fail.

A good arborist can predict and help prevent many unecessary failures.

Not too many know how. I've seen many a job passed off as weight reduction pruning where they just took off a few side branches closer in.

Yes, weight comes off, but leverage loads are not reduced by anywhere near what they could have been for the same amount of foliage taken from the ends.

I've done this type of pruning on thousands of trees over 15 years with great results.

I've also seen many i've recommended it to that didnt go ahead fail afterwards.

I wish there was a way of putting a timeframe to likely failure, but i still reckon thats an absolute guess. I'm watching a number of trees with massive identifiable defects in low/no target areas waiting for failure. It'll happen, one day.

I'm rambling now

The avenue was thick as before pruning on the inside. Both outers were w/red'd, from memory i think the side facing the water was not actually as over-extended as the other. Along with the avenue we did a row of about 5, a row of about 11, and a few stand alone specimens.

If your talking large mature+ cypresses generally the whole canopy needs treatment, top to bottom, all around. That is if there are targets, or you are trying to preserve the structure of the tree. Otherwise i find that in Victoria at anywhere from 50-80 years old they more or less begin to self destruct.

When they do, the failures are usually significant sized parts. This tends to cause panic and lead to removal.

Eric Frei

Great post Trevor however you are being very conservative with your calculations. Let me explain.

In engineering this is called a full moment cantilever, like a flag pole bolted to a vertical wall sticking out sideways.

The amount of force at the attachment point is way way higher than what you said. The easiest way for me to show this is using the diagram I just whipped up.

In buildings often balconies are cantilevered, with cranes they're well cantilevered, a weight behind the fulcrum. However trees don't have that, so the attachment point of that branch is under tremendous force, this is why it's so important to look for fibre buckling, stretched or broken/split bark, lesions, weeping or sap dripping etc.

Just today I reduced and thinned an over extended poinciana branch over the road. I cannot really remove the entire branch as it is some 20" dia and a co-dominant stem to the trunk at ground level.

So this is the second time I have worked on this particular tree. The branch was some 10m long, I reduced to maybe 8m and took off 1/2 it's foliage, it'll handle it, healthy as anything. I did that as the bark was split on the top of the branch right near the trunk where the maximum force is. The branch comes out from the trunk/ground at a 45 degree angle and slowly curves to near horizontal at 4m high then races out over footpath and road. It's a battle to lift it higher but we are getting there (trucks hit it!) but now it's back to the kerb at least.

Anyway, just thought I'd pop that in here, the forces on long branches are phenomenal, as per the diagram we haven't even considered bad weather.

In structures, buildings, machines etc you'll generally see supports, cantilevers, gussets, cables etc to get some mechanical advantage back and turn the contact point into a fulcrum so it only has to handle the weight not the torque.

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treeseer

took the words right off my fingertips. this use of VTA before every pruning cut is key.

fosheezy

hmmm some interesting points there, 2 of my flatmates are engineers so i'll get to work racking their brains about that attachment loading thing, lol i think they're fairly sick of tree questions because theres never a fixed value but whatever, keeps them honest.

i'm curious if that huge cypress that had most of it blown out about a year ago on burwood highway is still there, or had any more failures. driven past it lately?

TrevMcRev

Yep i think i know the one. At the rear of some units that back onto the main rd.

Still there. Big failure.

I quoted w/red & full removal on it about 3 years ago!
They didnt do it.

TrevMcRev

Thanks Ekka, your example makes sense, but the counter weight would result in equalibrium, a neutral load, a balanced teeter totter, at the pivot point.

The 1000kg at 1m neutralises the 100kg at 10m.
The 2000kg at .5m neutralises the 100kg at 10m.
The neutralising counterweight allows for the length on the other side with the load at the fulcrum being the combined weight on both ends plus the weight of the arm itself as a downward force at that point.
Say the arm itself also weighs 100kg, there is a balanced, but actual weight of 1000 + 100 + 100 = 1200 at the atachment.

The tree branch attachment is a fixed point, with no counter weight like the flagpole example.

Here i think the distance x weight = _____ principle stands true.

Without the 1m cantilever, the load is 1000kg at the attachment from the 10m arm with 100kg on it.

Again if the weight of the arm/branch is 100kg, and the foliage on the end is 100kg, we have only 100 + 100 = 200kg.

So it only actually weighs 200kg but is applying over 1000kg of force back at the attachment.

The engineering examples rely on design features to reduce and counter the levered load.

The tree relies on strength of the attachment, and strength and soundness of the material of the arm itself to cope with the load.

I'm sure there's more to it though....

Obviously in trees there are so many variables, branches are spread along the length of the lever, each making their own overall contribution to the load equation, lever angles vary, then winds, rain etc increase it all again.

Eric Frei

The cantilever example is to show not distances in meters but factors. In the first example it was a 10:1 factor like a lever or crowbar prying open a box. In the 2nd example 20:1 factor. Each time you get closer to the trunk with the counter weight the factor increases a lot and you need a lot more weight ... if you dont have that lever factor there then the join is taking the force, then likely a bending force to so equation converted to torque in nm, however you also introduce shear.

Have a look at hiab booms and franna crane booms. In close they can lift say 25T ... sticked out 1T or even less.

Imagine a L shaped bracket bolted to a wall like a bookshelf. If is narrow and you dump a heap of weight on it all it will try to do is shear the bolts of the wall. Now extend that bookshelf out 10m and put a lot of weight on it and it will try to not just shear the bolts but like a claw hammer pulling a nail out also try to lever them off (torque).

Interesting, dont really know the exact numbers but we know it's a lot. I did find some engineering stuff but like above all of it converts it to psi or nm etc and doesn't make a lot of sense to me/us in the way we are looking at it .... however the numbers are high as the engineers also look at deflection in the beam and shear stresses.

Maybe some engineer type around here will know what the exact forces are at the join.

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TrevMcRev

I think we pretty much agree ekka! Massive loading that not enough practical tree guys doing the assessments and physical pruning works have a grasp of anyway.

I have a mate who designs and builds roll cages for motorsport. I know they have some engineering software that tells them what loads placed at any point will exert what force.

I'll see if he can run some basic calcs on common tree strength scenarios with various angles and lengths. The actual force required to fail wont be relevant, but the ratios of variance between different designs should be somewhat reflective of tree structural design.

Like fosheezy said though, engineers hate to comment on the strength of such variable material as a living tree.

38mm dia 1.6mm wall thickness cromoly tube is a different story!

If this thread gets much deeper into this maybe a split off from pruning macrocarpas is an idea....