Big trees

We started work on an alignment of 27 large plane trees, which must stand around 35 tall. The trees are quite challenging, they were pollarded at various times during their lives and have extensive cavities, numerous fruiting bodies of Inonotus hispidus, large limbs infected by the Massaria fungus to name but a few. In the past they were pruned from a cherry picker, so it was clear from the get-go that this would make for some challenging climbs.

We got stuck in yesterday, after lengthy ascents, to my relief, the canopy structure turned out to lend itself quite well to the type of pruning we were supposed to do. This kind of challenge really makes you use all the tricks in your toolbox when it comes to work positioning or accessing pieces a long way out, lots of fun though.

So we worked our way through the first few trees, but once we reached number three and four, it became apparent that these were less forthcoming when it came to anchor points. Therefore we installed a line into tree number six which was overhanging the other two, at least height-wise, and accessed the other two from there. However, the points we were looking at were well offset from the centre and on long whippy limbs. Therefore, and also due to the fact that there were four climbers in the trees, we decided to put a number of remedial actions in place: First off, we re-routed the fall of the access line (orange in the image below), redirecting it over a limb, so as to put the stem on which the anchor point was on in compression, rather than loading it laterally. This is easy to imagine, if you consider the vector forces from the access line anchor point (yes, this is very much kitchen physics, described in layman’s terms, ignoring friction at the anchor point, I realise. But this is, after all, not the physicsmagineers’ blog) and imagine the two vector forces, F₁ and F₂ pointing away from it, the resultant force from adding the two together, F₃, halves the angle between the two lines – and this is what puts the stem in compression, as it lies in the same line as the direction of the vector force.

Next, we secured the out-lying anchor points back to one of the central stems.

We did this using arborWINCH, a Dyneema rigging line line, which is extremely light, but more to the point of course has a very low elongation. With this we braced back the anchor point in the middle tree to tree number six. We did this by rigging simple mechanical advantage systems, which we then tensioned up (see below).

The effect was quite surprising. 

The arborWINCH really limited the movement of the anchor points very efficiently, this brings considerable peace of mind with it, allowing you simply to focus on the task at hand, rather than having that niggling doubt in the back of your mind regarding the point you are tied into. 

You definitively need to use a Dyneema, or some other  material with low-elongation for this measure to be effective. Also, considering vector forces when defining anchor points can make a big difference, as you can see when you test load the point. Initially, we had this rigged straight up and down. When a two-person load was applied, there was considerably more movement than after re-routing the line.

Once we had done all this, we got to work on the two trees, working from the secured anchor points (climbing lines indicated in the image above in blue).

All of the above just goes to show how with a little bit of advanced planning and discussion, as well as investing a bit of time in set-up, can make a big difference to how safely and efficiently you can get the job done. I also think it is fun to working stuff like this out…