Am I running out of ideas to write about here?
Nope, out of time, more like, we are into the busy time of year and things seems to be coming thick and fast. On the plus side, this also brings with it things to write about on the blog by the bucket load, so consider yourself warned!
On Tuesday morning I got a phone call from the Zoo’s arborist that a Sophora had dropped a sizable limb over night, could we come and have a look and do what needed to be done to the tree in order to make it safe. It has to be said that this tree is right in the centre of the Basel Zoo, opposite the freshly-renovated restaurant, with seating of the picknick area below it on one side and a tree trunk lying on the ground on the other side that kids climb on, so it is a highly frequented location. Over Whitsun they had 10’000 visitors in the Zoo, most of these will have passed below this very tree.
When we arrived on site, we had a look at the tree: the point that the limb had failed at was maybe 25cm diameter, without any visible damage or any indicator as to what might have caused it, so after some discussion we decided to reduce the width of the canopy, removing weight out of the periphery and making failure less likely.
When I got to the tie in point, I installed my pulley saver around a sizable limb and started to move out on a horizontal limb below me. The tree felt… brittle – which is unusual. Sophora to my mind is a very a fibrous wood with high elasticity that will flex considerably before breaking. This is not what this tree felt like – at all — , it felt as though it was close to failure with very little flexing. Consequently, I returned back from the limb and installed a V-rig onto a second anchor point above the part of the canopy I wanted to make a start on and climbed out onto the limb again. I reached the position I wanted to make the cut in, maybe five meters out from the stem, when suddenly, without any warning, the whole limb failed. I took a pendular swing back into the stem, did not impact very hard and luckily a couple of scratches is all I took away from it.
After having established that all was ok, we spent maybe a quarter of an hour longer trying to work the tree, but it was becoming more and more apparent that the whole side facing the restaurant had this issue of being super-brittle, so in the end our conclusion was that the degree of weight reduction required to make it safe would be so severe that it would leave the tree a ruin, therefore we decided to call it a day and recommended to remove the tree. In a nutshell, the exposure time is very high, the probability of a person being below the tree is high – and the consequence of a potential failure is severe… so sad as it is, I really see no alternative.
This incident highlighted a number of points to me:
- Falling is weird. Being accelerated at 9.81m/s² hits you hard and it is outside of the range of what you are used to or expecting. My physical response to this fall was that my body went into shutdown mode for a moment and left me feeling absolutely drained.
- Trust your instinct. I had a gut feeling that the tree was not trustworthy and might behave differently than I was expecting it to. I came back off the limb to install the V-rig, had I not done this the pendular swing would have been much more severe – also, it was good for the load to be equally distributed onto two anchor points.
- When I fall I go totally limp – falling is a very strange state in which you are totally powerless and unable to do anything much, other than letting gravity take over and trying not to tense up, but rather to focus on how you are going to impact.
In retrospective we came to the conclusion that the reason for this failure – again, the point of failure was very similar and adjacent to the original one – was that the rays in the wood were filled with fungal mycelium (visible in the second image below as fine, white lines in radial orientation through the wood), with the lignin removed out of the xylem, reducing it to a mushy consistence. The phloem and cambium on the other hand were left intact, the limb was in leaf without any visible indicator of damage. This is worrying as these were limbs you would trust without any question as they looked absolutely fine. When the limb failed, I actually had very little weight on it, most of my weight being suspended by my climbing system, the limb essentially being loaded in compression.
Looking back I realize how an incident like this, despite the fact that it was managed well and luckily had no serious consequences, still leaves me feeling a bit rattled, the reason for this being that it illustrates the limits of our ability to manage all risks and foresee all possible issues. In one sense this is quite humbling, demonstrating the inherently chaotic nature of natural structures, and in another goes to show that a cautionary approach is never wrong. Potentially the outcome of this incident could have been a fair bit more serious.