Mountain pines in debris flows

During our hike on Tuesday we encountered several debris flows and got to know how and when they occur. In fact, our journey actually started on a debris flow, which rumbeld down the mountain on the 5th of August 2022, just one month before our camp started. Andrea Millhäusler and Ricco Blass, our guides for the day, explained us how heavy rain or rapid snow smelting can cause loose material to lose grip and flow down the mountain at up to 60 kilometers per hour. These debris flows consist mostly out of solid material and are powerful enough to destroy trees, sweep away cars and damage houses. Big rocks and boulders are "floating" on the finer material which, soaked with water, works as a slide for the rougher material. While we visited the flows, we were able to observe how differently the trees were damaged and decided to question this more precisely in our field work.

In what way does the circumference correlate to the visible damage (process) on standing mountain pines caused by the debris flows from 2018 and 2022 in the swiss national park near the Ofenpass?

We thought that bigger trees have deeper roots and are due to this more stable and less bendable. Because of this we assumed that bigger trees can better survive a debris flow unharmed than smaller ones.

We measured the circumference of the mountain pines on a average height of around 1m. Our sample trees were mostly located in the middle of the debris flows we examined in the swiss national park. In addition to this, we scaled the damage and defined for ourselves whether the debris flow was responsible for the damage or not. Our group took the samples in a range between 50 and 200 meters from the end of the debris flows.

Our results regarding the correlation between the circumference of the tree and the damage done by the debris flows were according to our hypothesis. As shown in the graph, the damage taken by the tree diminishes with the increase of the circumference. Bigger trees from a circumference of 45cm were often able to stay standing without the protection of other trees and without taking to much damage. Trees with a circumference of 70cm or more stayed generally unharmed with the exception of potential bark damage or some broken branches.

But some results were unexpected. A few trees with a circumference below 45cm kept standing contrary to our starting hypothesis and even some of the mountain pines with a circumference of 20cm stayed unscathed. The reason for this can be seen in the scenery on the right hand that shows that some larger trees can function as a shield for smaller ones.

In our hypothesis we forgot to take into consideration that the debris flow is not equally violent everywhere. By measuring the mountain pines, we quickly realised that the density of trees surviving the debris flows quickly decreases when you approach the source of the flow. In this map you can see that the trees that are lower and further away from the origin are less influenced by the debris flow than the ones that are closer to it.

In addition to this, the destruction the mountain pines suffer, is significally smaller when they stand on the border of the debris flow. This is based on the fact that the middle of an arm of a debris flow carries more stones than the outer parts. Because of the lower mass on the sides, the trees are less injured by the boulders.

Most of the results are easily comprehensible by applying a bit of logic thinking. Some though don't have a clear explanation, like one observation we made on the trunks of several mountain pines. About 30 centimeters above the ground (top of the debris flow) we saw streaks that looked like a combination between cuts and claw marks. We are not sure what caused this damage, but we can place some assumptions. The reason for these streaks we deem to be the most likely one, is the process that some stones that were part of the debris flow were catapulted back by others and scratched the trees. If the stones that came back were a little sharper than others, it is possible that they destroyed parts of the bark as seen in the picture. In the picture, the damage could also be caused by rocks that were big and heavy enough to penetrate and harm the bark although they are not very prominently formed. Lastly it is also a possibility that the trees were weakened by the debris flow and that some animals used this weakness to eat or harm the bark in another way. So the damage on the mountain pine was maybe caused by animals rather than directly by the debris flow.

The data we collected seems quite specific, but it can be used for broader topics. You could hypothectically analyze similair setttings in other valleys and this means that you measure standing mountain pines in a debris flows. When there are big size differences, you might be able to make a statement about the different health levels of the trees in various valleys. If trees in a valley stand with a lower circumference although the debris flow was basically the same strenght, you could deduct from this that the roots there go deeper or that the wood is more stable.

After our research, we were able to partially confirm our hypotheses. While bigger trees were less affected directly by the debris flow, some smaller ones survived pretty well too. This may have several reasons, like their flexibility or their positioning behind a bigger tree or a group of trees, which both might function as a buffer. The bark damage did, correlating to our hypothesis, not depend on the size but was more or less coincidental - rocks don't look out for the biggest tree. We also have to criticise our methods. For a stating result we would have to take more samples even tough 79 is a good start, it's not yet precise enough. Our scale is also a point which can or even should be criticised because it is based on our subjective observation and doesn't clearly exclude damage caused by other sources. We tried to exclude samples that we thought weren't obviously influenced by the debris flow, but this is of course not scientific.

In the end we can say that our topic was interesting, but that we would have needed more time to research the standing moutain pines. Our results were mainly satisfactory and we learned a lot doing the field work.