the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Mar 2026
Simulation of sliding deadwood logs in mountain forests: towards a quantitative hazard assessment
Abstract. Deadwood is an integral component of mountain forests, supporting biodiversity and contributing to protection against gravitational hazards. However, under specific conditions, deadwood may itself become a hazard when mobilised and transported downslope. Although sliding logs have been repeatedly observed in steep forests, a quantitative framework to assess their hazard potential has so far been lacking. We present a physics-based model to simulate the motion and runout of sliding deadwood logs in complex terrain. The model extends an existing rockfall simulation framework based on nonsmooth rigid-body dynamics with hard contact laws and Coulomb friction, explicitly representing deadwood log geometries and interactions with terrain, standing trees, and protective structures. Model calibration and evaluation are performed using two recent Swiss case studies in which deadwood logs up to 35 m in length travelled several hundred metres in a single rapid descent and impacted infrastructure. Simulations indicate that sliding deadwood hazard is favoured by very steep slopes >35°, wet surface conditions, and a narrow decay-stage window characterised by the loss of bark and branches to reduce sliding friction while still retaining sufficient structural strength. Sliding trajectories are strongly controlled by micro-topography, with preferential paths along gullies, while standing trees limit downslope propagation but increase lateral spread through repeated deflections. The proposed model highlights the importance of adaptive forest management in mountain forests and provides a quantitative basis for optimising the balance between the protective and hazardous roles of deadwood.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2026-130', Anonymous Referee #1, 14 Apr 2026
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AC1: 'Reply on RC1', Joël Borner, 06 May 2026
Dear anonymous referee,
We sincerely thank you for the positive and encouraging evaluation of our manuscript. We are grateful for the appreciation of the topic, methodology, model limitations, and the analysis of the two case studies. We also thank the referee for the constructive comments, which we fully agree with. We will update the manuscript as follows once we have received the second referee report and the editor’s decision:
- Figures: We will revise all figures with particular attention to the visibility of map symbols. We will increase the size of markers, use stronger outlines and apply higher-contrast colours. For the standing trees, which are currently shown to scale according to their DBH, we will add a clearly indicated scaling factor to make them more visible while preserving the relative differences in tree size.
- Relationship between height above ground and decay rate: We agree that height above ground is a potentially relevant factor that is often associated with, but not identical to, direct ground contact. We will expand the discussion accordingly. Although studies addressing a direct relationship between height above ground and decay rate are scarce, distance to the ground may affect local moisture conditions and accessibility for microbial colonisation, particularly at low heights and where dense vegetation, such as shrubs, tall herbs or grasses, surrounds the dead stems. This has, for example, been discussed and tested by Perez et al. (2022), who included sample distance to ground as a potential predictor of deadwood decay. While they did not find a significant effect, they noted that this variable was measured only at the time of sampling, although it is temporally variable, and that multi-year averages may better represent the conditions experienced by a log throughout the decay process. This is particularly relevant for our context, as branches may break and the stem may settle closer to the ground over time, potentially increasing moisture availability and contact with decomposer communities and thereby accelerating decay.
- Relationship between slope exposure and decay rate: In the revised version, we will also add slope exposure as a relevant but complex site factor. As correctly pointed out by you, exposure can influence both temperature and moisture, which may have opposing effects on decay rates. South-facing slopes may favour decay through higher temperatures and longer biologically active periods. However, they may also lead to faster drying of stems, potentially constraining decomposition, whereas north-facing slopes may favour decay through higher humidity and more favourable conditions for fungal activity (Bardelli et al., 2018).
References
Bardelli, T., Ascher-Jenull, J., Burkia Stocker, E., Fornasier, F., Arfaioli, P., Fravolini, G., Alves Medeiros, L. R., Egli, M., Pietramellara, G., Insam, H., & Gómez-Brandón, M. (2018). Impact of slope exposure on chemical and microbiological properties of Norway spruce deadwood and underlying soil during early stages of decomposition in the Italian Alps. Catena, 167, 100–115. https://doi.org/10.1016/j.catena.2018.04.031Perez, S. B., Fraterrigo, J. M., & Dalling, J. W. (2022). Interspecific wood trait variation predicts decreased carbon residence time in changing forests. Functional Ecology, 36, 674–685. https://doi.org/10.1111/1365-2435.13936
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AC1: 'Reply on RC1', Joël Borner, 06 May 2026
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RC2: 'Comment on egusphere-2026-130', Anonymous Referee #2, 31 Aug 2026
Dear authors,
thank you for your nice work. The article is almost fine as it is. Maybe, some very small comments:
- Tree falls sometimes occur induced by rockfalls when a falling block fells a fresh stem. The original rockfall then losses parts of its kinetic energy and transfers it to the tree stem. How does this effect change the situation for the simulation and the hazard situation?
- Dead wood or also freshly cut trees are not only a danger but provide a certain protection from rockfalls till a certain degree of degradation. You already cited the work of Ammann (2006). Some more findings are to be found in e.g. https://theses.hal.science/tel-05373164
- L36: could you add a short reference to section 2 that the reader knows that you will present the tree fall events in this section more closely ?
- L122 : Can you add the orientation of the two tangential directions? I would have expected only one direction in parallel to the relative motion.
- L137+196: Have you checked the influence of a possible partial elastic restitiution component? How large is it or is it really neglible ? I can imagine that the impact of a rigid rock block into the ground is fully plastic but how does a stem react that – expecially with a length of 25m – has a certain degree of bending flexibility ?
- It is mentioned several times in the article that the tree stem has no bark and branches. How do you reckon the influence of branches to enable the stem sliding downwards? The branches could act like skids of a slidge? Further, the branches might keep the stem in the air preventing getting stuck in microtopographic ground?
- L180-182: Please, be careful, you are not directly calculating the probability. Here, you determin the relative frequency. Only, if N_tot is large enough, the relative frequency approaches the probability. To determine the necessary N_tot you have to consider variance/standard deviation etc. Currently, your simulations are more a random test/spot sample.
- g. L285: the blank after the degree-sign is missing a couple of times in the manuscript.
- L249-297: This finding could be very important for the whole rockfall simulation “world”. A forest is not fully randomly distributed in the terrain but follows certain topographical laws. Could this be a significant improval for current models?
- L327: Stem impacts into net not only depend on the danger that a stem might slip through the mesh of the primary net. Moreover, they load the impact area more than a normal block due to the concentrated front area.
- Have you studied the influence of the DEM resolution on the sliding performace of the investigated trees? Especially, because you do expect the friction coefficient to be dependend on the DEM resolution.
Citation: https://doi.org/10.5194/egusphere-2026-130-RC2 -
AC2: 'Reply on RC2', Joël Borner, 08 Sep 2026
Dear anonymous referee,
Thank you very much for your positive and constructive feedback on our manuscript and for the many interesting points you raised. We found your comments very helpful for clarifying several aspects of the study and for identifying points that deserve further discussion. We address all comments individually below and indicate the corresponding changes to the revised manuscript where appropriate:
1. We agree that rockfall may represent an additional mobilisation mechanism. Unfortunately, observations of sliding deadwood events are scarce, and we are not aware of documented long-distance sliding events directly triggered by a rockfall felling a tree. The process is nevertheless plausible. As discussed in Sect. 6.1, mobilisation alone is not sufficient: low-friction conditions, steep terrain, and a suitable decay stage must coincide. A rockfall may therefore fell a healthy tree without causing substantial subsequent sliding, whereas impact on a sufficiently predisposed snag could provide the required mobilisation. Compared to the already mentioned mobilisation mechanisms, the transferred momentum would alter the initial condition/motion of the stem, but these triggering mechanics are not resolved by the present model, which idealises the initial conditions and focuses on runout after mobilisation, as discussed in the limitations section of the discussion. We clarify this point in the revised manuscript and explicitly include rockfall as a possible mobilisation mechanism. We rename Sect. 6.5 to “Model limitations and remaining uncertainties” and discuss this aspect there together with the possible influence of branches raised in Comment 6. These two mechanisms remain plausible but cannot presently be evaluated systematically from the available observations.
2. Thank you for pointing us to this very interesting work. In the revised manuscript, we include the findings of Olmedo (2015) in the discussion on forest management recommendations. In particular, the thesis provides a useful basis for our proposed measure of selectively felling and securing potentially hazardous trees across the slope, showing how such felled-tree structures can be configured to retain or enhance their protective effect against rockfall while preventing subsequent downslope mobilisation.
3. Yes, thank you. We have added this reference.
4. We clarify the definition of the local contact frame in the revised manuscript. The two tangential directions are mutually orthogonal and span the local tangent plane, while their absolute orientation within this plane is arbitrary for the isotropic spatial Coulomb friction law. During sliding, the resulting tangential contact force acts opposite to the relative tangential contact velocity within this plane.
5. Thank you for raising this interesting point. We agree that a long stem may exhibit substantial flexural deformation during impact and that this could influence its subsequent motion. Unfortunately, addressing this with the present model is not straightforward. The assumption of a normal restitution coefficient of zero was directly adopted from the underlying rockfall formulation. In the present nonsmooth rigid-body framework, Newton’s impact law is a kinematic impact law relating the pre- and post-impact relative normal velocities. It is not a constitutive representation of the stem’s flexural deformation. Simply increasing the restitution coefficient while retaining the stem as a rigid body would therefore not represent this flexural response and could instead introduce artificial rebound or bouncing behaviour. A more physically meaningful treatment could involve representing the sliding log as a deformable body capable of bending, storing strain energy, and subsequently releasing or dissipating part of this energy. Implementing such a formulation would require a substantial extension of the present rigid-body model and is unfortunately beyond the scope of this study.
Based on these considerations, in the revised manuscript, we remove the reference to negligible elastic deformation from the description of the restitution coefficient, as this wording is misleading. Instead, we discuss stem flexibility explicitly as a limitation of the rigid-body formulation. In the original manuscript, this limitation is discussed only with respect to the inability of the stem to break. We extend this discussion to note that long stems may also bend and temporarily store strain energy during impact, which may subsequently be dissipated or released again and thereby influence the post-impact translational and rotational motion.
6. This is an interesting thought. The idea that branches may keep parts of the log above the ground and thereby help it overcome small-scale roughness and obstacles is plausible. At the same time, branches introduce competing effects. We expect this mechanism to compete with the exceptionally low sliding resistance of a smooth, branchless, barkless and wet stem. In addition, branches may increase resistance through interlocking with terrain or obstacles (standing trees, deadwood), or even by penetrating soft ground. Given these competing effects, the influence of branches is therefore likely to depend strongly on terrain roughness, obstacles, forest density and branch or crown architecture.
Especially in forested terrain, one may expect branches to increase resistance due to the surrounding standing trees. For the events investigated here, we therefore still consider the very low sliding friction of smooth, barkless and wet stems to be the dominant factor. In these specific case studies, terrain roughness and discrete obstacles along the main sliding paths also appear to have played a secondary role compared with the low-friction ground contact. This does not prove that branch loss generally increases mobility, but it supports this interpretation for the presented events. At other sites with more pronounced roughness or obstacles, the balance between these effects may be different.
The effect may also depend on tree phenotype. More horizontally oriented branches and wider crowns could increase the likelihood of interlocking with rough terrain, whereas slender crowns with more downward-oriented or hanging branches may interact less strongly with the ground when the stem moves crown-first. Even within Norway spruce, the species observed in the presented events, substantially different crown types occur, ranging from strongly hypotonic “comb” types to strongly amphitonic “plate” types, with corresponding differences in second-order branch orientation (Geburek et al., 2008). We therefore expect the net influence of branches to be both site- and tree-specific. Since all observed events in our study involved branchless stems, this provides an indication that branch loss may be relevant, but not sufficient evidence to establish a general causal relationship. In the revised manuscript, we therefore add this discussion to Sect. 6.5, which, similarly to Comment 1, addresses plausible mechanisms that cannot be fully assessed from the available observations.
7. We have corrected the terminology in the revised manuscript and refer to the quantity as the relative frequency, or equivalently an estimate of the corresponding reach probability for a finite number of simulations, rather than as the exact probability itself.
8. We have added the correct blank spaces in the revised manuscript.
9. Thank you for highlighting this potentially broader implication. Our results suggest that the spatial correlation between forest structure and terrain-controlled preferential process paths may also be relevant for rockfall modelling, as rocks, similarly to sliding deadwood, tend to follow preferential paths controlled by the terrain. We add a cautious statement on this aspect to the discussion of the revised manuscript, as further research is required before drawing general conclusions. This represents an interesting direction for future work, particularly as increasingly detailed forest data and established methods for detecting individual tree locations and forest structure now provide the basis for systematically assessing this effect. For elongated deadwood objects, this effect plays an even more important role, since once the stem is deflected by a tree, it cannot simply pass around subsequent trees alternately on the left and right.
10. Thank you for this addition. We expand the discussion of impacts on rockfall barriers in the revised manuscript to note that approximately longitudinal impacts may concentrate the load over a relatively small frontal area and therefore produce a more localised loading of the net than typical block impacts.
11. We have not systematically investigated the influence of DEM resolution on the sliding behaviour in the present study. However, a short sensitivity test for the Gruobenwald case study showed that the relative reach frequency of the national road increased from 9.1% for the 0.5 m DEM to 9.5% for a 2 m DEM and 18% for a 5 m DEM when using the same friction coefficient of 0.25. This supports our expectation that runout and reach can increase with decreasing DEM resolution because progressively more microtopographic roughness is smoothed out. We add these results to the revised manuscript to illustrate the potential magnitude of this effect, but refrain from deriving a general relationship between DEM resolution and sliding behaviour, as the sensitivity is expected to depend strongly on the roughness present at a given site. For Gruobenwald specifically, the results suggest that the loss of relevant terrain detail occurs between 2 and 5 m resolution, while a 2 m DEM still reproduces the 0.5 m result closely.
References
Geburek, T., Robitschek, K., and Milasowszky, N.: A tree of many faces: Why are there different crown types in Norway spruce (Picea abies [L.] Karst.)?, Flora – Morphology, Distribution, Functional Ecology of Plants, 203, 126–133, 2008. https://doi.org/10.1016/j.flora.2007.01.003
Olmedo Manich, I.: Felled trees as rockfall protection devices : Experimental and numerical studies for design purposes, Theses, INSA de Lyon, 2015. https://doi.org/10.70675/b09b0561z2d0fz4887z8171zbed7af26793dCitation: https://doi.org/10.5194/egusphere-2026-130-AC2 -
RC3: 'Reply on AC2', Anonymous Referee #2, 08 Sep 2026
Thanks for the consideration of all my previous comments. Go for it.....
Citation: https://doi.org/10.5194/egusphere-2026-130-RC3 -
AC3: 'Reply on RC3', Joël Borner, 09 Sep 2026
Thank you very much for your feedback. We are glad that you are satisfied with the suggested changes to the manuscript.
Citation: https://doi.org/10.5194/egusphere-2026-130-AC3
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AC3: 'Reply on RC3', Joël Borner, 09 Sep 2026
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RC3: 'Reply on AC2', Anonymous Referee #2, 08 Sep 2026
Model code and software
Sliding Deadwood Geometry Generator Joël Borner https://github.com/joelborner/sliding_deadwood_geometry
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The authors address a very interesting and understudied topic: the movement of deadwood in mountain forests. I found the manuscript engaging and the results highly interesting. The methodology is very solid and clearly presented. The authors also demonstrate good awareness of the limitations of their model and provide a thoughtful outlook for future studies. I particularly appreciated the in-depth analysis of the two case studies used to validate the new model.
I strongly recommend this paper for publication, subject to a few minor comments:
- All figures could be improved. In the maps, some symbols are difficult to distinguish and would benefit from being thicker or more visible.
- Lines 257–259: I appreciate that tree species is discussed first, but I would also suggest mentioning height above the ground, which is closely related to ground contact. This variable is especially interesting, though also challenging, because it may change during the first years after tree fall, for example as branches break. In addition, slope exposure could be worth considering: does a south-facing slope versus a north-facing slope make a difference? This may also be linked to differences in temperature and humidity.
Nice work, and congratulations to all the authors.