the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Jan 2026
How does biotic weathering work? Influence of alpine plants on rock temperature and rock moisture
Abstract. Rock temperature and moisture are critical factors influencing rock weathering. In alpine environments, these parameters are determined by both macroscale factors, including climate, and microscale factors, including vegetation cover. We investigate the effects of alpine plant species with distinct architectures – among others Dryas octopetala L., Primula auricula L. and Carex firma Scop. – on rock temperature and moisture at rocky limestone slopes. The Arnspitze massif (German/Austrian border) was affected by severe wildfires in the 1940ies so that wide slope areas in the subalpine belt are still characterised by limestone outcrops free from forest cover.
Rock temperature and electrical resistivity (as a proxy for moisture) were monitored over three months at hourly resolution, complemented by small-scale electrical resistivity tomography (ERT) and microwave sensing (MW). Bare rock, soil-covered rock (< 10 cm), and plant-covered rock with different species were compared.
Plant cover was found to reduce the mean daily temperature amplitudes in the rock by 3.2 to 5.2 K compared to uncovered rock. Soil cover effects vary, influenced by soil thickness and microtopographic exposure. Varying rock temperature dynamics are attributed to plant architecture, with shading, canopy heating, decoupling from atmospheric conditions and rock moisture content hypothesized as key factors. Rock moisture increases under soil and plant cover, with reduced evaporation and altered drainage patterns assumed as driving mechanisms. ERT measurements reveal high spatial heterogeneity in rock moisture at the microscale, which is influenced by plant cover, and which is providing favourable sites for vegetation establishment. MW measurements show heightened moisture content under plants at shallow depth (few cm), while with further increasing depth, rock moisture decreases in plant covered rock, suggesting possible plant water uptake with different responses depending on species, growth form and root architecture.
Regarding biotic rock weathering we hypothesize that plant cover generally mitigates temperature weathering by reducing temperature extremes, but enhances chemical weathering and subcritical rock cracking through increased moisture. This underscores how sparse alpine vegetation potentially influences microscale weathering processes.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-6223', Martha-Cary Eppes, 17 Feb 2026
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AC1: 'Reply on RC1', Oliver Sass, 21 Mar 2026
A big thank you to Missy Eppes for her very positive and encouraging comments!
In a revised version we will include the mentioned additional literature and we will emphasise the significance of our work a little more. We will also ‘embellish’ the table containing the botanical names of the plants with small sketches illustrating their growth forms.
We will point out the representativeness of our results by highlighting that our findings are likely generalizable because the studied species represent widely distributed life forms common to rocky outcrop habitats, suggesting broader applicability to carbonate cliff systems beyond the study area. Implications for erosion rates: Yes, from a geomorphologist’s point of view, I’d like to get to the bottom of that in a planned project proposal. Our initial findings do not yet allow us to draw sound conclusions. The question of increased loose debris under plants is a chicken-and-egg problem, as sites with more loose debris are home to entirely different plant species.
Apologies for the incorrect description of subcritical cracking – I realised the inaccuracies after reading the manuscript again. We will clarify these points (chemo-physical process, influenced by moisture, water chemistry, temperature) in a revised version. After all, highlighting the overarching importance of moisture for sub.cr. provides us with even clearer arguments for the importance of our research and the relevance of the findings. Thank you for the pre-written paragraph which we are happy to use in parts. We will rephrase question 1 accordingly, and I will read Eppes et al., 2020 (GRL) for the second time ;-)
Citation: https://doi.org/10.5194/egusphere-2025-6223-AC1
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AC1: 'Reply on RC1', Oliver Sass, 21 Mar 2026
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RC2: 'Comment on egusphere-2025-6223', Anonymous Referee #2, 02 Mar 2026
The paper presents an interesting dataset showing thermal and moisture regimes in bare and plant covered rock surface. This dataset and analysis contribute to the understanding of the role of vegetation in rock weathering.
Below, I list a few points that need further clarification and justification.
Frost cracking
In the conclusion section, the authors claim that plant covered rock surfaces will have a reduced frost cracking based on their results. As measurements of temperature and moisture were done during spring and summer and without any reported < 0 day, how can they support this claim with their presented results? While it is very likely the same attenuation of temperature will occur during winter months, how can the author be certain while not accounting for effects such as the thermal effect of snow?
Thermal weathering intensity
The paper claims that the observed temperature attenuation of 3–5 K under vegetation reduces thermal weathering intensity, but this link is not sufficiently established. Thermal weathering is driven not only by the magnitude of diurnal temperature amplitude, but also by the rate of temperature change, yet the paper provides no data on the latter. The observed temperature attenuation is an interesting finding, but the causal chain between it and reduced thermal weathering intensity remains undemonstrated, especially at >0 degrees conditions throughout the study period.
Presentation of temperature data
The paper shows well how moisture responds temporally to rainfall events (Figures 5 and 6), a similar presentation of the temporal view of temperature would be interesting to assess the rate of temperature change and its relationship to claims made about cracking (“there is no doubt that the thermal effect of vegetation in our study area reduces the intensity of ciritical and subcritical cracking” ) that are only based amplitude of daily temperature.
Subcritical crack
Page 22 Line 460 - "Higher and deeper-seated moisture has the potential to promote subcritical cracking."
Page 20 Line 394 “there is no doubt that the thermal effect of vegetation in our study area reduces the intensity of critical and subcritical cracking.”
The link between the presented results and these claims is not sufficiently clear.
Microwave sensors and contact
The handheld MW sensor used in this study requires full contact with the measurement surface to produce relaible result. In the methodology section, how this inherent limitation of the sensor was mitigated while measuring on plant covered surfaces is not addressed. Could the variation in plant type and architecture influence results? This would be a useful clarification to add.
Citation: https://doi.org/10.5194/egusphere-2025-6223-RC2 -
AC2: 'Reply on RC2', Oliver Sass, 21 Mar 2026
We thank reviewer2 for his thoughtful comments that will help improve the paper.
Frost cracking: Due to the exposed character of the slopes investigated, snow cover in winter is relatively short. As reviewer2 states, it is highly likely that similar attenuation of temperature will occur in the winter months as long as there is no snow cover. In times of snow cover, surface temperatures at this altitude are usually attenuated to a relatively constant BTS (bottom temperature of snow cover) just below 0°C which means “equal conditions” between plants and no plants. Thus, freeze-thaw events almost exclusively occur in snow free periods; and thus we feel that we can confidently conclude that plant cover will reduce the probability of freeze-thaw events also during the snow-free periods of the winter months.
Thermal weathering: New analysis has been done with rates of temperature changes. We can clearly see that the mean maximum heating rate is around 10 K/hr for the rock sites and only 2-3 K/hr for the plant covered sites. The same is valid for the maximum cooling rate which is between -5 and -10 K/hr for the rock sites and mostly between -1 and -3 K/hr for vegetation sites. We have produced two preliminary new figures showing this (--> attached) (PRELIMINARY; boxplots for South Bottom are swapped). We will provide the numbers in the text and the figures in the appendix.
Temporal view of temperatures will be added in the appendix. Rate of temperature change is shown in the new heating/cooling rate figures.
Subcritical cracking: The two quotations selected by the reviewer on Page 20 Line 394 (thermal attenuation reduces sub.cr.) and Page 22 Line 460 (higher moisture promotes sub.cr.) do not contradict each other. Both effects are likely to occur and the question is which of them is more important on the long term. Due to the pivotal role of moisture for sub.cr. we believe that the moisture effect is dominant (see our reply to reviewer1). We will make this clearer in the discussion.
It is difficult indeed to find spots that are smooth enough to place the microwave sensors but it is possible (it took quite a while to find). The procedure for plant cover is addressed at the end of section 3.3.: “For measurements below plant leaves and soil, the cover had to be (temporarily) removed. The measurements were taken below the plant leaves for all species except Primula auricula L., where the plant leaves did not cover much space of the rock. There, measurements took place directly below the plant individual.”
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AC2: 'Reply on RC2', Oliver Sass, 21 Mar 2026
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This paper seeks to characterize the impact of vegetation on key rock physical parameters (temperature and moisture) that are known to strongly influence rock breakdown (weathering) overall. This topic, although seemingly obvious, is greatly understudied, particularly in the nuanced way that this paper seeks to address it. I applaud the authors for taking it on in such detail. They present novel and very interesting results. My main critique is that they are undersold in the write-up of the paper!
Dr. Sass has a strong reputation for development of novel methods for measuring rock moisture – a notoriously difficult metric to derive in the field. This study puts this expertise and experience to strong use with several measurement methods used as moisture proxies across several locations. The study design is strong and clearly laid out – with details regarding control of, for example, rock surface orientation. Then, they also got lucky! The measurement period included both a relatively we month and a relatively dry month allowing for examination of the vegetation-moisture relationships under a full range of conditions.
The 4 questions laid out at the beginning of the paper are gaping holes in our knowledge of how vegetation, rock, water and temperature are co-dependent. It makes for an exciting study! I think it could be strengthened even more by fleshing out ideas even more in the discussion and considering the following suggestions (Some more minor edits are also included in the pdf comments):
Overall, I think the authors undersell the significance of their work and their results. With some more context and inferences about applicability, the paper’s potential impact could be more clearly seen by the reader. For example, water held by rock (rather than soil) is a bit of a hot topic at the moment and has been addressed by a few groups (Riebe and others; and Holbrook’s group in a recent paper: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2025JF008424) . This work has potential to address unknowns highlighted in this prior body of work. I think the current work’s impact could be increased by bringing in those ideas.
As a non-biologist, I find it difficult to distinguish between the different latin names of the plants as I read along with the paper. Perhaps a visual key to them in a supplemental doc would be helpful – along with providing their common names. Or can you even have a simple sketch of their morphology in the table where you name them? That could go a long way to helping the reader interpret the results as they are presented. Ah! I see you get to just that in Figure 7. Perhaps include those little diagrams in the table also? (What amazing figures (7&8)! )
By including these, can’t you sort of increase your impact by talking about how representative they might be of other plants in other settings? I would love to see in the discussion how you expect the results to translate to other climates and plants and rock types a bit. I think these temperatures are very common across the globe for bare rock. What are the implications for erosion rates, for example? Or – how could you leverage your methods in a different site to get at the question of acceleration by moisture vs. deceleration by temperature range. Or even in your own study area. For example: Is there evidence that different parts of the rock are eroding more rapidly than others (ex: areas wth plants have more loose debris on the rock surface?)
Although this is not central to the paper, the concept of subcritical cracking is incorrectly or insufficiently presented in much of the paper. Some reorganization and re-emphaisis will help.
As the authors likely already know well, subcritical cracking is a generic term referring to a mechanism of rock bond-breaking at crack tips whereby very small stresses (caused by ANY weathering phenomenon) stretch the bonds at the crack tip to the point where the rock molecules become interactive with water and then, the resulting chemical processes allow the bond to break due to the very low stresses and the crack propagates. Because subcritical cracking is a chemo-physical process, any factor that influences chemical reaction rates will also influence subcritical cracking rates. i.e. moisture, water chemistry, temperature – INDEPENDENT of the level of the stress itself. In other words, you can have two rocks with the exact same stress on them from roots, from freezing, from whatever, and the wet one will crack faster due to the chemophysical bond breaking.
In the paper, this concept is not at all clear and subcritical cracking is inferred to occur with thermal cycling alone in one spot (intro) but then in response to chemical factors in another (the table). There are also very confusing references – like the Question 1: “Is the daily temperature amplitude in the rock reduced by vegetation and soil cover, potentially reducing direct and subcritical thermal cracking”. It is not clear as written what this might mean. I think it means that a lower temperature amplitude would both reduce the thermal stress (which it would), but also a lower temperature would decelerate thermal cracking even more due to the chemophysical nature of subcritical cracking (yes, that would be the idea as well). But I don’t think a reader will understand this without basically including the paragraph that I wrote above in their introduction (Feel free to use any part of it).
It would strengthen the paper to emphasize that moisture plays a role in the stress magnitudes and efficacy of weathering processes like temperature cycling and freezing, but ALSO plays a role in the bond-breaking itself – accelerating physical breakdown rates even when the stress magnitudes are identical. This ‘double role’ of water makes measurements like the ones presented herein even more important, potentially. I think it is worth separating them. Eppes et al., 2020 (GRL) lay these ideas out succinctly.
Ok – now I got to the end and see that you are invoking these ideas in your discussion and conclusion! Great. but I think the average BG reader will not be familiar with the distinction. Perhaps add a background section to the paper, moving some of the bio-details from the intro there as well?