| 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.
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?