Weathering is a critical process in nature, as it serves as the starting point for erosion processes and soil formation. The intensity of weathering is further determined by factors such as time, climate, topography, and vegetation (Derry, 2009). Freeze-thaw weathering and temperature weathering are typical processes of physical weathering occurring on steep rock slopes in alpine environments. In freeze-thaw weathering, the volumetric expansion of ice and ice segregation increase tensional stress at crack tips in the rock, leading to rock fracture (Hales and Roering, 2007; Deprez et al., 2020; Draebing and Mayer, 2021). In temperature weathering, warming and cooling of rock is induced by wind and solar radiation, resulting in thermal stress fatigue over time (Hall, 1999). Almost all weathering processes are accelerated by abundant rock moisture, be they physical (frost cracking, Mitchell and Sass, 2024; salt weathering, Godts et al., 2023), chemical (hydration, oxidation; Calabrese and Porporato, 2020) or biological (biofilm; Gaylarde, 2020). Moisture plays a “dual role” as it not only intensifies the abovementioned processes but is also of central importance for subcritical cracking (Eppes and Keanini, 2017), which is accelerated by rock moisture due to chemophysical bond breaking (Eppes et al., 2020). However, information regarding the internal moisture levels of bedrock under soil are limited (McAllister et al., 2017).

Rock weathering shapes ecosystem properties, thereby driving plant colonization and vegetation dynamics. In turn, it has been accelerated by plants for the last 400 million years (Porder, 2019). The contribution of plants to weathering is indirect by mediating energy and water fluxes, and direct by destructive root action (Table 1). Dense vegetation and soil cover generally protect surfaces by mitigating the erosive forces of wind and water (Dahanayake et al., 2024), while sparse vegetation provides less protection, increasing vulnerability to these forces (Starke et al., 2020). Furthermore, plant cover buffers temperature maxima and fluctuations in the rock subsurface (He et al., 2024). In the context of climate resilience of cities, the body of literature on vertical greening and its micro-climatic effects is steadily growing (see review in De Groeve et al., 2024). For example, ivy foliage was found to buffer thermal cycles, which considerably reduces the number and severity of freezing events (Coombes et al., 2018). However, studies from alpine rockwalls and related to biotic weathering are rare. The shadow of a single plant and the release of litter decrease the solar insolation at the surface, reduce the thermal radiation of the underground, and inhibit the influence of wind. Since specialized alpine plants accumulate heat in the canopy and topsoil, the temperature on rocks under low growing vegetation is decoupled from the atmosphere (Körner, 2021).

The influence of plant cover on subsurface moisture and the associated feedback loops are much less straightforward. Ni et al. (2019) found strong seasonal variations: water contents in vegetated semi-arid soils in summer were up to 50 % lower than in a bare soil due to higher evapotranspiration, while in autumn, water content in vegetated soil was up to 70 % higher. In an arctic-alpine setting, volumetric water content was found to vary by more than 50 % over meter-distances due to the spatial heterogeneity of soil-plant systems (Aalto et al., 2013). On calcareous slopes, different plant functional types exploit distinct water pools: a shallow-rooted shrub rely on variable, surface-level sources, whereas a deeper-rooted tree can also access more stable, deeper pools (Nie et al., 2019). Small-scale studies on moisture distribution in rock are very rare (e.g. Sass, 2004).

On sparsely vegetated rock slopes, cracks facilitate physical and chemical weathering processes. Plants can access continuous water supply and avoid drought when they are able to explore the cracks with their root system (Zwieniecki and Newton, 1995; Rempe and Dietrich, 2018; McCormick et al., 2021). Feedback loops occur as the scattered colonization of rocks by vascular plants favours the accumulation of soil and organic matter which can capture additional water (Kuntz and Larson, 2006). Many vascular plant species in alpine environments are adapted to a temporary drought phase with storage tissues in different plant parts like roots and leaves, yet constant water availability may be a precondition for their establishment (Körner, 2021). In cracks colonized by plants, the temporal and spatial patterns of water infiltration are altered compared to bare cracks, and infiltration rate increases. Once roots have entered the rock through a crack, biochemical weathering may be enhanced by moisture fluxes along the root (Pawlik et al., 2016). On the other hand, interception by the vegetation layer can temporarily retain water before it reaches the surface, with part of it being lost through evaporation (Klamerus-Iwan et al., 2020).

It is generally agreed that woody roots may split rock masses by physically prying them apart (Porder, 2019). Although the tensile strength of unweathered rock greatly exceeds the pressure achieved by radial root growth, fissures or chemically altered joints provide entrance routes (Pawlik et al., 2016) or concentrate roots within fractures (Witty et al., 2003). Additionally, roots release organic acids and mucilage at the fine root tips (Körner, 2021), which promotes chemical weathering (Liu et al., 2021; Larsen et al., 2023). In their literature review, Pawlik et al. (2016) state that knowledge on the process of root cracking is fragmentary. Present-day weathering caused by root action has been documented on two tree species. Taxus baccata L. expands its root system on fissures favouring cliff recession, then, once exposed to open air, stabilizes the vertical cliff face (Jackson and Sheldon, 1949). Quercus muhlenbergii Engelm. penetrates into the rock, accelerating dissolution and inducing rock displacement – processes not observed in similar adjacent rock outcrops with grass vegetation (Phillips, 2016). Studies on biomechanical weathering by vascular plants on alpine environments, that are not trees by growth form definition, are altogether missing.

Rock weathering, be it purely physical or supported by biochemical processes, provides niches for growth in rocky alpine environments, with the pioneer plants mediating weathering by modifying micro-climate. This co-evolution is a paradigm of biogeomorphological interaction (Viles et al., 2008; Marston, 2010; Eichel et al., 2016), on the fringe between the dominance of physical forcing to biogeomorphologic macroevolution (Corenblit et al., 2011, Fig. 8). In the deep time perspective, there is a large number of studies on the global consequences of enhanced weathering by the roots of emerging tree species in the Late Devonian (Algeo et al., 1995). However, there is a lack of local studies with a focus on the direct plant-rock relationship. It is unknown how strongly the cover of sparse, individual plants affects rock temperature and moisture, key parameters when evaluating the weathering dynamics of an area (Hall, 1999). We expect that plants will alter the residence time and the amount of moisture inside the rock, altering infiltration and evaporation. The latter can increase or decrease because plant cover can change solar radiation, air pressure, air humidity, and wind influence at the rock surface.

To fill this research gap, we investigated variations in rock temperature and rock moisture beneath seven different vascular plant species belonging to different functional types (life form, growth form, and root architecture). While life forms impose general constraints on plant stature, determining the position of perennating buds, the variety of growth forms captures finer-scale differences in above-ground architecture and ramification patterns, while root architecture affects water uptake (e.g. Körner, 2021). This combination is thereby expected to capture a range of plant–substrate interaction strategies relevant for rocky outcrop vegetation, extending beyond the specific biogeographic context of the subalpine rocky slopes in the European Alps investigated in this study. Specifically, our research questions include: i.

Is the daily temperature amplitude in the rock reduced by vegetation and soil cover, potentially reducing thermal stresses?

This study was carried out at the Arnspitze, an isolated mountain range northwest of Innsbruck, between the Karwendel and the Wetterstein mountains. These mountain ranges are part of the Northern Calcareous Alps, which extend across Germany (Bavaria) and Austria (Tirol). The highest peak of the Arnspitze range is the Great Arnspitze, with an elevation of 2196 m above sea level (a.s.l.) (Sass and Kloss, 2015). The study sites were located on the southern (Tirol) and eastern slope (Bavaria) of the Arnspitze (Fig. 1).

The Arnspitze lies between the suboceanic, cool, and humid northern side of the European Alps and the subcontinental, warm, and dry Inn Valley. The mean yearly precipitation in the nearby valley of Scharnitz (976 m a.s.l.) is 1734 mm and the mean temperature is 3.3 °C. Most precipitation falls in summer as rain (May–August > 200 mm per month); the driest month is February with 88 mm. The mean monthly temperature varies during the year from -7 °C in January to 12.6 °C in July (a range of 19.6 °C). In Winter and spring, dry foehn winds are typical of the region (Vacik et al., 2011).

The geology of the area is dominated by the light-grey Wetterstein limestone dating to the Middle Triassic, which is mostly massive or thick-bedded. With a calcium carbonate content of 95 %–99 % the Wetterstein limestone is very pure and is prone to intense karstification. As a result, there are no perennial watercourses and overland flow is limited even during heavy rainfall (Sass et al., 2010), so direct rainfalls are the main source of rock moisture (Sass, 2005).

Due to the low amount of solution residue, raw humus carbonate soils and shallow rendzinas with typical Ah-C or O-C profiles are formed (Sass et al., 2019). On the southern and eastern slopes of the mountain, wildfires burnt the forest and the organic layers, leaving these areas highly exposed. The erosion and deposition rates of debris caused by avalanches remain consistently high (Sass et al., 2010), primarily disturbing tall flora and preventing the formation of thick soil layers (Sass and Sarcletti, 2017). A subalpine and alpine low-statured flora characterized by the shrub Rhododendron hirsutum L. and the creepy woody plants Dryas octopetala L. has established on the steeper parts of the slopes, growing mostly in rock cracks and places with minimal soil accumulation (De Giuli et al., 2024). The plant species associated with the microclimatic measurements are listed in Table 2.

In total, six study sites were established on the eastern and southern slopes near the Arnspitze Hut. The sites ranged in elevation from 1790 to 2005 m a.s.l., with orientations varying between 92° E and 185° S (see Table 3). For each method applied (see Table 3), the aim was to obtain measurements from plant-covered and uncovered rock under as identical conditions as possible (e.g. exposure, inclination). At each of the four T/RM logger sites (see below), three measurement points were chosen close to each other. The first (reference) measurement point was always located in rock without plant cover; the second measurement point was in rock directly under the leaves or roots of one of the target species, or sometimes two species occurring in one place; the third measurement point was in rock with a varying cover situation depending on the location (see Table 3).

The novel combination of three different techniques of small-scale rock temperature and rock moisture measurement proved to be a promising approach for characterising the potential impact of different plant species on rock microclimate and weathering. The dataset is consistent, with the three techniques (T/ER logger, ERT, MW) yielding different types of information. For all three techniques, absolute calibration (to e.g volumetric moisture content) is difficult as samples used in the laboratory differ from the complex conditions (especially joints) encountered in the field. Empiric calibration in the field (by observing minima in long dry periods and maxima after long wet periods) is a more promising approach than calibration using samples in the lab; with longer measurement periods, we will be able to determine the possible value range. For the moment, higher/lower conductivity and microwave values are interpreted as “wetter” and “drier”, respectively.

The daily rock temperature amplitude is consistently reduced with vegetation across all sites. The attenuation is 3.2–5.2 K compared to uncovered rock. The difference tends to be greater under plant/soil complexes (D. octopetala or C. firma) compared to the effects of just leaves. However, pronounced individual differences between micro-sites have to be considered. Similar attenuation of rate and amplitude of daily temperature cycles by vegetation in the growing season was also observed by Gage et al. (2024) at the Niagara escarpment. As the amplitude of daily temperature fluctuations is a key factor in thermal stresses (Hall and Thorn, 2014; Ravaji et al., 2019; Ito et al., 2021) there is no doubt that the thermal attenuation by vegetation in our study area reduces thermal stresses. This is also supported by the considerable reduction in the rate of temperature change.

Although there were no subzero temperatures in our measurement period as we did not measure in winter, we assume that the number of freeze-thaw cycles will be reduced by vegetation and soil. It is very likely that the same attenuation of temperature will occur during winter months as most of the investigated species are evergreen. Temperature patterns in winter can be modified by snow cover which is, however, relatively short at the exposed sites of our study area. Currently we cannot estimate if annual minima during long-lasting freezing periods or the time spent in the frost-cracking window (Hallet et al., 1991; Murton et al. 2006; Mayer et al., 2023) will also be affected. Continuing measurements will clarify this point in the future.

Moisture in the rock increases with vegetation and soil cover, with a predominant effect induced by the presence of soil. The results suggest that in general plants and soil increase the humidity in the rock. The presence of soil, with or without the presence of plants, seems to be a necessary condition to create situations with a conductivity at least one order of magnitude higher than bare rock. The presence of leaf canopy alone does not have a clear effect. Results obtained with the microwave measurements suggest that plants in their integrity as overall organisms help retain moisture near the rock surface (0–3 cm) by shading the surface and by producing and trapping litter and organic matter, while causing drying at greater depths (0–25 cm) by their interception at leaves and branches and by their evapotranspiration. Moisture in the ERT profiles confirms this interpretation as it is heightened in the plant's rooting zone but lower under leaf cover (e.g., ERT profile EM under Dryas octopetala). Increases in humidity and the presence of liquid water lead to exponential acceleration of subcritical cracking (Eppes and Keanini, 2017; Eppes et al., 2020). Thus, a potentially enhancing effect on rock decay along soil- or root-filled clefts is very probable. Furthermore, laboratory investigation of rocks using acoustic emission sensors show that samples with high saturation subjected to freeze-thaw cycles produce many more acoustic signals (indicating crack formation) than dry samples (Mitchell and Sass, 2024), which means that water retention by plants potentially intensifies frost weathering. At present, it is uncertain whether the dampening effect of temperature or the intensifying effect of humidity has a greater overall impact.

Results show that changes in rock moisture are in fact dominated by rain events. The impact of melting snow will be evaluated in a follow-up study. Uncovered rock usually responds most quickly to rainfall, while measurement points with soil or vegetation cover show a subdued and slightly delayed response, which we mainly assign to leaf interception. On the other hand, moisture from rainfall events is retained longer at vegetation- and soil-covered sites. The consequences for weathering are equivocal. Quick moisture fluctuations on bare rock might contribute to rock decay (e.g. Yang et al., 2018; Dong et al., 2023) while longer dry periods might reduce weathering. Prolonged deep wetting of stone may facilitate the movement of salts or organic acids deep into micro cracks, thus promoting chemical weathering (McAllister et al., 2017).

The detected variations between life forms and species suggest that these play distinct roles. Tufted and mat-forming species retain moisture near the rock surface (0–3 cm) more effectively than subshrubby or rosette species, suggesting that this effect is driven by the architecture of aboveground organs, as we hypothesized. Unique species-specific impacts also emerge. The reduced moisture retention observed near the rock surface (0–3 cm) in Primula auricula may be due to the fact that this rosette species has leaves that are not pressed against the ground. Furthermore, this species exhibits an almost negligible effect on moisture at greater depths (0–25 cm), suggesting a relatively shallow root system, which is consistent with observations that its taproot is partially exposed in cracks (Murray, 1934). At greater depths in the rock (0–25 cm), distinctions between life forms become apparent, in agreement with Nie et al. (2019). In our study, chamaephytes and phanerophytes tend to exploit water deeper in the rock than hemicryptophytes. This observed pattern partially aligns with the correspondence between above- and belowground plant sizes (Beccari and Carmona, 2024), with larger species (Rhododendron hirsutum and Erica carnea) able to access water resources at greater depths compared to smaller species (Carex firma and Primula auricula). In addition, non-clonal species with a single main root axis, such as Primula auricula, are more likely to encounter impenetrable rock that hinders root expansion, thereby forcing them to rely on superficial water sources. In contrast, branched root systems enhance the ability to access water stored in fissures through coarse roots (Poot et al., 2012), and to forage at greater depths. The lack of available rooting depth data for our species pool prevented us from directly testing these patterns.

The process of biological weathering induced by vascular plants does not occur homogeneously on the rock surface. At the steep slopes of our study site, vascular plants primarily establish themselves in microtopographic features such as pockets, ledges, and cracks in the rock (Kuntz and Larson, 2006). Just with their lateral growth, the roots do not exert sufficient pressure on the surrounding rock to cause dense and massive rock to crack (Malik et al., 2019). However, through their presence, vascular plants contribute to the physical alteration of these microenvironments and the immediately surrounding space by accumulating organic matter and litter, locally modifying hydrology through evapotranspiration, intercepting part of the precipitation, and shielding the underlying rock from short-wave radiation with their aboveground organs. The magnitude of these effects depends on species identity, encompassing variation in growth form, life form, and root architecture. While our study is geographically constrained, the results may be extended beyond the study area. First, most of the selected species have distribution ranges spanning southern European mountain systems (except Rhododendron hirsutum, restricted to the Alps) and represent different Raunkiaer life forms (e.g. chamaephytes, hemicryptophytes), which are widely reported across rocky outcrop systems on carbonate substrates beyond the distribution ranges of the studied taxa (Larson et al., 2000; Raevel et al., 2013). Second, the species exhibit contrasting growth forms (e.g. reptant, caespitose, rosulate) and root architectures, thereby capturing a range of plant–substrate interaction strategies that may occur across different rocky habitats. These physical alterations are expected to influence weathering rates, increasing them due to higher moisture within the root system while simultaneously reducing them due to a decreased amplitude of surface temperature and moisture under the leaves. Which of these effects prevails cannot be answered yet from our preliminary study.

Vascular plant communities could therefore have both a stabilizing function for the substrate in which they persist or a destabilizing one for the rocks. This has interesting implications not only for defining weathering rates but also for understanding the dynamics of these plant communities. Bemis et al. (2026) ask the question: “Does vegetation colonize areas of favorable existing regolith, or does vegetation create a regolith that suits its needs?” On the steep slopes of our study site the answer is clear: Our investigated species root in cracks of the rock where they are able to influence weathering, while sites with more loose debris are home to a set of entirely different plant species with different root architecture. On mobile slope regolith plant roots are specifically adapted to stabilise scree, not to penetrate crevices. If rock plants generate significant amounts of regolith, they pave the way for other species, not for their own needs.

Temperature measurements, 1D- and 2D-resistivity and microwave techniques offer promising opportunities to assess how alpine plants mediate microclimate and rock weathering. The results provide insights into how species explore water resources in rocky substrates, providing a potential tool to estimate root depth in environments that are particularly difficult to sample.

The results show that diurnal temperature cycles and change rates are attenuated under plant roots and leaves, potentially reducing the efficacy of thermal and frost cracking. Water is retained and stored by plants, causing pockets of wet substrate under plant roots and soil (particularly under Carex firma and Dryas octopetala) while a pure leaf curtain on the rock has a much lower or reverse effect. Microwave measurements indicate that plants increase moisture in the uppermost centimetres of rock while at greater depth, rock beneath plants is drier (probably due to interception loss and transpiration). Higher and deeper-seated moisture has the potential to promote subcritical cracking. There are variations between different species and functional types, likely driven by differences in the architecture of both above- and below-ground organs.

The results underline a biogeomorphological co-evolution at alpine rocky sites, with weathering providing micro-sites for plants and plants mediating rock weathering.