During the last century, socio-economic transformations have led to a large-scale decrease in agro-pastoral activities across the mountain areas of Europe, resulting in many challenges for grassland management and biodiversity conservation (MacDonald et al., 2000; Valkó et al., 2018). For instance, in Switzerland, the last 30 years have witnessed a significant decrease in grassland area in alpine regions (Strebel and Bühler, 2015; Zehnder et al., 2016). The reduction of livestock farming has caused profound landscape modification and widespread shrub and tree encroachment in former meadows and pastures (Estel et al., 2015). For example, from 1985 to 2013, shrublands and forests increased by 10.6% across Switzerland, with the largest increases in the Alps (Abegg et al., 2020). Encroachment of montane grasslands by woody species has been even more pronounced in the Italian Alps (Orlandi et al., 2016). Shrublands now cover an area of 679 km² in Switzerland (Abegg et al., 2020), with about 70% of them composed by green alder [Alnus viridis (Chaix.) D. C.], which is among the most rapidly expanding shrub species in Central Europe (Anthelme et al., 2007). Alnus viridis is a pioneer shrub species that lives in symbiosis with the N₂-fixing actinomycete Frankia alni (Huss-Daniel, 1997). It is found mostly in steep, north and west-facing slopes, but it can easily expand into other habitats in montane environments, thanks to its efficient colonization ability and substantial seed production (Farmer et al., 1985; Caviezel et al., 2017). Its presence is strongly affected by land-use intensity, as land-abandonment is a key driver of its spread (Caviezel et al., 2017).

Encroachment by A. viridis can have several negative agro-environmental impacts on montane grasslands, which in turn reduce the provision of key ecosystem services for society. For instance, A. viridis encroachment prevents forest succession, and causes nitrogen enrichment in soils leading to increased nitrate leaching and higher risk of dissolved organic carbon leaching (Bühlmann et al., 2016; Hunziker et al., 2017). This can result in soil acidification, water pollution and gaseous nitrogen losses (Caviezel et al., 2014; Bühlmann et al., 2017). van den Bergh et al. (2018) also showed that A. viridis-encroachment increases the evaporative water loss. Moreover, A. viridis-dominated shrublands are characterized by a lower forage quality and host lower animal and plant diversity than adjacent grasslands (Anthelme et al., 2001; Bühlmann et al., 2014; Svensk et al., 2021). A recent study by Zehnder et al. (2020) showed that A. viridis encroachment resulted in a rapid reduction in plant species richness and grassland specialist species, with dense A. viridis patches hosting 62% less species than nearby open pastures. This decrease in biodiversity is linked to reduced light levels in highly encroached areas, as well as to soil nitrogen enrichment. Indeed, only a few tall and shade-tolerant plants with broad leaves, such as Adenostyles alliariae (Gouan) A. Kern and Cicerbita alpina L. (Wallr.), together with a few fern species, are able to adapt to these ecological conditions and dominate the understory vegetation of A. viridis shrublands (Svensk et al., 2021). Furthermore, in contrast to coniferous forests, A. viridis stands do not provide protection against erosion and avalanches on steep slopes, mostly due to the elasticity of their branches that easily bend under snow pressure (Caviezel et al., 2014). Their resistance to this environmental pressure provides an advantage for A. viridis in these areas, compared to other shrubs or trees that can easily break under such disturbance. Finally, the encroachment by A. viridis can also adversely impact landscape quality in montane areas, resulting in reduced attractiveness for tourists.

Because A. viridis predominantly establishes on steep slopes and marginal locations, with few roads, the mechanical removal of this shrub species can be technically difficult, expensive and time consuming. One alternative and sustainable nature-based solution to counteract shrub encroachment could be the use of targeted grazing (Elias and Tischew, 2016; Elias et al., 2018; Pauler et al., 2022). Meisser et al. (2014) monitored the grazing behavior of Hérens cows in A. viridis dominated pastures and showed that they did spend time in half or more encroached areas, despite their usual preference for open areas. However, many shrub species like A. viridis are not palatable for production-oriented livestock due to their low foliage digestibility. Stević et al. (2010) found that A. viridis leaves contained an average of 4.4% of tannins, which could eventually cause an astringent taste and reduce palatability (Kumar and Vaithiyanathan, 1990). Nevertheless, some robust livestock species and breeds have higher resistance to tannins, with ruminal bacterial populations that can better degrade lignified material, allowing them to feed on shrubs and digest their leaves (Berry et al., 2002; Marques et al., 2017). For instance, previous studies have shown that sheep and goats can efficiently feed on woody plants and significantly reduce their cover (Iussig et al., 2015; Álvarez-Martínez et al., 2016; Pauler et al., 2022). Some cattle breeds can also feed on shrub species (Zehnder et al., 2016; Pauler et al., 2019). For example, Highland cattle, a robust breed originating from Scotland, are able to graze on low quality shrub foliage (Pauler et al., 2020a) due to their low maintenance energy requirements and their more efficient use of nutrients from the vegetation (Berry et al., 2002). This has recently led, together with the low costs of their maintenance and care, to an increased rearing of this livestock breed in alpine regions (Pauler et al., 2020b). Previous studies have demonstrated their capacity to reduce woody plant species cover over time, with a turnover in plant diversity (Pauler et al., 2019, 2020a). In addition to the direct grazing of the leaves, Highland cattle are also able to damage shrub branches and trunks thanks to their long horns (Svensk et al., 2021), even if they do not directly debark trees as some robust goat or sheep breeds do (Pauler et al., 2022). Recently, Svensk et al. (2021) showed that Highland cattle can graze on steep montane pastures characterized by high A. viridis cover and associated low forage quality vegetation in the understory. Therefore, Highland cattle grazing could be an efficient and sustainable tool to reduce A. viridis encroachment and restore encroached pastures in the long-term.

Together with cattle feeding behavior, livestock management techniques are also key elements to increase livestock impacts on targeted shrub-encroached vegetation. For instance, livestock supplements could be used to attract them to underused and shrub-encroached locations (Probo et al., 2013, 2016). Different types of dietary supplementation exist, mainly composed of salt (i.e., mineral mix supplements) or sugar (i.e., molasses-based blocks, MB). Previous studies have already demonstrated the efficacy of mineral supplements in attracting beef cattle herds within steep montane shrub-encroached areas (Probo et al., 2013; Pittarello et al., 2016). For example, in a 5-year study, Probo et al. (2016) highlighted a significant reduction in shrub cover, together with the establishment of typical pasture species with higher forage quality, thanks to the effects produced by grazing, trampling, seed and dung translocation at mineral supplement locations. Dehydrated MB were also proven to be efficient in influencing cattle grazing patterns in unfavorable field conditions (i.e., steep slopes, far from water sources and usually undergrazed zones) and across large areas in the North American steppes (Bailey and Welling, 1999; Bailey et al., 2001). However, they have never been tested in montane shrub-encroached areas, despite their lower cost and labor compared to fencing and herding. The MB for cattle grazing management provides different advantages, such as an enrichment of cattle diets through mineral supplementation, that can also reduce the risks of foot pathology and mycotoxicosis, and an enhancement of the intake of forage with low palatability, thanks to their appetizing role (Mordenti et al., 2021). However, more information is needed on the efficiency of molasses-based supplements in changing livestock spatial distribution and increasing the cover of target plant species of functional groups, in montane environments. Indeed, previous studies (e.g., Tocco et al., 2013), showed that the analysis of functional group cover in the short-term can be essential to assess the effectiveness of targeted grazing techniques in a longer term.

In the present study, we placed MB on A. viridis-encroached pastures to attract Highland cattle herds and increase their effects on shrub vegetation. Specifically, we aimed to investigate: (i) the effect of strategically placed MB on Highland cattle spatial distribution and (ii) the impacts of targeted grazing by livestock around MB on A. viridis shrubs, understory plant functional groups and soil cover. We hypothesized that (i) Highland cattle would significantly use more the areas around MB, and (ii) their targeted grazing and trampling pressure would have a higher impact on A. viridis shrubs around MB, with an increased removal of leaves and damage on trunks and branches compared to control areas. Simultaneously, we expected (iii) the understory vegetation to be more affected around MB, with an increase in bare soil due to livestock grazing and trampling, which could lead to a potential re-colonization of those areas by typical pasture species in the long-term.

Overall, the strategic placement of MB significantly altered Highland cattle spatial distribution, and increased livestock use of areas around MB locations, compared to the previous year and to control areas. This result confirmed our first hypothesis that Highland cattle were attracted to MB and that they were able to use steep and highly encroached A. viridis areas, as recently pointed out by Svensk et al. (2021). The results on the effectiveness of MB are thus consistent with those found by Bailey and Welling (1999), who showed that they could attract herds into underused rangelands with poor quality forage. Those authors assessed the efficiency of dehydrated blocks in un-encroached zones with gentle to moderate slopes, while our study showed that such strategic placements can attract herds even in areas with steeper slopes and high levels of woody species encroachment, where grazing conditions could be more challenging. When analyzing the attractive effect at different spatial scales, a significant increase in the use of areas up to 50-m from MB locations, compared to control areas, was detected. This 50 m-scale effect of MB underlines the potential of such management techniques to increase livestock use over large A. viridis encroached patches. The effect of MB within 10-m buffers was even more significant, with substantially increased livestock use at a close range near the molasses-based blocks. Such measurements at different spatial scales highlight the influence of MB on Highland cattle spatial distribution, with this type of management enabling cattle to increase their visits to the surrounding encroached zones once they are attracted by the molasses. Other studies have also shown high potential for different strategic placements to reduce shrub-encroachment at different scales, and to attract herds into under-grazed areas (Bailey and Welling, 2007; Bailey and Jensen, 2008; Pittarello et al., 2016). In particular, Pittarello et al. (2016) found a significant attractive effect at 10- and 50-m around supplement blocks in dwarf-shrub encroached montane pastures, using a different supplement type (mineral mix supplements) for another cattle breed (Piedmontese breed). In our study, under comparable mountain topographic conditions, livestock use and supplement consumption were much higher, with 134 g of molasses consumed per animal per day, corresponding to ca. 80 g of minerals without taking into account the 40% of sugar from molasses content (see Supplementary Table 1) compared to 13.7 g of mineral mix supplement in Pittarello et al. (2016), suggesting a higher attractiveness and efficiency of molasses-based than mineral mix blocks in modifying livestock spatial distribution. Indeed, the average time spent around MB (i.e., 74 min cow^–1 day^–1) was much higher than the 18 min cow^–1 day^–1 around mineral mix supplements in a 50-m buffer found by Pittarello et al. (2016). Furthermore, the familiarity of the cattle with the paddock characteristics might be an important factor to consider when implementing such management in the long term, as the exploration behavior could increase over time and have an enhanced effect on vegetation.

Overall, the increased use by Highland cattle of areas with MB led to a significant removal of leaves and damage to the branches of A. viridis shrubs. Moreover, it resulted in a decrease in herbaceous cover and an increase in bare soil after grazing, compared to control areas. Further, the cover of medium and small herbs and woody plants was significantly reduced by the increased livestock use of MB areas. These impacts were related both to targeted grazing and to the mechanical damage caused by Highland cattle once attracted to the MB areas. Such damage may result from trampling, scratching and breaking branches while walking (see Supplementary Figure 2), as recently observed by Svensk et al. (2021). The significant effect of the interaction between MB and the distance of shrubs suggests a different effect of the distance between control and MB areas. Indeed, large paths created by the animals were observed between MB and the surrounding area, due to grazing activity and movement from MB to water sources. While we could not differentiate between effects of grazing and trampling on the vegetation, Highland cattle were observed to graze on species with particularly low forage quality, such as ferns (A. filix-femina, D. dilatata) and tall herbs (A. alliariae). This demonstrates the ability of this breed to feed not only on A. viridis shrubs but also on other unpalatable understory species, which is consistent with our second and third hypotheses. Previous studies have also shown that Highland cattle tend to be less selective with regard to forage quality compared to other breeds, and are able to feed on woody plants (Pauler et al., 2020a,b). Moreover, they tend to be more adapted to low quality vegetation and lower nutrient intake, as they continue to gain weight under such constraining conditions (Berry et al., 2002). A. viridis leaves and the understory vegetation have also proven to be a valuable fodder resource, but only for certain robust cattle breeds, such as Dexter cattle or Hérens cattle (Meisser et al., 2014; Zehnder et al., 2016). Even if only measured in the short-term, the intense impacts exerted on A. viridis shrubs and other woody species indicate the potential for Highland cattle to reduce woody species encroachment in the medium- and long-term.

Maintaining Highland cattle grazing and the strategic placement of MB over the long term could enable typical pasture species to colonize areas where vegetation cover was highly impacted and new gaps were created. Indeed, several studies have shown that long-term targeted grazing by goats led to increased light conditions in formerly highly shrub-encroached pastures, with the establishment of light-demanding, low-growing and less competitive grassland species (Elias et al., 2018; Silva et al., 2019; Köhler et al., 2020). Similarly, we can expect cows to alleviate light competition for the understory vegetation cover (thanks to the combination of increased bare soil and removed of A. viridis leaves) likely producing a shift in plant functional groups in the long term, and an increase in plant diversity (Hautier et al., 2009). Moreover, to avoid possible risks of localized erosion linked to excessive livestock trampling, it would be advisable to regularly move MB to different shrub-encroached areas during the grazing season and throughout the years. This would also allow the expansion of the livestock impacts on vegetation to a wider area (Probo et al., 2013). While our paddocks always had comparable stocking rates between years, they did not always have exactly the same stocking densities (e.g., paddock 1, in which in the second year a bigger herd grazed for a shorter period compared to the first year). Therefore, future studies with similar stocking densities among years would be useful to confirm the present findings. The assessment of the potential effect of social interactions within the herd (Stephenson et al., 2016) could also be useful to refine the effect of MB in the herd at an individual level. Moreover, as Highland cattle periodically traveled from open pastures to highly encroached zones (Svensk et al., 2021), the restoration of former pastures could be enhanced by seed translocation through endo- and epi-zoochory. Indeed, a recent study (Pauler et al., 2019) demonstrated that a higher number of epizoochoric plant species were found in pastures grazed by Highland cattle compared to pastures grazed by production-oriented cattle breeds, since the long fur of the Highland cattle resulted in more efficient seed dispersal. The same study also found that the less selective grazing behavior of this breed led to higher plant species richness in pastures, as Highland cattle consumed a larger range of species compared to other breeds. They were thus able to graze on less palatable dominant species and reduce their abundance for the benefit of other herbaceous species. Moreover, a high number of dung droppings was already observed around MB after grazing, which could further promote seed dispersal from adjacent pastures through endozoochory (Cosyns et al., 2005). Indeed, even if other management strategies such as controlled fires or clear-cutting have proven effective at reducing shrub encroachment in montane environments, targeted grazing seems to be best suited to restore montane pastures, especially considering the enhanced plant dispersion by livestock (Alados et al., 2019). In addition, the mechanical damage caused to A. viridis branches by livestock might increase pathogen attacks on this shrub species and therefore reduce its spread. For example, fungi species of the genus Phytophthora can spread through water and have the potential to infect bark lesions of a plant, and have been reported to have severe impacts on alder shrubs including A. viridis, sometimes leading to mortality (Pisetta et al., 2012; Bregant et al., 2020).

In conclusion, our findings demonstrate that the strategic placement of molasses-based blocks was efficient in attracting Highland cattle toward areas highly invaded by A. viridis, despite the harsh terrain (steep slopes). Highland cattle successfully fed on, and damaged, A. viridis shrubs around the MB, thus significantly altering the understory vegetation through grazing and trampling, in just one grazing season. Highland cattle could thus have the potential to effectively reduce A. viridis-encroachment in the long term, and, by increasing light conditions for understory species and translocating seeds, they could promote montane pasture restoration.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found below: https://doi.org/10.5281/zenodo.6491171.

The animal study was reviewed and approved by the Direction générale de l’agriculture, de la viticulture et des affaires vétérinaires (Canton de Vaud).

MS: methodology, data collection, formal analysis, and writing. GN and MPi: data collection, formal analysis, and writing. PM: supervision, methodology, data collection, formal analysis, and writing. DB and EP: data collection and writing. ML: formal analysis and writing. EA: supervision, formal analysis, and writing. MPr: conceptualization, funding acquisition, project administration, methodology, supervision, data collection, and writing. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.