Dominant shrub species are a strong predictor of plant species diversity along subalpine pasture-shrub transects

Abstract

Abandonment of pastures and successional shrub expansion are widespread in European mountain regions. Moderate shrub encroachment is perceived beneficial for plant diversity by adding new species without outcompeting existing ones, yet systematic evidence is missing. We surveyed vegetation along 24 transects from open pasture into shrubland across the Swiss Alps using a new protocol distinguishing different spatial scales, shrub cover of each plot (2 × 2 m) and larger-scale zonal cover along the transect. Data were analysed using generalized linear models of shrub cover, shrub species and environmental conditions, such as geology, aspect or soil. Most shrub communities were dominated by Alnus viridis (62% of transects) and Pinus mugo (25%), and the rest by other shrub species (13%). These dominant shrub species explained vegetation response to shrub cover well, without need of environmental variables in the model. Compared to open pasture, A. viridis resulted in an immediate linear decline in plant species richness and a marginal increase in beta-diversity (maximally + 10% at 35% cover). Dense A. viridis hosted 62% less species than open pasture. In P. mugo, species richness remained stable until 40% shrub cover and dropped thereafter; beta-diversity peaked at 35% cover. Hence, scattered P. mugo increases beta-diversity without impairing species richness. In transects dominated by other shrubs, species richness and beta-diversity peaked at 40–60% shrub cover (+ 23% both). A. viridis reduced species richness in a larger area around the shrubs than P. mugo. Therefore, effects of shrub encroachment on plant diversity cannot be generalized and depend on dominant shrub species.

Spatial Distribution of Highland Cattle in Alnus viridis Encroached Subalpine Pastures

“…Overall, plant communities with higher A. viridis cover had lower plant diversity and were dominated by a few broad-leaved species, like A. alliariae and C. alpina, which prefer N-enriched conditions. These results support the findings of Anthelme et al (2001) and Zehnder et al (2020), who also showed a decline in alpine plant diversity with A. viridis encroachment. The few species that survive in A. viridis-dominated areas are able to take advantage of the N-enrichment and increased shading, and they replace many typical pasture species that are highly light demanding.…”

“…Encroachment by A. viridis can have several negative agri-environmental impacts: in particular, A. viridis-dominated shrublands have lower plant and animal diversity and forage quality than open grazed areas (Anthelme et al, 2001). A recent study showed that A. viridis encroachment resulted in a rapid decline in plant species richness, with dense A. viridis patches hosting 62% less species than adjacent open pastures (Zehnder et al, 2020). Moreover, there is high nitrate and dissolved organic carbon leaching from A. viridis shrublands, which results in water pollution, soil acidification, hydrological drawbacks, and gaseous nitrogen losses (Bühlmann et al, 2016;van den Bergh et al, 2018).…”

A multi-kingdom metabarcoding study on cattle grazing Alpine pastures discloses intra-seasonal shifts in plant selection and faecal microbiota

“…With the vegetation surveys we recorded a total of 157 vascular species in the whole grazed surface of Ombretta, with a maximum of 49 and a minimum of 23 species per survey. This number was in line with what was found in other studies involving alpine pastures at the same altitude 88 , 89 , while the total number of species was higher. This is expectedly due to the spatial heterogeneity within the pasture area that increased species richness 90 , 91 .…”

Thinning the thickets: Foraging of hardy cattle, sheep and goats in green alder shrubs

“…In contrast to other shrub species, A. viridis fixes atmospheric nitrogen (N) by a mutualistic relationship with actinomycete Frankia alni . Thus, its expansion into subalpine grasslands has far‐reaching consequences for these initially nitrogen‐poor ecosystems and the wider environment: Due to the high N supply, A. viridis and a few fast‐growing understorey species, such as Adenostyles alliariae or Peucedanum ostruthium , out‐compete most other plant species and thus, biodiversity declines tremendously (Anthelme et al, 2003) and clearly more than with other shrub species (Zehnder et al, 2020). While N leaching in open pastures is close to zero, surplus N continuously leaches from A. viridis stands (Bühlmann et al, 2016).…”