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.
Highlights
IntroductionMany of them are created and modified over millenia by human activities, especially by forest clearance, mowing and grazing livestock in transhumance systems (Holtmeier 2009; Lauber et al 2013)
Grasslands cover large areas in mountains worldwide (Dong et al 2011)
Potential transects from open pasture to closed shrubland within the subalpine zone (1300 m a.s.l. to 2400 m a.s.l.) of the Swiss Alps were identified by first selecting all cells of the Swiss land cover map (BFS 2013) that were classified as pastures (NOAS04 classes 42–49) and were directly adjacent to shrubland (NOAS04 class 57, excluding dwarf shrubs)
Summary
Many of them are created and modified over millenia by human activities, especially by forest clearance, mowing and grazing livestock in transhumance systems (Holtmeier 2009; Lauber et al 2013). In the European Alps, forest established after the retrieval of glaciers around. The establishment of pastoralism increased species richness and reduced wood cover (Schwörer et al 2015). In the Alps, this has lowered the treeline by up to 350 m: Open land was created in the subalpine zone, where in contrast to the alpine zone, trees would dominate naturally (Carnelli et al 2004). Alpine and subalpine pastures range amongst the most biodiverse habitats worldwide (Wilson et al 2012) and provide important services to society (Tasser et al 2020)
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