Dominant shrub species are a strong predictor of plant species diversity along subalpine pasture-shrub transects
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ORIGINAL ARTICLE
Dominant shrub species are a strong predictor of plant species diversity along subalpine pasture‑shrub transects Tobias Zehnder1,2 · Andreas Lüscher1 · Carmen Ritzmann1 · Caren M. Pauler1 · Joel Berard3,4 · Michael Kreuzer2 · Manuel K. Schneider1 Received: 15 February 2020 / Accepted: 31 August 2020 © The Author(s) 2020
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. Keywords Biodiversity · Vegetation · Mountain · Succession · Shrub encroachment · Conservation
Introduction Grasslands cover large areas in mountains worldwide (Dong et al. 2011). 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 * Manuel K. Schneider [email protected] 1
Agroscope, Forage Production and Grassland Systems, Reckenholzstrasse 191, 8046 Zurich, Switzerland
2
Institute of Agricultural Sciences, ETH Zurich, Universitätstrasse 16, 8092 Zurich, Switzerland
3
Agroscope, Animal Production Systems, Rte de la Tioleyre 4, 1725 Posieux, Switzerland
4
ETH Zurich, AgroVet‑Strickhof, Eschikon 27, 8315 Lindau, Switzerland
10,000 years ago, but was already modified by humans from around 6500 before present (Schwörer et al. 2015). Since t
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