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

Green alder shrubs Alnus viridis increasingly overgrow European mountain pastures but hinder natural forest succession. This nitrogen-fixing, autochthonous invasive species has numerous negative effects including the loss of biodiversity and landscape aesthetic, eutrophication of soils and downstream waters, and greenhouse gas emission. Over centuries, A. viridis encroachment was impeded by grazing livestock, particularly goats. However, in modern agriculture, livestock numbers decreased on remote mountain pastures and goat farming became unprofitable. A grazing experiment tested if hardy breeds of the more economically attractive livestock species, cattle and sheep can replace goats as A. viridis antagonists. On a subalpine pasture heavily encroached by A. viridis, space use and debarking (bark foraging) by Dexter cattle, Engadine sheep and Pfauen goats were analysed using GPS tracking and vegetation mapping. Dexter cattle used space least evenly (Camargo evenness: cattle = 0.39; sheep = 0.52; and goats = 0.47) and preferred flat slopes and open pastures. They spent least time foraging in A. viridis stands (relative presence while foraging: cattle = 0.55; sheep = 0.76; and goats = 0.80). Dexter cattle did not debark any A. viridis, but damaged shrubs by trampling. Engadine sheep visited A. viridis stands nearly as often as goats, but preferred flat slopes, short vegetation and shrub edges more clearly than goats. This sheep breed debarked significantly more A. viridis branches (average 244 branches per paddock; 7.4% of all A. viridis branches) than goats (45; 0.8%). Goats preferred mountain-ash shrubs Sorbus aucuparia covering only 2% of the shrub layer. Synthesis and applications. Green alder shrubs, controlled by goats traditionally, overgrow valuable mountain pasture ecosystems today. Cattle, kept instead of goats, may slow down but not hinder shrub encroachment, because they avoid shrub stands and do not debark green alder. However, this study is the first to show that Engadine sheep are even better suited for green alder clearance than goats. Moreover, if management goals include forest re-establishment, Engadine sheep outperform goats, because they debark green alder (which hinders forest regeneration), but do not destroy elderberry-a valuable forest pioneer. Hardy breeds are an important tool to maintain biodiverse, open pastures, restore natural mountain forests and mitigate the negative environmental effects of shrub encroachment.