Climate change simulations in Alpine summer pastures suggest a disruption of current vegetation zonation

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Alpine summer grazing areas (high-altitude pastures) represent hotspots of biodiversity and high cultural heritage values. Low density forests (Alpine Larix decidua and/or Pinus cembra forests) and species-rich open land (Alpine and boreal heaths, siliceous Alpine and boreal grasslands) are protected according to the Flora-Fauna-Habitat-directive of the European Union. These habitats are threatened by the accelerated mountain climate warming and the expected upslope shift of vegetation zones. With climate change, tree growth accelerates and montane species spread to higher altitudes. The fate of low density larch forests, alpine heathlands and grasslands is unclear. We used the process-based wooded pasture model WoodPaM to simulate large-scale and long-term dynamics of the forest and tree line under several land-use scenarios and climate change. The simulation results for the central Alpine summer grazing area Furggtal (Canton Valais, Switzerland) showed a plausible projection of the upslope migration of vegetation zones due to climate warming in the very long-term. For the upcoming centuries however, the existing vegetation zonation was disrupted by the intermixing of tree species during the migration process. This unexpected result emerged from the process-based modelling of tree species specific dispersal and establishment in interaction with livestock grazing. This effect was most pronounced in simulations of grazing abandonment and current low intensity grazing of free ranging livestock, which failed to buffer climate change effects. The establishment of paddocks and thereby intensified grazing on the current pastures on the valley floor promised the conservation of valuable open habitats. We conclude that the ongoing rapid upslope shift of climate zones in mountain regions might not be followed by a similar shift of vegetation zones. Widespread colonisation processes might lead to a period dominated by very dynamic vegetation communities that might not zone along climatic gradients as it was in the past of comparable stable climate. Climate change may therefore impact more like disturbance on alpine ecosystems rather than to drive continuous range shifts. During this disruptive transformation, adapted grazing management in summer pastures can help to maintain continuity for valuable habitats but would require subsidies to establish infrastructure and to increase livestock numbers. • Rapid warming disturbs the ecological balance among larch forests, Alpine grasslands, and livestock grazing. • The upslope shift of climate zones in mountain regions might not be followed by a continuous shift of vegetation zones. • During a disruptive transformation period, colonisation processes might be dominated by dynamic vegetation communities. • Climate change might therefore impact like disturbance on Alpine ecosystems. • Adapted grazing management in summer pastures can help to maintain continuity for valuable habitats.