Positive interaction facilitates landscape homogenization by shrub expansion in the forest–tundra ecotone

Abstract

AbstractQuestionsChanges in vegetation structure, including shrub expansion, occur in forest–tundra ecotones in sub‐arctic regions. However, the community‐level processes driving vegetation change are poorly understood. We aimed to evaluate factors mediating the assembly processes for community initiators of vegetation change and determine the ecological processes driving vegetation changes.LocationKuujjuarapik/Whapmagoostui (55.31 N, 77.75 W), Quebec, Canada.MethodsVegetation was surveyed in eight 100‐m line transects with 16 quadrats (1 m × 1 m) per transect established in both forest and tundra habitats. To elucidate the assembly processes of the initiators of vegetation change and other community components of the forest–tundra ecotone, we evaluated β‐diversity among and within habitats and its causal factors (i.e. spatial and environmental factors). Interspecies interactions were estimated based on the patterns of co‐occurrence between all pairs of species.ResultsAlthough environmental and spatial factors significantly affected the community structure of the specific and shared species in the forest habitat, the variation in community structure within and among tundra habitats was not explained by environmental or spatial factors for any community components. Therefore, community assemblages of shared species in tundra habitat were independent from local conditions determined by environment and spatial location. However, we found a positive co‐occurrence pattern among the dominant shared shrubs to be a characteristic of the tundra habitat, and this relationship explained the patterns of community structure within tundra habitats.ConclusionsCommunity assemblages of initiators of vegetation change in tundra habitats are unrestricted by any external factors, such as dispersal limitation or environmental filtering. However, positive relationships in the abundance or occurrence of dominant shrubs may indicate a positive feedback loop between colonizing species and environmental modifications. This positive feedback loop may be a driving process in the expansion of shared species and vegetation change.