Biotic homogenization of upland vegetation: patterns and drivers at multiple spatial scales over five decades

Questions
Is there evidence for biotic homogenization of upland vegetation? Do the magnitude and nature of floristic and compositional change vary between vegetation types? What can be inferred about the drivers responsible for the observed changes?

Location
Upland heath, mire and grassland communities of the northwest Highlands of Scotland, UK.

Methods
We re‐survey plots first described in a phytosociological study of 1956–1958 to assess the changes in plant species composition over the last 50 yr in five major upland vegetation types. Using a combination of multivariate analysis, dissimilarity measures, diversity metrics and published data on species attributes; we quantify, characterize and link potential drivers of environmental change with the observed changes in species composition.

Results
Grassland and heath vegetation declined in species richness and variation in community composition, while mires showed little change. Previously distinct vegetation types became more similar in composition, characterized by the increased dominance of generalist upland graminoids and reduced dwarf‐shrub, forb and lichen cover, although novel assemblages were not apparent. Species with an oceanic distribution increased at the expense of those with an arctic‐montane distribution. Temperature, precipitation and acidity were found to be potentially important in explaining changes in species composition: species that had undergone the greatest increases had a preference for warmer, drier and more acidic conditions.

Conclusions
The vegetation of the northwest Scottish Highlands has undergone marked biotic homogenization over the last 50 yr, manifested through a loss of various aspects of diversity at the local, community and landscape scales. The magnitude of change varies between vegetation types, although the nature of change shows many similar characteristics. Analyses of species attributes suggest these changes are driven by climate warming and acidification, although over‐grazing may also be important. This study highlights the importance of the link between the loss of plant diversity and homogenization at multiple scales, and demonstrates that boreal heath communities are particularly at risk from these processes.