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Are diversity trends in western Scandinavia influenced by post‐glacial dispersal limitation?

Felde, Vivian A., Grytnes, John‐Arvid, Bjune, Anne E., Peglar, Sylvia M., Birks, H. John B.
Journal of vegetation science 2018 v.29 no.3 pp. 360-370
analysis of covariance, humans, lakes, land use, latitude, pollen, regression analysis, vegetation, Norway, Scandinavia, Sweden
QUESTIONS: What are the latitudinal diversity patterns in Scandinavia over the last 8 ka? Have they been consistent over time? What are the long‐term diversity trends at different sites? Is there a consistent rate‐of‐change between sites? LOCATION: Thirty sites in Norway, two sites in north‐west Sweden. METHODS: Pollen data from these 32 lakes. The sites lie within six of the vegetation zones of Moen (1998). The data are used to estimate pollen diversity using Hill′s effective species numbers N0, N1 and N2. Diversity relationships in time and along latitude are assessed using ANCOVA. Linear regression is used to investigate site‐independent long‐term trends. Patterns in the regression slopes are investigated between vegetation zones and along the latitudinal gradient. RESULTS: The latitudinal pattern of decreasing diversity with increasing latitude existed over the last 8 ka, although for the number of effectively common (N1) and abundant (N2) pollen taxa the relationship was slightly weaker between 8–6 ka. Long‐term trends show that diversity increases at most sites over time. However, the magnitude and rate‐of‐change vary between sites. Highest rate‐of‐change in N0 over time is observed in southern sites, although the spatial relationship of rate‐of‐change is not statistically significant. CONCLUSIONS: The constant increase in diversity over time may reflect additional taxa arriving and changes in abundances. Most sites change in the same direction, but at different rates and magnitudes. There is no clear spatial pattern in rate‐of‐change, and the general patterns of increasing diversity over time are consistent with the hypothesis of post‐glacial (Holocene) dispersal limitation. Drivers of these changes have not been rigorously investigated, but our results suggest that environmental and historical processes are not mutually exclusive. During the last 2–1 ka BP diversity patterns have probably been increasingly affected by human land use.