Post-glacial patterns in vegetation dynamics in Romania: homogenization or differentiation?

Aim- This paper examines eight fossil pollen datasets from Romania with the aim of exploring regional and elevational patterns in site similarity throughout the Holocene In particular, we aim to determine whether there are clear intervals of homogenization/differentiation and to ascertain the potential driving factors.Location- Romania.Methods- Qualitative (pollen diagrams) and numerical methods including principal components analysis and Bray-Curtis similarity analyses were used.Results- We found strong variability in the past vegetation dynamics during the Holocene Bray-Curtis similarity analyses show large fluctuations in vegetation similarity and distinct periods of homogenization and differentiation throughout the Holocene The magnitude and length of these periods appear quite variable in time, but the significant ones can be delimited as follows: (1) differentiation between 11,250 and 11,000-cal.-yr-bp, 10,000 and 9750-cal.-yr-bp, 6000 and 5750-cal.-yr-bp, 2500 and 2250-cal.-yr-bp, and especially over the last 200-years; and (2) homogenization between 9750 and 9500-cal.-yr-bp, and 2750 and 2500-cal.-yr-bp, with more stable periods between 9000 and 7750-cal.-yr-bp, 4750 and 3500-cal.-yr-bp, and 2000 and 1000-cal.-yr-bp.Main conclusions- First, periods of biotic homogenization that occurred before significant anthropogenic impact on vegetation demonstrate that not all homogenization is a product of anthropogenic change: it can also be driven by natural causes In fact, recent human impact (over the last 200-years) appears to have resulted in increased regional differentiation and not in homogenization - a result that contradicts most studies based on more modern, short-term records Second, both abiotic (climate and disturbance) and biotic factors are likely drivers of intervals of differentiation and homogenization We suggest that differentiation may be triggered primarily by climate changes and disturbances (mostly natural pre-2500-cal.-yr-bp and human-induced thereafter), whereas homogenization may be driven predominantly by biotic interactions (e.g immigration and interspecific competition) Third, this long-term study raises awareness that assessments of pattern in vegetation homogenization/differentiation may depend on the specific time period and length of investigation Long-term investigations through multiple generations are likely to yield particularly useful information on the mechanisms and effects of biotic homogenization. © 2010 Blackwell Publishing Ltd