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Spread and Expansion of Alnus Mill. In the British Isles: Timing, Agencies and Possible Vectors

F. M. Chambers and L. Elliott
Journal of Biogeography
Vol. 16, No. 6 (Nov., 1989), pp. 541-550
Published by: Wiley
DOI: 10.2307/2845209
Stable URL: http://www.jstor.org/stable/2845209
Page Count: 10
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Spread and Expansion of Alnus Mill. In the British Isles: Timing, Agencies and Possible Vectors
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Abstract

The conventional hypothesis states that Alnus glutinosa (L.) Gaertn. spread to the British Isles from refugia in central and eastern Europe, reaching South-east England via the Low Countries after 8500 BP. Recent findings cast doubt on this view, suggesting additional, earlier, spread from a western refugium in or near western France. An examination was made of pollen data from 174 published and some unpublished sites to test these two hypotheses. Such analysis demonstrates that the conventional model is misleading, but that the alternative is also deficient. Re-appraisal of data in the light of modern views on the pollen representation of tree taxa suggests a wider range of possible first arrival times. Indeed, Alnus, in Britain and Ireland in the Devensian (Midlandian), with long-term survival of small isolated populations is not necessarily impossible. A native inoculum might have existed, and it may no longer be necessary to invoke Post-glacial spread from the Continent. Nevertheless, a major expansion of existing Alnus does seem to have taken place in the British Isles in the millennium from c. 7500 BP. Probable agencies in expansion include disturbance, such as by beaver and human activities. Vectors in long-distance spread are discussed; it is concluded that dispersal by birds has previously been underestimated.

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