Eilif Dahl (1998)

Pp. xii + 297. Cambridge University Press.

?60 (hardback), ISBN 0-521-38358-7.

This book is Eilif Dahl's ‘Swan Song’. Ipse dixit. Sadly, he did not live to complete it, but that task, with the exception of a final intended chapter on aquatic plants, was ably taken in hand by his widow Gro Bulden and his friend and colleague Professor John Birks of Bergen University. It is a slender volume at the price, and almost half consists of five tabulated appendices. There is one photograph, a happy and informal snapshot of the author, and 82 text figures, mostly species distribution maps. However, the work is clearly written and there is much solid material here – the compilation of a lifetime's labour in the disciplines of geology, climatology, plant ecology and the systematics of both vascular plants and cryptogams. Eilif Dahl has brought his questing mind to bear on the process by which studies in physiological ecology can be made to illuminate the many problems of plant species distribution across his chosen area of northern Europe and Iceland. Plant geography is full of fascinating correlations but, as Dahl is at pains to insist, a correlation is not an explanation and a physiological mechanism for it has to be sought and tested by experiment. This he has done to a greater extent than most. There are extensive references covering all published works of relevance up to 1994.

After introductory chapters on the relevant aspects of climate, soils, Pleistocene history and the methodology of the discipline, the core of the book consists of a detailed examination of the four most generally accepted floristic elements – Atlantic (Oceanic), Thermophilic, Boreal and Arctic Alpine. This occupies 69 pages. The various distribution patterns, such as endemic, disjunct and centric, within these elements takes up a further 38 pages.

There is a final short chapter on anthropochorous plants. Several species are rather confusingly listed under two separate floristic elements. Thus, Betua nana appears as belonging to both the Scandinavian boreal subelement and the montane element. There is no explanation but no doubt in this case it is due to the difficulty of separating species whose requirement is for cold winter temperatures from those requiring low summer temperatures since both factors restrict distribution in similar ways. B. nana certainly requires further autecological study as it is exclusively a plant of acid peat bogs in Scotland but mainly a member of free-draining shrub communities in Norway.

The immense value of this work lies in the way it brings together, for the first time, an up-to-date analysis of plant distribution in northern Europe, embracing bryophytes and lichens as well as vascular plants, and also both tested and untested hypotheses on the underlying physiological and historical mechanisms of these distributions. Dahl was a ‘periglacial survivalist’ and many of his early observations of unglaciated pockets on the western seaboard of Norway and Scotland, at first treated with scepticism by geological colleagues, have been accepted subsequently as correct. The jury is still out on whether vascular plants could have survived there and Dahl fairly presents the arguments of colleagues who consider ‘the nunatak hypothesis’ unnecessary to explain present known distributions. Fossil evidence remains neutral on this point.

Appendix II holds valuable reference material; it lists all northern European species of Flora Europea (over 2200 excluding subspecies), indicates whether they occur in the British Isles and Fennoscandia and gives the climatic parameters that Dahl has used in establishing his correlations, the rationale of which is thoroughly discussed in Chapter 6 and Appendix I.

But this book is not just for academics. There is much information for the more practically minded ecologist to apply to his or her own field problems. For example, how many foresters or forest ecologists in this country are aware that the seeds of Scots pine and Norway spruce lose their viability if repeatedly wetted and dried at temperatures too low for germination? Seeds of Norway spruce are shed in autumn and protected from the damp by snow cover in continental areas but not in oceanic coastal regions. Those of Scots pine are shed in winter or early spring, late enough to survive the critical cool wet period. Hence, pine but not spruce is well established along the Norwegian west coast, although spruce grows well if planted there. Armed with this information the situation in Scotland can be reasoned as follows: here we observe that lack of pine regeneration is seldom due to failure of seed viability, while planted spruce shows occasional regeneration in a region which is even more oceanic than western Norway. However, spruce cones fall unopened from the trees in autumn and seeds are thus able to remain dry until the occasional spring, or summer drought periods may release still viable seeds on the drier substrates. Pine seeds are normally shed in spring, but prolonged wet weather may delay this until seedlings are unable to develop sufficiently before winter, or the seeds themselves may encounter the same problem as spruce seeds shed in autumn in coastal Norway.

If you have any interest whatever in the byways of European plant geography, this book is worth reading – and then buy a copy for your own shelves, for you will want to refer to it repeatedly.