112 HUGO SJORS 



The Boreo-nemoral region in America is a tract of land where Boreal 

 conifers {Picea glauca, P. mariana, Abies halsamea) are well distributed and 

 frequently tend to be predominant, even if now largely replaced by secondary 

 hardwood forests. The latter are extensively formed by trembling aspen 

 {Popuhis tremuloides) which, like the white birch {Betula papyrifera), is a 

 "neutral" tree of little zonal importance. Also some southern hardwoods have 

 expanded considerably into the widespread secondary forests, notably sugar 

 maple {Acer saccharum). Other southern tree species common in most of the 

 Boreo-nemoral region are Quercus borealis, Tilia americana, Populus grandi- 

 dentata, Betula lutea, Fraxinus nigra, Ulmus americana, and the conifers 

 Pinus strobus, P. resinosa. and Tsuga canadensis. Some of these even reach 

 quite a distance into the Boreal region farther north (see below). 



Most of the "Acadian Forest Region" (Rowe, 1959) in the Maritime 

 Provinces of Canada can be classed as part of the Boreo-nemoral zone. It is 

 rich in hardwoods in the lower hills, whereas conifer forests are often pre- 

 dominant on the coast, in the valleys, and in the interior upland. Red spruce 

 {Picea rubens), a non-Boreal species, is often dominant. 



The spruce-fir forests of the northeastern U.S.A. are found at increasingly 

 higher elevation towards the south (Shantz and Zon, 1924) and are here 

 mapped as Montane (Fig. 2). They merge into the Boreo-nemoral zone 

 towards the north and are not truly Boreal. Picea rubens is one of the chief 

 species. 



In Europe, conditions are simpler. The European vicariants of Pinus 

 strobus (i.e. P. peuce), and P. resinosa (i.e. P. nigra) are confined to the south 

 for historical (not climatic) reasons and do not interfere. P. silvestris occurs, 

 chiefly on sandy soils, even in some parts in the Nemoral zone, but in the rest 

 of the Nemoral area, the forests were deciduous almost everywhere before 

 conifers were planted. Passing into the Boreo-nemoral zone, there is a very 

 sudden shift to predominant Boreal conifers: Norway spruce {Picea abies) 

 and Scots pine {Pinus silvestris). Hornbeam {Carpinus befulus), beech {Fagus 

 silvatica) and durmast oak {Quercus petraea) occur only in small western 

 parts of the Boreo-nemoral. Most of the other southern hardwoods continue 

 their distribution farther north (or rather northeast). The Boreo-nemoral 

 region in Europe, contains much Quercus robur, Fraxinus excelsior, Ulmus 

 glabra, Tilia cor data, Acer platanoides (not A. pseudoplatanus), Corylus 

 avellana, etc. Although generally subordinate, these southern hardwoods may 

 be of great local importance, particularly on calcareous soils (e.g. on the 

 island of Oland, Sweden, which is crossed by the borderline between the 

 Nemoral and Boreo-nemoral regions). 



The Boreo-nemoral region in Europe is narrow in the west and the east 

 but quite broad in the central part. It reaches from southernmost Norway over 

 most of the southern third of Sweden (the southwestern and southern 

 coastal parts belong in the Nemoral zone). The southwestern corner of Finland 



