598 PLANT GROWTH AND PLANT COMMUNITIES 



vegetations are highly differentiated, many of them through impov- 

 erishment. The contemporary Canary laurel forest is floristically poor 

 (Ceballos y Ortuiio, 1951) in comparison with French Pliocene forests 

 (Depape, 1922); the temperate rain forests of the Antilles have none 

 of the deciduous-forest elements ( and may have diverged before their 

 emergence ) . The contemporary stands of evergreen cherry, Rhododen- 

 dron, Laiiriis, Mijrica faija in Southwestern Europe, are badly cut up 

 and invaded by "stronger" associations, when their component species 

 are not quite absorbed into the matrix of the moister oak (or even 

 beech) forests. This is the usual fate of such species as Viburnum 

 tinus, Ilex aquifolium, Ruscus aculeatiis, Prunus lusitanica, Laurus 

 nohilis. Rhododendron ponticum. These species, especially the first 

 three, can be considered bona fide members of modern associations. 



On the other hand, in some of the areas of high moisture ( such as 

 parts of the Maritime Provinces of Canada), we may be witnessing the 

 persistence of an early Northern Mixed Mesophytic forest, where 

 Picea, Abies, Tsuga, Thuja, Acer, Fagus, Tilia, Fraxinus all occur to- 

 gether in fluctuating numbers. I am aware that this has often been in- 

 terpreted as a postglacial merger of Boreal, Great-Lakes, and Decidu- 

 ous-Forest elements, and there is no denying that the descent of the 

 glacial sheet did repeatedly press together narrowing bands of these 

 three zones, and that a persistent cool-moist period may well have 

 blurred for some time the barriers between spruce-fir forest, hemlock- 

 hardwood forest and deciduous forest! 



I have attempted elsewhere (1953, 1956 a, b) to show how cli- 

 matic and ecological gradients, alternately narrowing and broadening, 

 may cause a phenomenon which I have called "cornering" and which 

 consists in reducing the area where the critical physiological require- 

 ments of a species can be met to such an extent as to send it sliding 

 down the scale of the four thresholds defined above. In this event, for 

 instance, a broad zone of Tsuga-Thuja forest, extending in North 

 America from the Pacific to the Atlantic between a Picea-Abies zone 

 to the north and a Sequoia zone to the south, is gradually squeezed 

 out as continentalization of the climate proceeds. Some of its species 

 (such as Ilex aquifolium. Viburnum tinus, etc., in Europe) remain as 

 members of a new climax Acer-Fagus-Tsuga; others slip down to serai 

 stages ( Thuja ) . 



On the ecological scale, at a more immediately observable level, 

 there are a number of species which are regionally located within a 

 certain climax area {Quercus rubra, Pinus resinosa) but which at this 

 time nowhere find a climatic-edaphic compensation system that will 

 allow them optimal development. This will keep them down as sub- 

 dominants or companions in a community. Rey ( 1960 ) has developed 

 a very ingenious system for calculating these equivalences, and his 



