Strand and Dune Flora of the Pacific Coast 179 



drop from 14 to 10. At the Mexican boundary the species density 

 is 7; at Socorro, 5; and beyond Bahia de Sebastian Vizcaino but 

 I species, Abronia maritima, remains. As far as Socorro, no com- 

 pensating strand plants appear to replace those dropping out, 

 except that Ailenrolfea occidentalis (Wats.) Ktze. and species of 

 Salicornia tend to become builders of foredune hillocks. Certain 

 tropical strand species appear farther south— for example, Ipo- 

 moea pes-caprae — but the extent and character of the strand 

 flora of the Mexican coasts is not at present ascertainable. 



In more general view, the region of high species density may 

 be considered as extending from southern Vancouver Island to 

 Monterey Bay. Yet this is by no means the region where the 

 greatest number of individuals of the type appear. Foredune 

 hillocks, accurate indicators of the number and vigor of these 

 species, are almost absent on the Oregon coast. They become 

 gradually more evident southward, and attain their greatest 

 development in southern California, where species are compara- 

 tively few but individuals are abundant. 



Two factors may provide an explanation. Constant and severe 

 summer winds in the north are inimical to foredune formation 

 both because they make accumulation of sand difficult and be- 

 cause they increase transpiration from the plants. Again, in the 

 north, frost kills back the plants in winter, so that accumulations 

 made during the summer are dissipated, whereas in the south 

 growth is continuous and the hillocks constantly tend to increase 

 in size. Particularly important in the south are the frost-sensitive 

 succulents Abronia, Carpobrotus, Convolvtdus, and Franscrta, 

 which there are able to flourish throughout the year. 



Effects of barriers. — For the most part, upon the coast of the 

 United States and Baja California the strand-dune environment 

 occurs in strips of varying length separated by bluffs, cliffs, bays, 



