DUNE VEGETATION AT SAN FRANCISCO 197 



Associations 



Vegetation of the dunes is not readily catalogued into definite 

 communities. One may, however, distinguish a mixed scrub 

 association in the vegetation mentioned as occurring in sheltered 

 places that are not especially moist. The lower areas, domi- 

 nated by willows may be called a willow thicket association. 

 These "associations," as also the pioneer groups and the relic 

 mounds all belong to a single sand-dune formation.^ 



•*..s*^ 



V? 



r^ 



Fig. 4. A patch of Franseria undermined and destroyed by blowing away of 

 the substratum. 



* Later stages of the mixed scrub and of the willow thicket, while not occur- 

 ring in the neighborhood of the dunes, may be seen at Land's End, 3 km. to the 

 north and beyond the Cliff House. Here there are cliffs 20 to 40 meters high, for 

 the most part of fairly stable rock, but in places giving way to soft sandstone. 

 Intermittent streams or storm waters have eroded this sandstone and carried 

 down the material, forming alluvial fans. These fans differ in soil character 

 from the dunes in that they have a certain amount of humus. Upon these fans 

 there develops a vegetation-complex closely like that of old dunes. There are 

 the same shrubs and herbs but in somewhat different proportions. The yellow 

 lupine is especially prominent; Eriophyllum also is common. Where ravines 

 occur in sandy soil the willow thicket becomes well developed, but here with the 

 increased amount of humus there is a far richer flora than in the dune area. 

 Many additional species enter the association as: Riibus viiifolius, V icia gigantea, 

 Heteromeles arbuiifolia, Diplacus glutinosus, Micromeria chamissonis, Sambucus 

 glauca. 



