Of particular interest in the Valley Grassland is the type of 

 habitat known as a vernal pool, sometimes locally called hog wal- 

 lows. These pools occupy depressions in the grassland area that 

 fQl with water during the winter. As the pools begin to dry up in 

 the spring, various annual plant species begin to flower (Plate 

 lOB). The result is local patches of color that may persist for 

 relatively long periods of time until the pools dry up. A number 

 of plants are restricted to these vernal pools of the Valley Grass- 

 land; these include several species of meadow foam {Limnanthes 

 spp., Limnanthaceae), downingia (Downingia spp., Lobeliaceae), 

 goldfields (Lasthenia spp., Compositae), and other colorful 

 genera. 



In Miocene and Pliocene times the area now occupied by the 

 Central Valley was a large inland sea. This sea diminished subse- 

 quently, although it persisted into recent times in the form of ex- 

 tensive lakes and marshlands that occupied the Central Valley 

 well into this century. One consequence of this relatively recent 

 availability of the valley floor for occupation by plants is that 

 many of the plant species that are restricted to the Central Valley 

 are of recent evolutionary origin: the evolution of these species 

 was associated with the appearance of a new and ecologically dis- 

 tinctive land area for occupation by land plants. The surrounding 

 upland areas have been occupied by plants for a much longer 

 period of time than the Central Valley and support evolutionarily 

 older plant species and plant communities. 



Riparian Woodland (Plate IOC, D) 



Since the climatic regime over much of California is an arid 

 one, the local occurrence of permanent standing or running water 

 has a striking influence on the vegetation. The many large streams 

 and rivers that flow out of the California mountains are generally 

 lined with deciduous trees, shrubs, and herbs that are restricted 

 to the banks of these water courses. Often these plants follow the 

 streams or rivers out into the Valley Grassland. In various portions 

 of California stream- and riverbanks are occupied by trees such 

 as Bigleaf Maple {Acer macrophyllum). Black Cottonwood {Pop- 

 ulus trichocarpa), and White Alder (Alnus rhombi folia). At lower 

 elevations and on the valley floors, water courses are lined with 



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