NO. 3 DUNKLE : PLANT ECOLOGY, CHANNEL ISLANDS 259 



island. This is significant as it has been demonstrated that the wind is 

 the controlling factor, with precipitation a subsidiary factor, in the 

 distribution of insular plant communities. While a study of meteoro- 

 logical records shows clearly that while there is a general climatic 

 similarity for the entire insular area, there are wide extremes between 

 the southern tip of San Clemente and the northern tier of islands. These 

 differences are even more pronounced in the areas of maritime climate 

 on the adjacent mainland. 



There is considerable uniformity in both precipitation and tempera- 

 ture in the area between Point Vicente and Cabo San Quintin in Baja 

 California, with a rather sharp break north of the former and south of 

 the latter. Accordingly, these seem logical points for a division of the 

 maritime climate into three types. 



The northern or semi-humid maritime would be separated from 

 the central or arid maritime by the isohyet of thirteen inches of annual 

 precipitation. The division between the arid and the desert maritime 

 would be on the isohyet of six inches. 



The northern or semi-humid maritime would be separated from the 

 central or arid maritime by the isohyet of thirteen inches of annual 

 precipitation. The division between the arid and the desert maritime 

 would be on the isohyet of six inches. Figure 5 shows the approximate 

 area of the maritime and the proposed division of this climatic type. 

 South of Cabo San Quintin much of the dominant growth is cacti and 

 other low shrubs typical of desert conditions, with the individual plants 

 well spaced. North of Cabo San Quintin, extending to Point Vicente 

 there is much grassland on the rolling slopes with typical chaparral on 

 the steeper slopes, with a general scarcity of trees at lower elevations 

 with the exception of the riparian woodlands. North of Point Vicente 

 trees become increasingly evident and cacti become rare, while more 

 mesic plants such as Myrica, Umbellularia, and Holdodiscus appear. 



Among the islands, the counterparts of the sub-divisions of the 

 maritime climate on the mainland are recognized as follows : San Miguel, 

 Santa Rosa, Santa Cruz, and the western island of Anacapa^ would 

 fall in the semi-humid maritime; Santa Catalina, Santa Barbara, San 

 Nicolas, and the northern two-thirds of San Clemente would be placed 



lAn eight-year record on the eastern island of Anacapa shows an average 

 annual precipitation of 12.58 inches, and this period includes two rather ex- 

 ceptional wet years. The middle island of Anacapa possesses much the same 

 aspect as the east island, and Opuntia prolifera is to be found on both islands. 

 The vegetation of the western island of Anacapa has a different aspect from the 

 two eastern islands. Due to the greater heights of Santa Cruz and the west 

 island of Anacapa it would appear that the two eastern lie in a rain shadow. 



