NO. 3 DUNKLE : PLANT ECOLOGY, CHANNEL ISLANDS 273 



from the higher volcanics and gradually become shallower as the break 

 of the cliffs is approached. The soils of the terraces are relatively deep 

 and underlain by silty clay. At the immediate edge of the cliffs the soil 

 is only a few inches deep and much coarser than on the terraces. On 

 wind-swept headlands the soil is extremely shallow and consists mainly 

 of lag gravels. Soils in all parts of the island are alkaline in character, 

 the pH varying from 8.0 to 8.6. 



Table 3 presents data on wind force, angle of slope, and mechanical 

 analyses of different soils. 



The percentage of gravel in the soil is highest and the percentages of 

 silt and clay lowest where the slope is greatest or the wind velocity 

 highest. A clear-cut descending sequence in the percentage of particle 

 size is evident under either of the preceding conditions. The fine soils 

 of the terraces have been enriched by the decomposition of the luxuriant 

 vernal growth of grasses and forbs, as well as by the droppings of the 

 thousands of sea birds which have nested there for untold years. The 

 covering of grasses, forbs, and shrubs is so dense that there can be but 

 little run-off from the seasonal rains. This fact is evidenced by the small 

 amount of water erosion. 



Climatic Agencies 



Because of its central location among the other islands the wind, 

 temperature, fog, and precipitation are intermediate in respect to these 

 conditions on adjacent islands. The climate is more oceanic in character 

 than that of the nearby mainland. The thermograph records show an 

 average annual temperature range of less than 3° C. (5.4° F.). This 

 small range is partially accounted for by the more frequent periods of 

 warm, east wind on the island during the late fall, winter and early 

 spring. 



Wind must be considered the controlling factor in the climate of 

 the island. The direction of the wind, rather than the height of the 

 sun determines the temperature. The velocity of the wind determines 

 the areal limits of the plant communities. The prevailing direction of 

 the wind is from a little north of west. There is usually, however, a 

 weak land breeze during the early morning hours, and the seasonal 

 pressure gradients from September to May bring a dry, warm, east 

 wind over rather extensive periods. The average wind velocity recorded 

 on the central ridge at an elevation of 133 meters (435 feet) from 

 September 11, 1940, to December 6, 1941, was 16.8 miles per hour, 

 while that recorded on the eastern bluff at an elevation of a little less 



