NO. 3 DUNKLE : PLANT ECOLOGY, CHANNEL ISLANDS 325 



attack. This has resulted in very narrow beaches or steep cliffs and 

 bluffs bordering the islands. The bluffs possess a large number of en- 

 demics and shelter many remnants of the indigenous flora. Owing to high 

 wind velocity, the evaporation rates on the bluffs are higher than in the 

 interior of the islands. The soils of the bluffs are shallow and the soil 

 particles coarse. The variety in slope angle and exposure occasions great 

 variation in the communities of the upper slopes. In communities of the 

 shore and splash zone there is less variation because of the predominating 

 influence of the sea. 



Canyon, terrace, and hillside communities are differentiated in ac- 

 cordance with the pattern of similarly situated mainland communities, 

 though on the islands the differences are somewhat more sharply ac- 

 centuated owing to the higher velocities of the island winds. Woodlands 

 exist only in areas protected from wind and the direct rays of the sun, 

 and where the average annual precipitation is over 13 inches, or where 

 there is underground water available. Grasslands occur on fine-textured 

 soils on terraces, gentle slopes, and uplands exposed to moderate winds. 

 In many parts of the grassland, where it has not been too heavily pas- 

 tured, there are tree, shrub, and semi-shrub savannas. Northerly and 

 southerly exposures of canyons are sharply differentiated in respect to 

 their evaporation rates, plant life-forms, and floristic composition of the 

 plant communities. 



The succulent and thickly pubescent plants of the sea bluffs and the 

 trees and shrubs with sclerophyll foliage maintain a more or less uniform 

 appearance throughout the year. Generally, however, the winter Is a 

 season of growth, the early spring of flowering, while summer and fall 

 are rest periods. There is little undergrowth in closed communities. 



The limitations of this study have left many interesting problems 

 unsolved. Too little time has elapsed since heavy grazing has been 

 discontinued to present a clear picture of the tendencies of plant succes- 

 sion. Furthermore, it is probable that further field work on San Cle- 

 mente and San Nicolas islands will disclose additional evidence of plant 

 variation. The scope of the present investigation has not included in 

 detail the nature of the physiological and morphological modifications 

 resulting from insular isolation, nor how genetic or environmental fac- 

 tors have influenced variation. Neither has attention been given to modi- 

 fications which enable many of the plants of sea bluff and headland to 

 maintain growth and vigor during the long dry season. 



