THE BROAD-SCLEROPHYLL VEGETATION OF CALIFORNIA. 17 



II. CLIMATIC RELATIONS. 



Schimper (80) has shown clearly the constant relation between 

 sclerophyll dominance and climate. He says: 



"The mild temperate districts with winter rain and prolonged summer drought 

 are the home of evergreen xerophilous wood}'- plants, which, owing to the stiffness of 

 their thick, leathery leaves, may be termed sclerophyllous woody plants. 

 Wherever original conditions have not been altered by man the sclerophyllous trees 

 and shrubs of districts with a moist winter always form dense and continuous wood- 

 land, which in most cases consists principally or exclusively of shrubs, but which 

 occasionally becomes true forest, although of low or middle height only." 



As to the advantage of the evergreen habit, he states that the 

 vegetation is subject to short but frequent irregular periods of rest, 

 sometimes due to cold, sometimes to drought; that short periods 

 only afford simultaneously optimum conditions in temperature and 

 moisture; the absolute optima for these two factors being entirely 

 separate in time. It is therefore decidedly advantageous for the 

 plants to be prepared to do their assimilative and vegetative work 

 at all times. The regions which possess such a climate and support 

 such a vegetation are, according to Schimper, the Mediterranean 

 shores, the southwest extremity of Africa, southwestern and much 

 of southern Australia, central Chile, and California. More detailed 

 and localized works (4, 7, 8, 25, 78, 91) confirm Schimper's conclusions. 



After such a thoroughly adequate, even though brief, treatment 

 by the pioneer author, it is necessary here merely to particularize 

 somewhat concerning the region under discussion. 



The map (plate 2) is in part adapted from Reed and Kincer (77). 

 The iso-lines indicate the percentage of total precipitation occurring 

 in the half year April 1 to September 30. By comparing it with 

 the map (plate 1) a remarkable correspondence will be at once 

 evident between the region where the summer precipitation is less 

 than 20 per cent and the region where broad-sclerophyll species 

 are numerous. Moreover, the area of less than 10 per cent summer 

 precipitation corresponds closely with the center of distribution of 

 the sclerophylls, as described in the section devoted to that topic. 

 Seasonal distribution alone, however, is not sufficient to explain the 

 region of dominance of the broad-sclerophylls ; total precipitation is 

 only slightly less important. If the total is very high, or if atmos- 

 pheric conditions are such as materially to reduce evaporation during 

 the dry season, it may be possible for species that are less xerophytic 

 to control; if the total is very low, true desert species will replace 

 the broad-sclerophylls. I have therefore plotted, in a generalized 

 way, the total precipitation in the area with less than 20 per cent 

 summer rainfall. The iso-lines selected are of course arbitrary, but 

 they nevertheless mark out in a striking way the areas dominated 

 by the three great vegetation types. Where the total precipitation 

 is more than 30 inches, the vegetation is conifer forest of some type. 



