THE CHAPARRAL COMMUNITY. 



29 



and also decreases the evaporation from the soil surface. The reduc- 

 tion of the evaporating power of the air within a Symphoricarpos area 

 at a height of 17 cm. as compared with that 2 meters beyond in the 

 prairie is well illustrated by the following data, giving the average 

 daily evaporation in a clump of Symphoricarpos and in the adjacent 

 grassland, both near the crest of a loess hill : 



Date. 



Prairie. 



Scrub. 





c. c 



c. c. 



July 1 to 7, 1918 



33.1 



25.6 



July 29 to Aug. 5, 1918.. 



46.0 



33.8 



Aug. 26 to Sept. 4, 1918. 



18.0 



14.6 



Not only is the evaporating power of the air lowered by the presence 

 of the scrub, but among its stems drifts of snow find lodgement and 

 upon melting add considerably to the moisture content of the soil. 

 The run-off is greatly reduced as a result of the rich mulch of fallen 

 leaves and large quantities of wind-transported plant debris lodged 

 among its stems. Only a few years are required, owing to the more 

 favorable conditions for the formation of humus under the shrubs, to 

 fill the former prairie soil with a rich humus mulch not unlike that of 

 the woodland. Thus by cumulative favorable reactions the scrub 

 is frequently able to extend its area of dominance and also to prepare 

 a fine nursery for the seedhngs of trees, thus initiating a new stage in 

 the normal succession. Repeated determinations of water-content to 

 a depth of 3 feet show that the thicket soil is moister than that of the 

 prairie. 



The reactions of Rhus upon the habitat are similar to those de- 

 scribed for Symphoricarpos. Being a taller shrub, its effect upon the 

 bordering grassland is more pronounced. It invades the prairie by 

 means of rhizomes. These have been traced at a depth of only 4 to 

 8 inches to a maximum distance of over 20 feet from the edge of a 

 thicket, their course being marked by the presence of erect shoots. 



The evaporating power of the air is greatly reduced under Rhus as 

 compared with that in the prairie^ as is shown by the following experi- 

 ment: From May until September, 1916, non-absorbing atmometers 

 placed just 6 meters within the sumac thicket gave evaporation losses 

 averaging about 10 c.c. per day as compared with average daily losses 

 of 22 c.c. from similar instruments 24 meters outside the thicket in the 

 prairie. Likewise, the water-content of the soil at the scrub station 

 as compared with that of the grassland was found to average about 

 6 per cent higher throughout the season to a depth of 10 inches. The 

 deeper layers of soil to 5 feet were invariably much moister in the 

 sumac community. 



