96 



THE STUDY OF PLANT COMMUNITIES • Chapter V 



graduated rod. The 10:1 ratio makes accurate readings possible 

 to 0.01 inch. Exceptionally heavy rains may overflow the tube, 

 and the water in the large cylinder must then be poured over 

 into the emptied tube for measurement. Two types of recording 

 gauges are in use. 19 One registers increments of fall as a small 

 bucket fills, tips, and records; the other, a weighing type, records 

 accumulative precipitation as it falls. 



Chestnut-Chestnut i 

 Longleof- 



r^V^ Tall Grass 

 \/ / /\ Short Gross 



E353 Bunch Grass 



5223 Marsh Grass EZZg ChOPOTTOl 

 WZZZZ Desert Savanna EZ22 Pacific 



Sagebrush 



Douglas Fir 



[Xm Creosote Bush ^^ °^ heT Western 

 ESS3 Greasewood Forests 



K/H 



Forests 

 Oak- Hickory 



^^ Oak- Pine 



FlG. 43. Isoclimatic lines of vapor pressure deficits and vegetation areas 

 of the United States.— From Huffaker. vlt 



125 



For generalized field studies, the precipitation records from the 

 nearest weather station may be quite satisfactory. However, there 

 may be wide local variations, especially if the topography is 

 irregular, and, in mountainous regions, only local measurements 

 have real significance. In addition, under forest stands, the pre- 

 cipitation reaching the soil will vary from stand to stand because 

 of variation in interception. Thus, for intensive work, it is desir- 

 able to maintain rain gauges at each site of study. Although stand- 

 ard gauges are desirable, it is possible to obtain satisfactory 

 records for comparison by using straight-walled jars or cans of 

 equal diameter. 



Snowfall is measured at a point where the wind has not caused 

 drifting or disturbance, and the equivalent value in rain is com- 

 puted from samples of the snow. Depending upon the density of 



