Species formerly found only in especially favorable sites became more 

 abundant on typical sites, and species usually found in dry situations de- 

 creased in the communities once dominated. Coupland noted that the same 

 area of gasssland could change dominance from Bouteloua-Stipa-Boutela to 

 Stripa-Agropyron (Coupland 1959). 



In 1977 rainfall distribution in the study area was more erratic than 

 usual due to the absence of wide scale synoptic systems. Instead, most of 

 the precipitation came from thunderstorms, resulting not only in a spotty 

 distribution but also increased overground flow and less water in the sub- 

 soil. Duley (1939) and Duley and Kelly (1939) have pointed out that not 

 only are the timing and amount of precipitation important, but so are the 

 intensity of rainfall, topography, and condition of the soil and ground 

 cover. An inch of effective precipitation might account for eighty to one 

 hundred pounds of production (Whitman and Haugse 1972, Smoliak 1956). 



Precipitation data are not available for the productivity sampling area. 

 Piatt and Griffiths (1964) point out that environmental data for any but the 

 area under study cannot be used directly, but precipitation data from three 

 areas surrounding the sampling area can give a general indication of possible 

 precipitation at the sampling area. Table 28 shows the 1977 precipitation at 

 three sites, identified by their location relative to the productivity samp- 

 ling area. 



Table 28 1977 Precipitation at Three Sites 



Location Relative to Productivity Sampling Area 



23 km SW 60 km NNW 35 km ESE 



May-June 



Precipitation 5.65 cm (2.62 in) 8.71 cm (3.43 in) 12.42 cm (4.89 in) 



April-July 



Precipitation 11.56 cm (4.55 in) 10.29 cm (4.05 in) 15.57 cm (6.13 in) 



The mean precipitation and confidence limits (0.1) for the May-June pre- 

 cipitation data shown above are 3.65 in. +1.93 in. (9.26 cm. +4.93 cm.) and 

 the coefficient of variation is thirty-two percent. 



Weaver and Albertson (1956) note that if spring-summer rainfall is re- 

 duced to less than seventy percent of normal, which is about 6.9 inches 

 (17.6 cm) in this case, "the effects upon vegetation are usually so serious 

 as to cause a drought." Using this definition, the productivity sampling 

 area experienced a drought during 1977. The growing period terminated by 

 exhaustion of available soil moisture (Whitman, 1975) ended quite early in 

 1977. 



Smoliak (1956) found that May plus June precipitation accounted for 

 seventy-four percent of the variation in forage yield in Alberta prairie 

 dominated by Stipa Comata , Agropyron smithii , Bouteloua gracilis . 



66 



