PLANT SUCCESSION IN WASHINGTON 



293 



the cedar, make up the characteristic uiulergrowtli, while further 

 up the slope the ground is almost bare. Three instrmuents 

 were installed here on June 14 and readings were taken on alter- 

 nate weeks luitil September 19. All of the atmoineters were 

 placed in situations rather free from undergrowth, so that the 

 readings are perhaps higher than representative conditions 

 warrant. During the same period two atmometers were main- 

 tained at station No. 11 hi the fir-tamarack association. This 

 association also occupies a north slope somewhat above and 

 east of the cedar association. The Douglas fir and tamarack are 

 the principal trees, but a considerable amount of white fir and 

 some spruce (Picea englemayini) are also present. The under- 

 growth is very sparse. The few species represented include 



Fig. 10. Diagram showing the comparative evaporation rates in the butte- 

 mountain plant formations and associations on the basis of the average daily 

 amount from May 7 to September 5 (except in the cedars, which is from June 

 14 to September 19). 



some of those enumerated at station No. 10. Unfortunately 

 one instrument was taken a week before the end of the season 

 so that only a single graph was obtained from data taken here. 

 This together with those obtained in the cedars is represented in 

 figure 9 where all readings on the prairies, in the pines, in the fir- 

 tamarack association and in the cedars are respectively combined. 

 Inspection of these combined graphs together with figure 10, 

 where the average daily rate of evaporation is shown for each 

 association, bears out the former finding that plant succession 

 is closely related to evaporation. 



The plant formations and associations, their order of succession, 



