PLANT SUCCESSION IN WASHINGTON 275 



The major plant groups of the buttes and mountains, in which 

 evaporation was measured, in a succession extending from the 

 prairie to the cHmax mesophytic forest, are: the prairie, open 

 yellow pine association, Douglas fir-tamarack association, and the 

 cedar association. This list, however, is not complete. For 

 example, pure associations of yellow pine and Douglas fir (Pseu- 

 dotsuga viucronata) often occupy a position intermediate in suc- 

 cession between the yellow pine and Douglas fir-tamarack 

 association; while the white fir {Ahies grandis) may come in with 

 or even precede the tamarack. 



Livingston,- in his researches with his porous cup atmometer, 

 has shown that the evaporating power of the air is a rather 

 satisfactory summation of the atmospheric factors which deter- 

 mine the growth of plants during that portion of the season 

 free from frost. Accordingly, in the spring of 1913, a number of 

 observation stations were established in various plant associations 

 and by means of the Livingston porous cup atmometer the rate 

 of evaporation was determined during the ensuing growing sea- 

 son. The cups were mounted in bottles of convenient capacity, 

 closed with tightly fitting cork stoppers that were perforated 

 for the atmometer tubes and for bent capillary tubes which 

 served to equalize the atmospheric pressure within the bottles 

 with that of the external air, without causing loss by evaporation 

 or permitting rain water to enter the reservoir. The bottles 

 were sunk into the soil so that the evaporating surface of the 

 cups was in all cases 17-23 cm. above the surface of the soil. 

 Readings were taken at weekly intervals, or more frequently 

 except at the distant stations, which were visited on alternate 

 weeks. At each reading the bottles were refilled with a weak 

 (2% solution) of formahn from a 100 cc. graduate to a file scratch 

 on the neck. New instruments were employed, all of which 

 were standardized to the same unit before using and restandard- 

 ized at the end of the season. In correcting the readings a 

 gradual alteration was assumed and interpolations were made 

 for the coefficients to apply to the various readings. At most of 



- Livingston, B. E., Evaporation and Plant Habitats. Plant World, 11: 1-10, 

 1908. 



