Southeastern Washington and Adjacent Idaho. a7 
Eight miles east, near Viola, Idaho, on another butte slightly 
lower than Kamiak, but with plant communities very similarly 
distributed, records from a third set of stations were obtained. 
Here again the south side is prairie, which the pines, occupying 
the crest and northwestern slope, are gradually invading. The 
sheltered northeast side is clothed with a forest of Douglas fir and 
tamarack. These conditions are representative of numerous 
other situations (Fig. 46). Only as the mountains become higher 
and rainfall increases, does the prairie give way to a forest 
growth. The prairies at Kamiak and Viola are not greatly un- 
like those upon the basalt as far as kinds of species and their dis- 
tribution are concerned, but, because of differences in soils and 
greater exposure to the wind, the vegetation bears a more xero- 
phytic stamp and there are fewer plants per given area, and con- 
sequently more soil surface exposed. 
Fig. 6 gives the evaporation rates in the various plant com- 
munities on Kamiak Mountain, and also that of the pine con- 
socies at Viola, Idaho. A glance at the chart shows at once a 
direct relation between the evaporating power of the air and the 
stage in succession. The station on Kamiak in the Festuca con- 
sociation was on a south slope and only about 60 feet below the 
edge of the pines. The station in the pines was in a decidedly 
xerophytic habitat, as is shown by the evaporation graph, which 
closely follows that of the prairie. The ground was covered with 
pine needles which almost excluded undergrowth and the at- 
mometers were shaded only indifferently by the open growth of 
the trees. The evaporation data in a more typical yellow pine 
habitat located on the northwest slope of the butte at Viola is 
plotted on the same chart, for the sake of comparison. It corre- 
sponds in general with evaporation records secured from a second 
station in the yellow pines located at the north base of Kamiak 
Mountain, but from which complete data were not obtained. 
Two stations were maintained in the shrub community. One 
of these was located on the south side of the mountain and on a 
northeast slope which was occupied by a very typical consocies of 
Opulaster pauciflorus, into which the pines were invading. The 
evaporation graph is given in the chart. It may be noted that 
27 
