46 The Botanical Survey of Nebraska 



EVAPORATION 



A continuous record of the evaporating power of the air was 

 obtained from July 25 to September 16, in all of the forest sta- 

 tions and also in the prairie on the knoll shown in the transect, 

 figure 16. The atmometers were placed as usual at a height of 

 only 15-17 cm. above the soil surface and in that portion of the 

 aerial stratum occupied by the leaves of tree seedlings. The data 

 are shown in figure 17. An examination of these data shows a 

 remarkable uniformity in the general trend of the graphs which 

 also point out plainly the decreasing evaporating power of the air 

 which reaches its minimum in the linden consocies. 



SUMMARY 



1. This paper contributes data which still further substantiate 

 the conclusions drawn from previous investigations that evap- 

 oration rates and soil moisture conditions in the various plant 

 communities studied vary in general directly with the order of 

 the communities in succession, the communities nearest the climax 

 being the most mesophytic in both respects. 



2. The prairies and woodlands in the vicinity of Lincoln are 

 considerably more xerophytic than those in the neighborhood of 

 Peru. This fact is strikingly revealed in a comparison of the 

 ecological factor data from the two areas, and it is also strongly 

 reflected in the composition of the plant communities in the two 

 places, although the two areas are only about 60 miles apart. 

 Available soil moisture was exhausted in the prairie station at 

 various depths in the vicinity of Lincoln on eighteen different 

 dates in 191 7, from May to September, while the same condition 

 was recorded for only four dates, late in July and August, in the 

 prairie station at Peru. 



3. Ecological conditions are shown to change rapidly as the 

 forest-prairie ecotone is traversed in Nebraska. Habitats rapidly 

 become more xerophilous and many of the more mesophytic 

 species drop out entirely between Peru and Lincoln. It is thus 

 shown that the natural extension of our native woodlands is 

 greatly hindered, possibly altogether prevented in so far as any 



