1 6 The Botanical Survey of Nebraska 



able moisture in the various plant communities vary directly 

 with the order of those communities in succession. Long periods 

 during which there is a deficiency of available moisture in the 

 upper two or three feet of prairie soil are efficient barriers in the 

 way of a general invasion of prairies by trees in the absence of 

 the usual intermediary and ameliorating successional stages. 

 Even an infrequent depletion of soil moisture at the depths from, 

 which trees obtain their water supplies is a factor powerfully op- 

 posing the success of forest growth in a prairie soil and climate. 

 The severe winter droughts of 1916-17 and 1917-18 and the 

 extensive winter killing resulting therefrom throughout the cen- 

 tral portion of the prairie province are particularly illuminating 

 in this connection especially in view of the fact that the deep- 

 rooted .perennials so characteristic of our prairie associations 

 were not noticeably affected by those exceedingly xerophilous 

 periods. Such extreme limits of environmental factors are likely 

 to prove peculiarly fatal to vegetation not possessed of the full 

 complement of anatomical and physiological peculiarities neces- 

 sary to cope with these extremes. 



SOIL TEMPERATURE 



Soil temperature is probably an important factor in the develop- 

 ment of vegetation in the prairies, especially during the earlier 

 ecesic phenomena. The precise role performed by soil temper- 

 ature in ecology will not be clearly understood until much more 

 investigation has been conducted with reference to that par- 

 ticular factor. However, it is doubtless safe to state that the 

 temperature of the soil, especially at the surface, is actually the 

 determining factor in the failure or success of many very young 

 seedlings, and it may, indeed, inhibit germination itself. This is 

 particularly true in the case of arid and semi-arid habitats. The 

 period during which soil temperature extremes may prove fatal 

 to the establishment of plants in the field, and the relative resist- 

 ance of species to soil temperature extremes are problems that 

 should be investigated in connection with all studies of succession. 



Soil thermographs were operated continuously at the prairie 



22 



