1908J HARVEY— PRAIRIE-GRASS FORMATION 87 



of physiographic development (Bessey '99). And again, as the 

 prairie existed as a climatic formation long before these fires, either 

 of Indian or Caucasian origin, swept the prairies, it would appear that 

 this fire factor has been overestimated by many and is in no sense to 

 be regarded as the fundamental factor. That there is, nevertheless, 

 an extremely slow advance of the forest usually through the medium 

 of its forerunner, the shrub association, whose pioneer Symphoricar- 

 pus occidentalis is followed by Rhus glabra, is very evident. 



While the above factors are seemingly adequate to account for 

 tree absence upon the western upland prairie, yet in light of the 

 prairie as a natural climatic formation it would seem more proper to 

 make the problem one accounting for the occasional presence rather 

 than the general absence of trees. A future paper is planned to 

 discuss the factors controlling this encroachment. 



The period of growth resumption 



Not until about the first week in March are climatological condi- 

 tions at all favorable to an awakening of vegetation, and then only 

 on infrequent days; but the month as a whole is marked by the open- 

 ing of flower and leaf buds of trees and the beginning of the germina- 

 tion of prevernal annuals and the formation of the basal rosettes of the 

 perennials, though some tide over from the previous season. The 

 prevernal bloomers naturally make most rapid progress, aided by 

 their geophytic habit. Not infrequently the temperature falls below 

 freezing and killing frosts ordinarily result. Light snow storms, 

 which rarely occur, may temporarily retard growth. The conditions 

 become progressively more favorable and pass insensibly into the 

 period of the prevernal flowers. 



The usual snows and rains of the early part of the month assure 

 abundant moisture. The chresard, which is about 18 to 20 per cent, 

 in the early days of the month, decreases to about 14 per cent, at the 

 middle, and to about 12 per cent, toward the last of the month, thus 

 giving an average chresard of some 15 per cent. The different 

 exposures of the prairie hills, knolls, and ravines progressively recover 

 from the effects of winter in the following order: southeast to south- 

 west, southwest to northwest, northeast to southeast, and northwest 

 to northeast. The frost clings to the northern exposure in ravines, only 



