58 THE VEGETATION OF THE UNITED STATES. 



occurrence. The region in which this tree was formerly most abundant 

 is characterized by stands of 3,000 to 4,000 feet board measure per 

 acre, the area of next greatest abundance by stands of 1,000 to 2,000 

 feet board measure, and the third by stands of 1,000 feet board measure 

 or less. It has been possible for us to determine the climatic condi- 

 tions for each of these areas separately. The location of the areas of 

 different abundance within the area of geographical range is so irregu- 

 lar for Pinus tceda as to make our correlations difficult and to suggest 

 the strong importance of soil influence in determining the stands of 

 this tree in the different parts of its area. There is no other case, 

 however, in which the ecological distribution of any plant has been 

 so carefully worked out, and we are consequently unable to make use 

 of other maps showing the distribution of plants over regions in which 

 soil conditions are not so important in determining their relative 

 abundance. 



THE ECOLOGICAL DISTRIBUTION OF LIRIODENDRON TULIPIFERA. (PLATE 9.) 



There are several deciduous trees for which the areas of commercial 

 abundance have been determined, and we have selected one of these 

 for use in our cUmatic correlations. Mr. George M. Lamb, of the 

 United States Forest Service, has courteously given us the data for 

 the map shown in plate 9, indicating the geographical range and com- 

 mercial range of the tulip tree {Liriodendron tuUpifera). This map 

 subdivides the range of the tree much less satisfactorily than the map 

 of Mohr for Pinus tceda, since it indicates only the two regions of 

 relative abundance. The simphcity of this map, however, makes it 

 extremely useful for our purposes, especially in view of the relatively 

 small number of climatological stations from which we liave data. 



THE ECOLOGICAL DISTRIBUTION OF BULBILIS DACTYLOIDES. (PLATE 10.) 



In the absence of any previously published maps showing the ecologi- 

 cal distribution of plants other than trees, we have endeavored to con- 

 struct such a chart for buffalo-grass {Bulhilis dadyloides) . This has 

 been made on the basis of all available descriptive literature and 

 has been submitted for criticism to several botanists familiar with the 

 Great Plains region, to all of whom we are greatly indebted for informa- 

 tion. The map shown in plate 10 is designed to indicate the area in 

 which buffalo-grass was formerly a very common element of the Grass- 

 land, the area in which it was of frequent occurrence merely, and the 

 area'in which it is of scattered or rare occurrence. The geographical 

 range of this species coincides in a general way with the distribution 

 of the Grassland vegetation, although it does not range quite so far to 

 the northwest and extends beyond its limits at the southeast. The 

 area of its optimum occurrence lies in South Dakota, Nebraska, western 

 Kansas, and extreme western Oklahoma, in the heart of the Grassland 

 area. 



