228 NATUEAL HISTORY BULLETIN 



wrought his changes. The topography of our region has not 

 changed to any marked extent within the same time. Exposure 

 to evaporation, — to sun and wind, — is therefore much the same 

 as when prairie and forest appeared in their normal relation. 

 Why then do trees grow on the prairies of today ? 



As a matter of fact trees do not always grow well on the 

 prairies. With the exception of the cottonwood individual trees 

 or single rows of trees do not m the long run prosper on the 

 prairies. Larger groves made up of greater numbers of trees 

 set out at one time in a manner which would be impossible by 

 natural seed-dispersal, often thrive well, but even they suffer on 

 their exposed sides, as is illustrated in plate X. Figure 1 illus- 

 trates a walnut grove on the south side of a road at Mr. Patrick 

 ^IcGuire's house near George, Iowa. This is the interior of the 

 grove protected from excessive evaporation by the portions of 

 the grove to the south and west. The trees here are prosperous 

 and promise well for the future. Figure 2 illustrates the ex- 

 posed southwest corner of the same grove, the trees being of the 

 same age throughout. So far as could be determined soil, drain- 

 age, topography, etc., were the same, but the interior trees were 

 sheltered by their less-fortunate companions. 



Such a grove, if left to its own resources, would in time die 

 out, and during cycles of dry seasons, such as those of 1893, 

 1894 and 1895, thousands of trees did perish not only where 

 planted singly or in single rows, but also in larger groves, and 

 a large part of the trees set out during such dry years fail com- 

 pletely. 



The writer is convinced from observations made during a 

 period of many years that on the prairies man, by his care of 

 artificially planted trees, by cultivation, re-planting and pro- 

 tection, is just able to throw the balance in favor of the trees, 

 and that but for his efforts the trees would again disappear 

 from the greater part of the area which was once prairie. 



It is interesting to note that where a grove becomes estab- 

 lished, and thus provides the shelter which elsewhere is fur- 

 nished by topography, the original prairie flora disappears 

 entirely, and its place is taken by a typical mesophytic flora. 

 An excellent illustration of this change is given by the Whiting 



