Prairie Region. 517 
Once a prairie soil was formed, trees were unable to encroach upon it, ex- 
cept where by erosion in the stream valleys the dense turf was broken, or at 
the base of cliffs, and elsewhere, where loose talus, or stony ground prevented 
the grass vegetation from making a close mat. 
e experience of orchardists and farmer tree planters in the prairies has 
demonstrated that the eastern trees, on good soil cannot compete singly with 
the prairie grasses, no matter how great a supply of moisture is furnished 
either by rainfall or irrigation. HARVEY') suggests that it is the absence öf soil 
fungi that constitute the mycorrhiza upon which the forest trees depend for 
food supply, the difficulty of seed germination in a dense sod and finally 
prairie fires that account for the preservation of the typic grass-land formations 
against the encroachment of trees. There is only one way in which eastern 
trees may conquer the prairie grasses and that is by advancing in solid pha- 
lanx. This method of encroachment is necessarily very slow and accounts 
for the lack of time since the glacial epoch for the trees to extend their supre- 
macy over the whole prairie region. Another cause that operated in the past 
to prevent trees from extending into the prairies was the presence of millions 
of buffaloes in the region. The buffalo severely grazed the grasses and kept 
them in close mats. The buffaloes likewise attracted the Indians, who set fire 
to the grass, thus confining the trees to the river banks, mountains and broken 
lands. With the settlement of the country, the other external factors remain- 
ing unchanged, as before, the cultivation of the ground and the destruction 
of the graminaceous plants with deeply penetrating matted fibrous roots per- 
mitted the trees and shrubs to gain ground upon a territory formerly covered 
with prairie plants. In a former page (p. 235), we have given some of the 
proofs of this kind of invasion. A prairie is a heavily grassed area (a closed 
formation), destitute of forest growth, but existing in the midst of a wooded 
region, where the climatic conditions are favorable to the growth of timber, 
but where on account of the exclusiveness of the grasses and their success 
against all competitors coupled with ecologic, physiographic, edaphic and 
historic influences rather than want of water and soil conditions the spread 
of forest trees has been prevented. 
The boundaries of the prairie region cannot be given in an ab- 
breviated statement. The map will show the approximate location of this 
grassy country. Phytogeographically this region is made to comprise the 
great plains, as well, as the prairie region proper, notwithstanding 
that this may be a oa of the views of some eminent phytogeographers, 
who would separate the two as quite distinct floristically considered. Turf- 
forming plants are the most important vegetation-form for the characterization 
ı) Floral Suecession in the prairie-grass Formation of southeastern South Dakota. Botanical 
Gazette XLVI: 81— 108. According to experimental studies at Woburn Fruit Farm, 
is injurious to fruit trees through an active poison resulting directly or indirectly from the growth 
of the grass, See Livinoston, BURTON E,., Studies on the Properties of unproductive Soils; 
Bulletin 36. U.S. Bureau of Soils 1907! 
