86 BOTANICAL GAZETTE [august 



taries, reached the prairie region they were unable to occupy the high 

 lands, but occupied the flood plains and adjacent slopes. There is 

 no evidence to indicate that the then existing topographical and 

 climatic conditions differed radically from those of today. Again these 

 elements have today their most widespread distribution, a condition 

 at once intelligible when related to physiographic development in the 

 working-back of streams and the increase of flood plain. Two salient 

 points may now be noted: the initial and the continuous subsequent 

 preemption of the upland by the prairie formation, and secondly the 

 continuation of initial climatic conditions. The initial causes which 

 operated to restrict tree invasion and ecesis upon these prairie-covered 

 uplands would thus seem to have their duplication in those natural 

 factors which operate to that end at the present time. 



Under the discussion of ecological factors of the region it will be 

 shown that there is a coincidence of factors operating most antago- 

 nistically against tree growth upon the upland. The almost entire 

 absence of fungi upon the prairie cannot but be significant. The 

 roots of most of these deciduous trees are obligatively provided with 

 symbiotic mycorhiza and the absence of their specific fungus would 

 preclude advance. Hence fungal infection of the prairie soil must 

 precede or at least accompany forest encroachment. Again, the dif- 

 ficulty of seed germination, almost impossible either because of a dense 

 sod or a lack of soil moisture, successfully checks invasion. In 

 consideration of these precarious climatic factors, peculiar edaphic 

 conditions, and the fact that if planted upon the prairie trees thrive, 

 I am led to the view that the question of non-invasion upon the prairie 

 proper is primarily and initially one of pre-occupation and the inability 

 of seedlings successfully to withstand the extremely severe conditions 

 of the first winter's exposure. In the positive and coincident inter- 

 action of unfavorable biological and climatic conditions may be 

 found a cause sufficient to account for tree absence upon the prairie 

 and the slow migration of tree species into this region. 



While these causes are all sufficient, yet we must not disregard a 

 secondary and artificial though highly cooperative factor, which in 

 recent times must have served in many places to prevent tree estab- 

 lishment. I refer to prairie fires; yet even in the absence of prairie 

 fires for half a century the prairie stands uninvaded except in cases 



