June, 1922] HANSON — • PRAIRIE INCLUSIONS IN FOREST 
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Discussion 
The maintenance of prairie inclusions against invasion by the shrub 
and the forest by which they are surrounded has been discussed by a few 
writers. Shimek (7) states that the chief factor is the exposure to evapora- 
tion as determined by temperature, wind, and topography. He beHeves 
that the ''determining causes of relative prairie and forest distribution 
evidently lie in the atmosphere rather than in the soil" (7, p. 24), and that 
prairie fire was an effect rather than a cause. Vestal (8, pp. 122, 123) says: 
The essential condition is the great insolation and exposure to the dry summer winds 
from the south and southwest, making for local xerophytism. This is apparently a static 
rather than a dynamic feature of the environment, since both habitat and xerophytic vege- 
tation may persist indefinitely, even though there is a slow lateral migration as the valley 
widens. It is probable that the dryness occasioned by the slope to the south is in most 
places not in itself sufficient to preserve the prairie from forest encroachment, for forest is 
able to establish itself in quite xerophytic habitats in the vicinity, and has in fact done so 
over most of the south-facing ravine slopes. Other physical factors aid in the original 
exposure afforded by direction of slope. One is instability of surface, due partly to steep- 
ness, partly to the meagerness of protection against erosion afforded by the open and sparse 
ground-cover. Others are accidental and artificial factors which destroy or check forest 
growth, such as fire, cutting, grazing, and trampling. These operate in places only tem- 
porarily, but in other places recur frequently enough to permit the continued existence 
within the forest of small but rather numerous patches of prairie, with more or less shifting 
boundaries, wherever the basic condition of southward exposure is fairly extensive. 
Pool, Weaver, and Jean (3), in studying the vegetation in the vicinity 
of Peru, Nebraska, collected considerable factor data on a rather large 
prairie fragment. They state (p. 27) that 
Except for fires, grazing or other disturbance much of this subclimax grassland would 
undoubtedly pass through a scrub stage in succession and culminate in forest while still 
other extensive areas would probably remain covered with chaparral. 
But in the summary they state that 
The high saturation deficit and low soil moisture content (often reaching the non- 
available point) of the prairie sites in eastern Nebraska constitute barriers over which the 
forest trees can scarcely pass. We probably have herein the most ready explanation as to 
why our natural Nebraska woodlands are confined to the moist slopes of rather narrow 
valleys, and also the most probable answer to the oft-repeated question as to the treelessness 
of the prairies in general [p. 47]. 
The factors given for the maintenance of prairie areas are: A, Cli- 
matic: (i) temperature, by increasing evaporation; (2) wind, by increasing 
evaporation and by mechanical effect on tissues; (3) light (insulation) 
causing greater evaporation. B, Edaphic: (4) topography, steepness of 
the slope causing instability of the soil; (5) low water content. C, Biotic 
and accidental : (6) fire; (7) grazing and trampling; (8) cutting. 
It does not appear that fire, grazing, trampling, or cutting is an impor- 
tant factor in the maintenance of these small prairie inclusions, because on 
