442 



Illinois Natural History Survey Bulletin 



Vol. 2b, Art. 5 



determine the occurrence of prairie on the 

 upper slope of a bluff. The west- to 

 southwest-facing position of the slope, 

 which exposes the slope directly to the hot 

 rays of the afternoon sun and to prevail- 

 ing southwest summer winds, and the alti- 

 tude of the bluff and width of the adja- 

 cent bottomlands help to provide the ex- 

 tremely xeric conditions under which this 

 type of prairie thrives. Rapid and exces- 

 sive drainage, due both to slope and to 

 permeable loess substratum, is a reinforc- 

 ing condition. 



5. The vegetation of Illinois hill prai- 

 ries is the bunch-grass type, with Atuiro- 

 pogon scoparius the dominant species in 

 most stands. Routeloua curtipendula is 

 usually present, in a few prairies dom- 

 inant. In a few hill prairies, Sorghastrum 

 niii/iiis and a few other bunch grasses oc- 

 cur frequently and ma\ doininate small 

 areas within the prairie, or, rarely, the 

 entire stand. 



6. For detailed studies of the vegeta- 

 tion, plots were staked in unpastured and 

 in pastured variants of the same prairie 

 slope. Data were obtained from plot sizes 

 ranging from 1 256 milacre to 9 milacres. 

 The smallest size found to be effective 

 was 1/64 milacre. The largest area staked 

 in each prairie was 75 milacres. One mil- 

 acre in each prairie — pastured and unpas- 

 tured — was mapped. All plant individ- 

 uals were charted and counted in each of 

 these milacres. Species lists were compiled 

 for plots of the several sizes. 



7. In the mapped milacre of the un- 

 pastured prairie, the estimated number of 

 plants was 1,949; 1,404 of these were An- 

 dropogon scoparius. In the mapped mil- 

 acre of the pastured prairie, the estimated 

 number of plants was 1,,H1 ; 849 of these 

 were A. scoparius. 



8. The ground space occupied by 

 plants in the mapped milacre of the un- 

 pastured prairie was 1,884.1 square 

 inches, of which 1,781.0 square inches 

 were occupied by .indropogon scoparius. 

 In the mapped milacre of the pastured 

 prairie the ground space occupied by 

 plants was found to be 1,403.3 square 

 inches, of which 1,107.0 square inches 

 were occupied by A. scoparius. In the un- 

 pastured prairie, at ground surface, 69.96 

 per cent of the mapped milacre was bare 



loess; in the pastured prairie, 77.63 per 

 cent. 



9. In the mapped milacre of the un- 

 pastured prairie, foliage covered 4,988 

 square inches, approximately 80 per cent 

 of the milacre. In the mapped milacre of 

 the pastured prairie, foliage covered 2,525 

 square inches, about 40 per cent of the 

 milacre. In both pastured and unpastured 

 prairie, Andropogon scoparius was the 

 species with the largest foliage area. In 

 the unpastured milacre, approximately 20 

 per cent of the ground was not covered by 

 foliage; in the pastured milacre, about 60 

 per cent, 



10. The available space per plant in 

 the unpastured milacre was found to be 

 3.22 square inches; in the pastured mil- 

 acre it was 4.68 square inches. 



11. For the study of frequenc)' of oc- 

 currence of species in plots within hill 

 prairie, a good distribution of species was 

 obtained with quadrat sizes of 1/64 to 

 1/4 milacre in unpastured prairie; 1/16 

 to 1 4 milacre in pastured prairie. 



12. From species-area curves, the 

 smallest representative area — the smallest 

 one-piece area having some claim to be 

 representative — was determined for the 

 unpastured prairie as 0.76 milacre, and, 

 for the pastured, 1.26 milacres. The min- 

 imum area for assignment to type — an 

 area that is large enough to include all the 

 important and a moderate number of mi- 

 nor species — was determined as 3.80 mil- 

 acres for the unpastured and 6.30 mil- 

 acres for the pastured prairie. The fair- 

 sized stand — an area that is 50 times as 

 large as the smallest representative area 

 and contains twice as many species — was 

 38.0 milacres for the unpastured and 63.0 

 milacres for the pastured hill prairie. 



13. The flora of 36 hill prairies was 

 used as a basis for presence studies. The 

 "constants" (species in 29, 80 per cent, or 

 more of the 36 locations) of this type of 

 prairie in Illinois were Andropogon sco- 

 parius, Erigeron strigosus, Boutelnua cur- 

 tipendula, Petalostemum purpurcum. Eu- 

 phorbia coroUata, Pensteinon pallidus, .7. 

 gerardi. Verbena stricta, and Kuhnia eu- 

 patorioidcs. Twenty-five species were 

 found in 18 or more of the 36 prairies 

 used in this study of presence. Species 

 most characteristic of the hill prairie type 



