THE TRUE PRAIRIE. 



123 



hills of south-central South Dakota, where it stretches like fields of wheat as far 

 as the eye can reach. Like Stipa spartea, it often meets and mixes with Andro- 

 pogon scoparius or A . furcatus in low prairies or subclimax regions. In such 

 places, as well as in local areas of higher rainfall, Andropogon furcatus and A. 

 nutans often become controlling. When this is the case, however, the com- 

 munity is always to be regarded as subclimax. It need occasion no surprise 

 to find extensive outposts of subclimax grassland in the true prairies, if account 

 be taken of the close requirements of the dominants and the considerable 

 variation in normal rainfall at places not very distant from each other. Thus 

 Lincoln and Peru, Nebraska, are less than 60 miles apart, but one has a rain- 

 fall of 28 inches and is in the true prairies, while the other with a rainfall of 34 

 inches lies in the subclimax prairie (plate 22, a) . 



Factor relations. — Koeleria is a bunch-grass, while the other four dominants, 

 as well as the subclimax Andropogon furcatus, are sod-formers in the prairie. 

 All of the latter become bunch-grasses with the decreasing rainfall, such as is 

 characteristic of the sandhill areas to the westward. Their water relations 

 have been worked out for but a few regions as yet, but enough has been done 

 to indicate the general requirements. These agree well with the growth-form 

 and with the successional sequence, as well as with the physiographic relation 

 where this controls the distribution of water. Studies of water-content in the 

 Stipa-Koeleria prairies at Belmont, north of Lincoln, from April 22 to May 25, 

 1901, gave the following results at 5, 10, and 15 inches: 



The three levels, which were represented by 7 stations, correspond with 

 Bouteloua, Stipa-Koeleria, and Andropogon respectively. Weaver and Thiel 

 (1917 : 15) found the water-content of Stipa-Koeleria prairie near Minneapolis 

 to range for the most part between 5 per cent and 15 per cent during the sum- 

 mers of 1915 and 1816. In the low prairie of Andropogon furcatus and some A. 

 scoparius, the variation in 1915 was 27 to 45 per cent, and in 1917 chiefly from 

 20 to 55 per cent. In the Belmont prairies in 1916, the range of soil-mois- 

 ture in the high prairie was chiefly between 10 per cent and 25 per cent 

 on the slope, while on the ridge it fell for the most part between 5 per cent and 

 15 per cent. The high prairie at Minneapolis and Belmont gave an evapora- 

 tion rate nearly twice as great as that of the low prairie. The evaporation 

 rate on a ridge of the Belmont prairie was usually 2 to 4 c.c. higher than on 

 the slope. 



Sequence of dominants. — These results confirm the water sequence as indi- 

 cated by the successional and topographic relations. The subclimax Andro- 

 pogons have the highest water-content requirement, a fact further attested 

 by the readiness with which they are invaded by scrub or woodland. Agro- 

 pyrum follows closely, often being nearly equivalent to Andropogon scoparius. 

 After it come Koeleria, Stipa spartea, and S. comata in this order, with the 



