Figure 1-3. Essential re- 

 quirements are not adequate 

 for corn on this hill; grass- 

 land would not only make 

 better use of the resources 

 but also improve them. 

 Michigan. (U.S.D.A. Soil 

 Conservation Service.) 





^^l^Mi^'h^ S' f- J" 





tial requirements, for the soils there are quite acid and very 

 deficient in phosphorus, nitrogen, and exchangeable bases com- 

 pared to soils from unaltered rocks where Artemisia tridentata and 

 its associates grow. The pines, Pinus ponderosa and P. jeffreyi, and 

 some herbaceous montane plants are able to grow in mineral- 

 deficient soils, however, even though the precipitation is consider- 

 ably less than in the pine forests of the Sierra Nevada Mountains 

 to the west.^^ 



Many grasses possess definite length-of-day requirements. A 

 strain of side-oats gramagrass {Bouteloua curtipendula) (Figure 1-2) 

 from southern Texas requires intermediate or short days, with an 

 upper critical photoperiod of 13 to 14 hr, for vigorous growth. A 

 strain of this same grass from North Dakota requires long days, 

 flowering vigorously in day lengths of more than 14 hr — even 



Ecological Clmairacteristics of Species & Popvmlatioims 



13 



