PLANT SUCCESSIOlsr AND BANGE MANAGEMENT. 



47 



mean temperature being 42°, and when the nocturnal temperature 

 has dropped from 2° to 4° below freezing. Few perennial plants, 

 even though restricted in their distribution to the subalpine zone, 

 are capable of germination at such low temperatures. Being entirely 

 dependent upon seed for their perpetuation, obviously only those an- 

 nuals persist or are conspicuous which are capable of germinating 

 and becoming established at the earliest advent of spring. 



Tolmie's orthocarpus 

 (Orthocarpus tolmiei). 



Douglas knotweed 

 {Polygonum douglasii). 



Tansy mustard 

 (Sophia md^a). 



Fig. 18. — Plants characteristic of the first or early weed stage. 



Since the ruderal-early-weed stage represents the lowest or most 

 primitive herbaceous cover possible, it will be instructive to compare 

 briefly the conditions of growth of this type with those of the highest 

 herbaceous successional cover, the wheat-grass consociation. For the 

 purpose of ready comparison the average, maximum, minimum, and 

 optimum depths of the roots of the most characteristic ruderal-weed 

 species and of the chief wheat grasses are tabulated in Table 5. 

 Wliile the depth of penetration of the roots of wheat grasses as a 

 whole is appreciably greater than in the ruderal weed species, small 

 wheat grass obtains its water supply from much the same soil stratum 



