desertorum, A. intermedium, A. smithii, A. cnstatum, Bromus inermis, and 

 Elymus junceus, after seven years' growth, had a more favorable 

 effect on the soil structure, permitting greater penetration of 

 water, than the warm-season grasses: Andropogon gerardi, Boutelom 

 curtipendula, B. gracilis, and Buchloe dactyloides. An additional fac- 

 tor was involved in the low intake of water where Buchloe was 

 growing, for the numerous fine roots entrapped air bubbles thus 

 reducing the infiltration. ^''^ 



Efficiency of Species in Utilizing the Available 



Resources of the Environment 



This efficiency is attributable to the genetically induced effec- 

 tiveness, under various environmental conditions, of physiological 

 processes, which are summed or integrated in the principles con- 

 cerning requirements and the ecological amplitude. Many annual 

 species have greater capacity than perennials for growth on de- 

 pleted sites in early stages of succession, apparently because of 

 their wide ecological amplitude, great plasticity in size, and small 

 requirements, but it appears that the plants in the early stages 



Figure 1-7. Russian thistle (Salsola kali) is efficient in uti- 

 lizing resources of various habitats from drought-stricken 

 Belds and grasslands to embryonic dune areas. Note dust 

 storm in distance. March, near Fremont, Wyoming. 

 (U.S.D.A. Soil Conservation Service.) 



I 



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