INDICATOR SIGNIFICANCE OF NATIVE VEGETATION 275 



relation between vegetation and environment must be correctly 

 interpreted. 



It should be clearly recognized that the use of native vegeta- 

 tion as an indicator of forest sites is not without certain limita- 

 tions. The use of native shrubby, and particularly herbaceous, 

 vegetation as forest site indicators is not exactly comparable to 

 using native open-land vegetation as an indicator of the crop 

 productivity of an open-land site. In the case of the forest 

 site the presence or-absence of an over-wood must be recognized 

 as an influential factor. The use of shrubby and herbaceous 

 vegetation as indicators of forest sites encounters more obstacles 

 in the dense Douglas fir, lodgepole pine, and Engelmann spruce 

 {Picea engelmanni) forests than in the comparatively open west- 

 ern yellow pine forests. In the dense forests herbaceous plants 

 are rather scarce except in partial openings or where the stand 

 is below its normal density. The vegetation of the denser 

 forests also varies in its composition from place to place, not 

 only according to the favorableness of the site, but also with the 

 density of the crown cover and consequently the amount of 

 shade in the forest. The floor of the fir, spruce and lodgepole 

 pine forests is more hea\41y and continuously shaded than that 

 of the densest stands of western yellow pine. The factor of 

 shade is of great importance in conditioning the character of 

 the shrubby and herbaceous vegetation of dense forests. 



While engaged in the remeasurement of a set of permanent 

 sample plots aggregating 26.4 acres on the San Mateo Division 

 of the Datil National Forest in west central New Mexico in the 

 summer of 1915 the writer was impressed with the variation in 

 the vegetation occurring on the different w^estern yellow pine 

 sites. The San Mateo Mountain range extends in a gen- 

 eral north and south direction and rises to a height of 9500 

 to 10,500 feet above sea level. The north and west por- 

 tions of the mountains slope quite gradually to the adjacent 

 rolling plains. The east and south portions of the range 

 break off quite abruptly to the plains. The majority of the 

 steeper east and south aspects bear such species as Jimi- 

 perus monosperma, J. pachyphloea, Pinus edulis, Quercus chry- 



