▁including ▁al pine ▁situations ▁with ▁sand ▁or ▁gra vel ▁substr ate , ▁similar ▁substr ates ▁in ▁flat ▁t und ra ▁low lands , ▁and ▁also ▁fen ▁habit ats ▁upon ▁organ ic ▁substr ate ▁where ▁some ▁sh ading ▁from ▁adjacent ▁sed ges ▁or ▁shr ubs ▁may ▁occur . ▁ ▁The ▁Young er ▁D ry as ▁and ▁Old er ▁D ry as ▁stad ial s ▁are ▁ge ological ▁periods ▁of ▁cold ▁temperature ▁that ▁are ▁named ▁after ▁D ry as ▁oct op et ala , ▁which ▁fl our ished ▁during ▁that ▁time ▁and ▁is ▁used ▁as ▁a ▁foss il ▁indicator ▁of ▁those ▁periods . ▁ ▁Species ▁D ry as ▁compr ises ▁three ▁species , ▁but ▁the ▁genus ▁is ▁in ▁need ▁of ▁tax onom ic ▁revision : ▁ ▁D ry as ▁drum mond ii ▁Richard son ▁ex ▁Hook . ▁– ▁Dr um mond ' s ▁av ens ▁ ▁var . ▁drum mond ii ▁Richard son ▁ex ▁Hook . ▁– ▁Y ellow ▁av ens ▁▁ ▁var . ▁t oment osa ▁( F arr ) ▁L . O . ▁Williams ▁– ▁T oment ose ▁av ens ▁ ▁D ry as ▁integr ifolia ▁V ahl ▁– ▁Ent ire - le aved ▁av ens ▁ ▁sub sp . ▁cham isson is ▁( S pr eng .) ▁Sc og gan ▁– ▁White ▁av ens ▁ ▁sub sp . ▁c ren ul ata ▁( J uz .) ▁J . ▁K oz he vn ▁– ▁C ren ulate ▁av ens ▁ ▁sub sp . ▁integr ifolia ▁V ahl ▁– ▁Ent ire - le aved