Achillea millefolium and Fragaria virginiana were less 

 susceptible but were nearly eliminated after 400 passes. 

 Leaves close to the ground allow these two forbs to sur- 

 vive low levels of trampling. Tough, flexible, straplike 

 leaves and a tufted growth form combine to make 

 Xerophyllum tenax, a forb, the most resistant of all 



plants studied (fig. 15a, b). It is important to note that 

 even this most resistant species exhibited no threshold 

 below which trampling had little effect and above which 

 effects were pronounced. Instead, cover loss was immedi 

 ate, the rate of loss declining as trampling intensity 

 increased. 



Figure 15. — Vegetation of the 200-pass lane on the Abies lasiocarpa/Xerophyllum 

 tenax habitat type, (a) before and (b) after treatment. Xerophyllum tenax and moss 

 survive, but Vaccinium scoparlum and V. globulare do not. 



25 



