OC 100 80 60 40 20 



Relative Cover After Trampling (percent) 

 Resistance Index 



Figure 9 — Resistance, tolerance, and resil- 

 ience of abundant species in four vegetation 

 types in Wasfiington's Cascade Mountains. 

 Resilience is indicated by thie perpendicular 

 distance from the diagonal line of equal re- 

 sistance and tolerance. 



likely to grow faster. Mosses were intermediate in 

 response between the graminoid, Carex nigricans, and 

 the forbs. They were moderately resistant but highly 

 resiUent and, therefore, highly tolerant. 



For other species, I prepared tables of mean cover 

 before and after trampling and 1 year after trampling 

 for each level of intensity. These tables allowed me to 

 evaluate the relative resistance of many of the less 

 common species. The minimum nvmiber of passes that 

 reduced cover by 50 percent was used to classify each 

 species' resistance as: high (500 passes or more); mod- 

 erate (200 passes); or low (75 passes or less). For ex- 

 ample, Vaccinium scoparium cover declined 20 percent 

 on 75-pass lanes (from 2.0 percent to 1.6 percent) and 

 75 percent on 200-pass lanes (from 1.6 percent to 0.4 

 percent). Consequently, it was classified as a moder- 

 ately resistant species. Tolerance was classified, in an 

 analogous fashion, using the maximvun number of 

 passes that could be tolerated and still have at least 

 75 percent of original cover 1 year after trampHng. 

 Tolerance classes were high (500 passes or more), mod- 

 erate (200 passes), and low (75 passes or less). Resil- 

 ience was evaluated by examining the amount of re- 

 covery on lanes where trampling had reduced cover 

 to nearly zero. Resilience classes were high (if cover 

 1 year after trampling was more than two-thirds of 

 cover before trampling), moderate (if cover after 1 year 



was between one-third and two-thirds of original 

 cover), and low (if cover was less than one-third of 

 original cover). 



This classification (table 7) suggests some general 

 tendencies. The shrubs — as noted before — had mod- 

 erate to high resistance, but because of their low resil- 

 ience they were unable to tolerate much trampling. 

 The graminoids also had moderate to high resistance. 

 But their resilience was moderate to high. Conse- 

 quently, graminoids were the group of plants most 

 capable of tolerating the level and type of trampling 

 administered in this study. The forbs had low to mod- 

 erate resistance. With a few exceptions, their resil- 

 ience was moderate to high. Tolerance was generally 

 moderate, although some forbs had low tolerance and 

 others had high tolerance. Abies lasiocarpa seedlings 

 had low resistance, resilience, and tolerance. Mosses 

 had moderate or high resistance, resilience, and toler- 

 ance, depending on the vegetation type. 



Beyond these broad generalizations about shrubs, 

 graminoids, and forbs, other plant characteristics 

 affected response. For example, the differences in 

 resistance between the two forbs, Veratrum viride 

 (green false hellebore) and Leptarrhena pyrolifolia 

 (false saxifrage), probably reflect differences in the 

 architecture of their leaves and stems and the tough- 

 ness of their tissues. Veratrum is tall, with stems 

 that are leafy and swollen with fluid, or brittle if dry. 

 Stems readily snap at the base, eliminating all of the 

 biomass aboveground. Leptarrhena is short, with 

 leathery leaves confined primarily to a basal rosette 

 (a dense cluster of leaves arranged like the spokes 

 of a wheel at the base of the stem). Trampling may 

 crush the erect flower stalk, but the leaves will sur- 

 vive moderate trampling. 



The vegetation matrix within which a species occurs 

 also affects its resistance. The abihty to survive tram- 

 pling often increases if the adjacent plants are resis- 

 tant. These resistant plants can protect fragile plants 

 from trampling (Cole 1988). For example, Potentilla 

 flabellifolia had moderate resistance in the Phyllodoce 

 and Carex types, but low resistance in the Valeriana 

 type. Potential explanations for variation in resUience 

 are less apparent, but the variation likely reflects dif- 

 ferences in environmental conditions, as well as plant 

 characteristics such as growth rates, means of regen- 

 eration, and the protection and toughness of peren- 

 nating tissues. 



EFFECTS OF TRAMPLING 

 IN THE ROCKY MOUNTAINS 



Another set of study sites were located in the Arapaho 

 and Roosevelt National Forests, on the east slope of 

 the Rockies in Colorado. One site was located along 

 Bennett Creek, a tributary of the Cache la Poudre 



14 



