The Carex species were quite resistant, but not very 

 resilient. The grasses, Holcus lanatus and Panicum 

 boscii, were not very resistant, but they were highly 

 resilient. Their resilience made the grasses substan- 

 tially more tolerant than the sedges. The forbs and 

 ferns were generally not very resistant; however, their 

 resilience and tolerance varied greatly. The most re- 

 sistant of the forbs was Phlox stolonifera, which is a 

 prostrate chamaephyte with evergreen leaves. Phlox 

 was the least resilient of the forbs, probably reflecting 

 its slow growth dependent on perennating buds located 

 aboveground where they are susceptible to damage. 

 One of the most resihent forbs was Geranium macula- 

 turn, the forb that was least resistant. 



Table 25 classifies the responses of less common spe- 

 cies as low, moderate, or high. This table reinforces 

 the lack of resistance of most species, as well as the 

 great variability in resilience and tolerance. In con- 

 trast to other areas, even the responses of graminoids 

 varied greatly. This was the only place with grami- 

 noids of low resistance or low tolerance. The low tol- 

 erance of the Carex species may result from (1) being 

 tall rather than having the resistant caespitose or 

 matted growth forms and (2) growing under forest 

 canopies where low light levels reduce growth rates. 



DISCUSSION AND MANAGEMENT 

 IMPLICATIONS 



Among experimental trampling studies, this one 

 was unique in (1) the number of vegetation response 

 variables examined, (2) the wide variety of vegetation 

 types examined, and (3) the four different regions of 

 the country examined. This makes it possible to de- 

 scribe the rate and magnitude of various vegetation 

 responses to trampUng. It provides an opportunity to 

 assess the variability of responses among vegetation 

 types and to explain the variabiUty. Finally, it is pos- 

 sible to evaluate whether or not response varies by 

 region. 



Vegetation Responses to Trampling 



The first question we can address is which of these 

 types of vegetation change is the most sensitive indi- 

 cator of trampling disturbance? Which response to 

 trampling occvirs most rapidly and which is most pro- 

 nounced? The physiognomic changes in vegetation — 

 reduction in height and cover — always occur more 

 rapidly and are more pronounced than the floristic 

 changes — reduction in species richness and change 

 in species composition. Height reduction generally 

 occurred more rapidly than cover loss in types domi- 

 nated by tall forbs {Geranium, Leersia, Potentilla, 

 Amphicarpa, and Dryopteris) or by resistant grami- 

 noids (Carex nigricans, C. bigelowii, C. pensylvanica, 

 and Kobresia). Cover loss occurred more rapidly than 



height reduction only in the shrub-dominated Phyllo- 

 doce type. There was little difference in the six other 

 vegetation types. In general, the types of change that 

 occurred most rapidly also were most pronounced. 



In most vegetation types, species were lost more rap- 

 idly than the composition changed. This would be ex- 

 pected where a few relatively uncommon species are 

 particularly fragile. They may be quickly eliminated 

 by trampUng, but their loss has httle effect on overall 

 composition. In Kobresia, for example, 75 passes re- 

 duced species richness 40 percent vnthout significantly 

 changing composition. Sparsely distributed forbs, grow- 

 ing vothin the matrix of resistant graminoids, were 

 ehminated by Ught trampling; however, their loss had 

 little effect on similarity values based on plant cover. 

 Where the dominant species is particularly fragile, 

 composition would be expected to change more rapidly 

 than richness. This was the case in Dryopteris, where 

 significant changes in composition occurred after 25 

 passes, while a significant decrease in richness did 

 not occur until after 200 passes. 



One year after trampling, richness and composition 

 were almost always closer to original levels than cover 

 and height. In most vegetation types, both richness 

 and similarity made similar recoveries. However, in 

 some types the dominant species were much more or 

 much less tolerant than other species (Phyllodoce, Vac- 

 cinium, Carex bigelowii, and Lycopodium). In such 

 types, richness recovered more quickly than composi- 

 tion. The magnitude of height reduction and cover loss, 

 after 1 year of recovery, were also similar in most types. 



With only a few exceptions, a vegetation type's dura- 

 bility could be assessed using any of these response 

 variables. However, height reduction and cover loss 

 are the most sensitive measures. They change most 

 rapidly. Cover is easier to measure. It is also easiest 

 to interpret and probably best describes observable 

 changes. This validates the reUance of most trampling 

 studies on this single response variable. 



Variation Among Vegetation Types 



Another important issue we can address is the 

 magnitude of variation in the responses of different 

 vegetation types. If variation is pronounced, impacts 

 potentially co\ild be reduced by confining trampling 

 to more tolerant vegetation types. 



In all four regions cover loss differed significantly 

 among three of the four vegetation types. Many differ- 

 ent statistics could be used to portray the magnitude 

 of variation. The most resistant type, Carex nigricans, 

 lost no cover after 75 passes, while the least resistant 

 type, Dryopteris, had just 14 percent relative cover 

 after 75 passes. Alternatively, we can compare the 

 number of passes that caused a 50 percent cover loss — 

 20 passes in Dryopteris and 600 passes in Carex. The 

 Carex type can absorb 30 times as much trampling. 



46 



