Experimental trampling of vegetation. II. Predictors of resistance and resilience.
Experimental trampling, to simulate recreational use, was conducted in 18 vegetation types (including coniferous and deciduous forests, dwarf scrub, meadows and grasslands) in five separate mountain regions in the United States. Each type was trampled 0-500 times and vegetation response was assessed 2 weeks and 1 year after trampling. The response of vegetation to trampling is expressed in terms of three indices: resistance, tolerance and resilience. Resistance and tolerance were determined from the vegetation surviving 2 weeks and 1 year after trampling, respectively. Resilience compared the change over the remainder of the year with that during the first 2 weeks after trampling. Plant morphological characteristics explained more of the variation in response to trampling than the site characteristics that were assessed: altitude, overstorey canopy cover or total groundlayer vegetation cover. Resistance was primarily a function of vegetation stature, erectness and whether plants were graminoids, forbs or shrubs. The most resistant plants were caespitose or matted graminoids; the least resistant plants were erect forbs. Resilience was primarily a function of whether plants were chamaephytes, with perennating buds located above the ground surface. Chamaephytes were much less resilient than other plants. Tolerance, which measures the ability of vegetation to withstand a cycle of disturbance and recovery, was correlated more with resilience than resistance. Consequently, the least tolerant plants were the chamaephytes. The most tolerant plants were caespitose, matted and rosette hemicryptophytes, and geophytes. The resistance and resilience of individual species were negatively correlated, particularly for chamaephytes and graminoids. For herbaceous species with perennating tissues located at or below ground level, tolerance was more highly correlated with resilience than with resistance. For chamaephytes, tolerance was more highly correlated with resistance.