/ This article reviews some of the more important factors found to influence the susceptibility of plants to trampling impacts associated with recreational use of natural areas. A three-way interaction mediates plant responses to impacts: plant x environment x stress level(s). Plant responses vary in part according to the genetic constitution of the plant, life and growth form, the adaptive flexibility of the plant, and anatomical differences inherent to growth habit and morphology. Other factors that influence plant sensitivities to impacts are the habitat environments in which plants grow, since a number of conditions such as moisture excesses or deficiencies, nitrogen or oxygen starvation, late frosts, etc., cause physiological injury and may increase plant sensitivity to impacts. Among the environmental factors that may increase or lessen plant sensitivities to impacts are soil moisture levels, canopy density, elevation, aspect, microclimate, soil drainage, texture, fertility and productivity. Seasonal influences also bear consideration since environmental changes and phenological and physiological events are mediated by time of year. Stresses are caused by both direct and indirect forms of impact and vary according to season of use, frequency and amount of use, and the type of activity. These interactions are further complicated by evidence that inter-and intraspecific competition, antagonism, and commensalism may influence differences in the sensitivity of plant communities to impacts.Recreational use of natural areas causes ecological change. Very little is know about the long-term biological effects of such change; however, in a comparison of the ecological consequence of fire, grazing, and recreation, Cole (1981) has characterized recreation impacts as localized but severe. The magnitude of change is related to the amour of use, the areal extent of use, the length of time use has occurred, and the type of ecosystem. The importance of these changes is difficult to predict since this branch of impact ecology has dealt with this matter for only a comparatively short period of time. According to some investigators, severely impacted areas in alpine, subalpine, arid and semiarid ecosystems may require fifty to hundreds of years to recover (Stohlgren 1982, Reid 1979, Willard and Marr 1970. The importance of these changes also depends upon point of view since it relates to management objectives set by those responsible for the management of natural areas and the preservation and amenity values attached to these areas.The weight of evidence suggests that factors which give balance to ecosystem organization are highly sensitive to minimal disturbance and stress levels. Among the more damaging effects of trampling reported to date is the destruction of endangered, rare, and critically rare plant species (Hamilton 1983), loss of both biological and physical integrity of the impacted
