HUMAN TRAMPLING EFFECTS ON REGENERATION AND AGE STRUCTURES OF PINUS-EDULIS AND JUNIPERUS-MONOSPERMA

We examined effects of human foot traffic on age structures and densit ies of seedlings and saplings of Pinus edulis (Colorado pinyon) and Ju niperus monosperma (one-seed juniper) in a heavily used urban park, Ga rden of the Gods, Colorado. Age structures show no stand-destroying di sturbances, but they do contain small peaks 85-95 yr ago (minimum age) , which have been interpreted as responses to heavy grazing. For Pinus edulis reverse J-shaped age structures indicate a strongly reproducin g population, while flat age structures of J. monosperma show low pres ent reproduction. Young trees showed strong preferences for establishi ng under existing trees and shrubs and not among herbs or on bare soil . Pinus edulis seedling density was reduced by 73% in heavily trampled areas compared to lightly trampled areas. However, there were no diff erences in density when only the area protected by rocks, shrubs, or t rees was considered. This indicates that direct effects such as physic al damage and soil erosion kill young trees, and indirect effects, suc h as lower seed production, do not cause the lower densities in heavil y trampled areas. Pinus edulis saplings and J. monosperma seedlings an d saplings showed no differences in density across trampling intensiti es. In heavily trampled areas of Garden of the Gods Park, recent incre ases in use have apparently reduced Pinus edulis seedling establishmen t enough that long-term regeneration is threatened. Managers of all pi nyon-juniper woodlands must recognize that in areas strongly impacted by foot traffic, and also presumably by similar disturbances such as v ehicle traffic, sufficient regeneration likely does not occur to repla ce trees. The areal extent of severely disturbed areas should be limit ed, and managers should seek to avoid further degradation of less dama ged areas.