Alpine and polar tundra habitats are easily disturbed in late spring and summer during the thawing of the surface layers of soil, above an impermeable layer of ice. If vegetation is disturbed erosion is likely and regeneration is a slow process. Damage in winter often takes the form of compaction resulting from the action of skiers and vehicles. Compacted snow does not protect the vegetation in the same way as does natural snow cover. Upland areas suffer from pollution as their vegetation cover intercepts wind-blown clouds carrying contaminants and plants are under stress growing near the environmental limits. Agriculture in alpine regions is closer to the 'margins' than at lower altitudes and so is likely to have a greater effect upon the environment.