The coexistence of weeds and restricted native plants on serpentine barrens in southeastern Pennsylvania

Three plant species restricted to serpentine outcrops in southeastern Pennsylvania, USA, coexist with weedy congeners on these outcrops. The contrasting strategies by which each of the two types persists on the outcrops were examined by comparing growth rate, competitive ability, seedling mortality, mineral uptake, phenology, and leaf structure. In general, weeds had higher potential growth rate, more mesic leaf structures, lower seedling mortality on normal soil, lower Ca uptake, and earlier and/or more abundant seed production. Both types showed poor competitive ability when grown with ryegrass. Some, but not all, serpentine populations of weeds differed from nonserpentine populations in traits that may be adaptive to the serpentine habitat. The growth patterns of weeds are suited not only to the temporally open conditions of disturbed areas, but also to physically severe sites where the greatest stresses are seasonal. When moisture and nutrients are most abundant, weeds grow faster than serpentine—restricted plants and allocate biomass to structures allowing rapid growth and early reproduction. During periods when moisture and nutrient supply are low, growth of serpentine—restricted plants is favored over weeds. Within a particular range of moisture and nutrient availability, both types can coexist.