The Coexistence of Weeds and Restricted Native Plants on Serpentine Barrens in Southeastern Pennsylvania

Abstract

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.

The biology of Canadian weeds. 114. Symphyotrichum pilosum (Willd.) Nesom (Aster pilosus Willd.)

“…In the United States the hexaploid cytotype occurs on limestone outcrops, glacial-lake sediment deposits, and soil-covered unglaciated bedrock plains, as well as till deposits (Semple and Chmielewski 1985). Variety pilosum has also been reported from loam soils in the New Jersey Piedmont (Allen and Forman 1976;Jackson and Willemsen 1976) and the Shenandoah Valley, Virginia (Bowers 1993) and southeastern Pennsylvania serpentine soils (Hart 1980 Seedlings of var. pilosum grown in an Illinois field in a soil mixture of nutrient-rich silt loam and silty-clay loam all produced flowering stems, whereas only 11.1% of the plants grown in pure sand produced flowering stems.…”

“…Magnesium concentrations in dried shoots of S. pilosum originating from serpentine soils were approximately 34 and 13% when grown on serpentine and non-serpentine soils respectively. Magnesium concentrations in dried shoots of S. pilosum originating from non-serpentine soils were approximately 30 and 19% when grown on serpentine and non-serpentine soils respectively (Hart 1980).…”

“…Fall temperatures after dispersal are too low for germination (Keever 1950), which is delayed until the spring. Spring germinated individuals of S. pilosum may flower during the first season of growth provided that both intraspecific and interspecific competition are low, and resource availability is high (Hart 1980;Bazzaz 1990). Flowering occurs August through October in northern populations and until December in southern populations.…”

“…Those that do are typically shorter (< 50 cm) than normal, thin stemmed, and commonly bear fewer than 100 heads per plant. In general, the number of heads produced per plant is variable, ranging between 100 and 1100 and dependent in part on plant age, the number of available terminal apices of branches, plant bushiness, the duration of the long-day period prior to the start of the short-day period, total photosynthetic light energy, site age, and soils (Peterson and Bazzaz 1978;Hart 1980;Kadman-Zahavi and Yahel 1985).…”

VARIATION IN ALCOHOL DEHYDROGENASE ACTIVITY AND FLOOD TOLERANCE IN WHITE CLOVER, TRIFOLIUM REPENS

“…Alternatively, flood-tolerant individuals are poor competitors under nonflooded conditions. The existence of some trade-offs for being flood-tolerant is not completely surprising; many studies have demonstrated an inverse relationship between the ability of plants to tolerate abiotic stress and their ability to compete (Grime, 1979;Hart, 1980;del Moral, 1983;Keddy, 1990;Carter and Grace, 1990). However, T. repens growing under greenhouse conditions has no apparent trade-offs for being flood tolerant; the relative growth rate without flooding is not correlated to the relative growth rate with flooding.…”