The "biennial" plant series Verbascum thapsus, Oenothera biennis, Daucas carota, and Tragopogon dubius are all found in abandoned agricultural fields, but they differ in their time of first appearance, peak abundance, and length of time populations persist. In experimental field studies these species showed differences in their abilities to colonize various types of ground cover. Seedling emergence, survival, and growth were highest for all four species in patches of bare ground. The small—seeded species, Verbascum and Oenothera, survived only in bare ground. In contrast, the large—seeded species, Daucus and Tragopogon, survived in several ground cover types. Relative coverage of three ground cover types and differences in seedling performance in these cover types were compared in a 1—yr and 15—yr old field. Verbascum and Oenothera became established only in the 1—yr old field where bare ground was abundant and not in the 15—yr old field where these patches were rare (°1% of area). When bare—ground patches were created experimentally in the 15—yr old field, both species were able to establish, supporting the conclusion that it is availability of bare ground rather than age of field per se that confines these two species to recently disturbed areas. In contrast, Daucus and Tragopogon became established in both fields, although seedling emergence, survival, growth, and reproductive output were higher in the 1—yr old field. Under nonexperimental conditions, however, these two species are not generally found in newly abandoned fields. Their absence is due to the fact that they produce seeds that, although broadly dispersed, do not survive in the soil for >1—2 yr and are not often present in a newly distributed area. Once dispersed to and established in an area, they can persist for several generations because their seedlings can survive in several types of ground cover, including those with thick vegetation.

One of the underlying assumptions of both theoretical and empirical community ecology is that the processes determining community composition and abundance of species are interactions specific to particular pairs of species. However, we argue that, in sessile plants at least, competitive interactions are not usually species‐specific and that there exists a large degree of equivalence of the effect of species of similar growth form on the ability of any particular species to establish within a community. This null hypothesis of equivalence of competitive effects is based on three characteristics of plants: homogeneity of resource requirements among autotrophs; low encounter probabilities between individuals of any particular species pair; and the predominance of size asymmetries between competing individuals (e.g., seedling‐adult interactions.) We present an experimental design to quantify competitive interactions among plant species under field conditions and therefore enable statistical comparisons of competitive abilities among species. The competitive effect of one “neighbor” species on one “target” species is measured as the slope of a regression of performance of target individuals on biomass (or other measure of amount) of its immediate neighbors. Use of the design to test for equivalence of competitive effects and other advantages are described.

Mathematical models are developed to examine the population—level response of an herbaceous plant species (teasel, Dipsacus sylvestris Huds.) which was experimentally introduced into several habitats and monitored for 5 yr. Models based on morphological stages (size) rather than chronological age give more satisfactory results. Populations growth rates (λm) range from 0.63 to 2.60, which are likely typical for fugitive plants. Values are interpreted as responses to both external and internal factors. Grass litter, and the presence of other dicotyledonous species, and the overall primary productivity of the rest of the community are important factors determining the success or failure of an attempted colonization by teasel. Individual plant and population—level growth rates seem to be determined independently.

In field populations of the biennial plant (Dipsacus fullonum L.) the probability that an individual will die, remain vegetative, or flower during a particular growing season is highly correlated with the size of its vegetative rosette at the end of the growing season of the preceding year. Further, a rosette forms a flowering stalk only after attaining a critical size. Correlations of various plant fates with rosette size are independent of rosette age; hence size provides better predictions of plant fate than age.

We investigated the effects of size of opening in the vegetation and litter cover on seedling establishment of two species of goldenrods (Solidago spp.) in an abandoned field in southwestern Michigan, U.S.A. Seeds of S. canadensis and S. juncea were sown into clipped plots, ranging from 0 cm (control, unclipped) to 100 cm in diameter, with and without litter. Seedling emergence, survival and growth were followed for one year. Soil moisture was not significantly different among the opening sizes, but, within a size, tended to be lower when litter was removed. Light intensity at the soil surface was positively related to opening size early in the growing season, but later in the growing season reached a maximum in intermediate-sized openings and then leveled off.Litter strongly inhibited seedling emergence in both species. Emergence of S. canadensis seedlings was lower in 0 and 10 cm openings than in the larger openings, while emergence of S. juncea seedlings was lower in the largest openings (100 cm) than in all the smaller openings. In contrast, seedling growth and probability of survival increased with diameter of opening for both species. Some seedlings of S. juncea did survive in complete vegetation cover (controls, 0 cm openings) while seedlings of S. canadensis survived only in openings of at least 30 cm diameter. Thus, S. juncea had a smaller minimum opening size for seedling establishment than S. canadensis, although both species grew and survived best in the largest openings made in the experiment.