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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.
Gross et al. (Sun,) studied this question.