Factors Affecting Interaction and Distribution of Haplopappus Divaricatus and Conyza Canadensis in North Carolina Old Fields

Haplopappus divaricatus and Conyza canadensis commonly appear in first—year old fields of North Carolina. Haplopappus is very rare in fields of the North Carolina Piedmont, but occurs in varying densities on the coastal plain (0—83.8% relative density in 26 coastal plain old fields). Within the coastal plain, there is no east—west or north—south trend in the distribution of Haplopappus, and no correlation with the most recent grown crop is found. Fields disturbed in fall or winter, and fields with deep sandy soils and soils with low clay content, low moisture retention, and low available nutrients had high densities of Haplopappus and correspondingly low densities of Conyza. Haplopappus plants transplanted from the coastal plain to the Piedmont survived and produced large numbers of viable seeds, but no seedlings grew the following year around these transplants. Transplants from greenhouse to coastal plain and Piedmont fields survived and grew, but those in the coastal plain had greater dry weights. Haplopappus seeds planted in three plots in the Piedmont produced only one plant which set viable seeds. In greenhouse experiments both species were more affected by competition in sandy soil than in heavy soil. Haplopappus was more sensitive to interspecific competition, but decreased nutrients reduced the competitive ability to Conyza plants more than that of Haplopappus. Drought stress, which did not affect old or young Conyza plants, reduced the growth of young Haplopappus plants. Haplopappus showed less sensitivity to specific nutrient deficiencies than did Conyza. In the field and in greenhouse flats Haplopappus seeds did not germinate in heavy soil. The presence of added organic matter in sand reduced the germination percentage of Haplopappus, as did decay products of Conyza shoots. Haplopappus seeds in petri dishes on a temperature—gradient bar germinated at lower temperatures than did Conyza seeds. Germination percentages of Haplopappus were also greater when less water was available in the substrate. It is concluded that fall and winter disturbance of old fields kills many Conyza seedlings and opens bare soil. Because Haplopappus seeds germinate at low temperatures and in bare soil, this species is able to invade these areas. If no such disturbance occurs, few Haplopappus plants will mature, because Conyza plants are better competitors. Haplopappus apparently is prevented from becoming established in the Piedmont by the inability of its seeds to germinate in heavy soil and by the superior competitive ability to Conyza plants in the richer Piedmont soils.