Interference of winter annuals with Ambrosia artemisiifolia in early successional fields

Suppressed individuals of the summer annual Ambrosia artemnisiifolia L. com- monly occur in large numbers in fields dominated by the winter annuals Erigeron annuus and E. canadensis. Ambrosia artemisiifolia exhibits plasticity in response to intense competition, its population levels remaining constant throughout the growing season. Experiments in the field and greenhouse revealed a complex interplay of factors between the associated summer and winter annuals leading to the suppression of A. artemisiifolia. The microenvironmental regime of A. artemisiifolia in the presence of winter annuals is generally less severe than that in the absence of the winter annuals. Those aspects of the physical environment as integrated by plant water potential did not constrain the growth of the summer annual. Manipulation of the competitive interactions of A. artemnisiifolia with the winter annuals produced no evidence that any single factor is ultimately responsible for the suppression of this summer annual. Stunting results from a diminishing nutrient supply, decreasing light intensity, and altered light quality which result from the interference of E. annuas and E. canadensis. Allelopathy plays a less important role in the response of A. artemisiifolia than does com-- petition. The relative availability of resources defined by the competitive relationships among the plants is critical for the normal growth, development, and reproduction of A. artemnisiifolia, and not the absolute amounts of these limiting resources under the conditions studied. The inability of suppressed plants to partition a greater proportion of resources to re- production than nonsuppressed plants emphasizes the importance of the plastic response of A. artemnisiifolia in enabling diminutive plants to reproduce successfully. The result of the interaction between A. artemnisiifolia and Erigeron spp. is that recruitment of A. artemisiifolia to plant populations which develop after disturbance is assured and that there is a diversity of genotypes, further adapting the species for successful establishment in an unpredictable environment.

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