Interactions among Colonizing Annuals: Is There an Effect of Gap Size?

Small—scale gaps are often colonized by more than one plant, yet few individuals. We examined interactions among co—invading plants within small gaps in an old—field community as a function of the size of the gap being invaded. Several colonizing annual species were grown, as single individuals or in conspecific or heterospecific pairs, in artificially created gaps of 10, 20, and 40 cm diameter in a Poa pratensis L. background in two successive years. The identity of the neighbor influenced plant survivorship for one of the two years studied, but had no effect on the probability of becoming reproductively mature, or on levels of seed production. In contrast, vegetative growth was influenced by neighbor identity in both years. Both positive and negative plant—plant interactions were found; the presence of a neighbor did not always result in competitive reduction in the target individual. Conspecific neighbors were found to reduce vegetative biomass to a greater extent than heterospecific neighbors for one of the two years studied, primarily due to the sensitivity of one species, Setaria faberii, to conspecific neighbors. We found no evidence of a consistent competitive hierarchy present in this system. Finally, although increasing gap size had significant, positive effects on survivorship, growth, and reproduction, the size had being colonized had no effect on plant—plant interactions between co—invading individuals. In this system the number and identity of invading individuals, as well as the characteristics of the gap itself, interact to dictate the composition of the invading community and the relative performance of each individual colonist. This complexity may contribute to increased diversity in plant communities in which small—scale disturbances provide opportunities for earlier successional species to persist in localized patches within later successional vegetation.