Interspecific competition changes reproductive output but does not increase reproductive costs in a grassland perennial

In plants, it is hypothesized that allocation trade-offs may appear only when expenditures like seed production are high or external resources are scarce. In this study, we tested whether reproductive costs are more pronounced under enhanced interspecific competition. In a common garden, we investigated phenotypic correlations between sexual reproduction, clonal growth and storage structures in the grassland perennial, Succisa pratensis. During the past 50 years, habitats of this species have faced an expansion of clonal grasses that increase competition intensity. We simulated this process by growing five populations of Succisa from high- and low-production habitats with its clipped and non-clipped competitor, Agrostis capillaris. In addition, we experimentally removed flower heads of Succisa plants from one population grown with and without A. capillaris. We demonstrated costs of sexual reproduction by flower-head removal (resulting in increased plant size and relative allocation to belowground structures) but not by phenotypic correlations. We found no evidence that reproductive costs increase in a competitive environment and the opposite pattern was shown in both approaches used. However, high competition intensity reduced relative investment to flower-head production. In plants from low-production habitats, competition also reduced the absolute number of flower heads and belowground biomass as a result of smaller plant size. We assume that populations from low-production habitats are more prone to extinction as they have a reduced likelihood of local persistence and of escape to more suitable habitats during advancing succession. (C) 2009 Gesellschaft fur Okologie. Published by Elsevier GmbH. All rights reserved.