Plant Ecology and Evolution 144 (2): 138–147, 2011 doi:10.5091/plecevo.2011.549 Adaptive strategy of a spreading gynodioecious plant species (Origanum vulgare, Labiatae) in a riparian corridor Kris Van Looy 1,2,* , Olivier Honnay 3 & Peter Breyne 1 1 Research Institute for Nature and Forest, Kliniekstraat 25, BE-1070 Brussels, Belgium 2 Current address: Cemagref, Research Unit aquatic environments, ecology and pollutions MALY, river hydroecology team, 3 bis, Quai Chauveau - CP 220, FR-69336 LYON cedex 09, France 3 Laboratory of Plant Ecology, Biology Department, University of Leuven, Arenbergpark 31, BE-3001 Heverlee, Belgium *Author for correspondence: kris.van-looy@cemagref.fr INTRODUCTION River landscapes offer opportunities for the study of popula- tion dynamics in relation to the processes of fragmentation and colonization. For some species, reproductive and lower- ing strategies are well adapted to beneit from the frequent habitat disturbances, and thus are the key to survival. One such species that is extremely well recovering along the River Meuse is Origanum vulgare L. This species is currently tak- ing advantage of the newly established natural management of the loodplain in the framework of a large-scale river res- toration programme. The observation of the contrast between strongly spreading populations of this species on the one hand, and remnant declining populations on the other hand, was the starting point for this study on the genetic and popu- lation structure of the species along the river reach. We aimed at understanding the mechanisms that were responsible for the successful recovery of the species in a restoration context, at the landscape level. Because O. vulgare is a gynodioecious species, the lowering strategy may be responsible for the ob- served contrast in population dynamics between the different populations. All rights reserved. © 2011 National Botanic Garden of Belgium and Royal Botanical Society of Belgium – ISSN 2032-3921 REGULAR PAPER Backgrounds and aims – Origanum vulgare L. is a successfully recovering gynodioecious species in the ecological restoration project along the Common Meuse (Belgium). Strong contrasts exist between spreading – mostly newly established – populations on the one hand, and remnant, declining populations on the other hand. The lowering strategy of the species was assumed to be related to these differences in population status. Two alternative hypotheses for small, isolated populations were postulated: either they contain more sterile individuals due to their presence in marginal conditions, or they show more hermaphrodites for reproductive assurance. Additionally, we aimed at relating neutral genetic variation of the populations with their status and sex ratio. Methods – An analysis at two levels was performed to get a consistent image of the differences in lowering strategy among and within populations. For the population-level survey, 25 populations were sampled for lowering strategy and for genetic analysis. For the within-population analysis, the survey focused on the expanding populations. Five expanding populations on the riverbanks were fully surveyed for the sex ratio of all individuals. Statistical analysis focussed on relationships between the populations’ sex ratios and population genetics, size and dynamics, and stand conditions related to management intensity and isolation. Key results – Both hypotheses were supported, but at a different level. At the population level, the smaller populations showed more sterility. But within the expanding populations, isolated parts showed less sterility. For the observed sexual expression at population level, no signiicant correlation was present between sexual expression and population dynamics, isolation or management intensity. For the within- population differentiation, a relationship with reproductive assurance in expanding populations was shown. For the observed differentiation in sex expression, indications are present for the lower strategy being a developmental plasticity. Conclusion – For O. vulgare, the gynodioecy proves a successful plastic strategy for expanding and new establishing populations to cope with pollen limitation and inbreeding depression. Key words – riparian corridor, colonisation, reproductive assurance, adaptation, plasticity.