Alternate reproductive strategies of Populus nigra influence diversity, structure and successional processes within riparian woodlands along the Allier River, France

Abstract

The European black poplar (Populus nigra L.) is a riparian tree that dominates the early successional stages of alluvial floodplain forests throughout Continental Europe. River management and hybrid poplar plantations have contributed to the loss of this foundational species, and measures to conserve and restore the ecologically rich and diverse natural woodlands must sustain poplar reproduction. P. nigra is able to reproduce through either generative (sexual) or vegetative (asexual or clonal) processes and this enhances its chances for preservation in future forests. This research was undertaken to quantify the dominant reproductive strategy of natural black poplar stands, and to determine how this influenced woodland structure and successional processes within the floodplains along the lower Allier River. Two approaches were integrated: (1) analyses of the degree of clonality of stands based on DNA genotyping, sapling excavations, and the uniformity of poplar height and spatial distributions; and (2) retrospective analyses of successional processes to investigate spatiotemporal correspondences with the reproductive strategies. Structural stand patterns indicated the dominant reproductive strategy, with vertical heterogenity indicating clonality, while generative stands were more uniform. One half (50.2%, or 164 ha) of the floodplain study area supported woodlands with P. nigra stands or individuals. Of this area, 42% had apparently reproduced generatively with limited clonality; 43% had dominantly vegetative regeneration with high clonality; and 15% included generatively established, solitary old trees. The generatively and vegetatively reproduced stands displayed generally similar establishment rates but differed in their spatial patterns, with generative seedling bands or patches largely limited to barren streamside sites, while vegetative stands were more diversely distributed. The reproductive types also differed in successional dynamics, with generative stands providing more persistent successional phases with longer residence times. Thus, sexual reproduction enables woodland colonization and introduces genetic diversity, while clonal reproduction expands the population and structural diversity. Both reproductive strategies should be encouraged to provide healthy, diverse and resilient riparian woodlands.