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The degree of canopy closure can shape the dynamics of understorey plant populations that rely on clonal and sexual recruitment. Populations are expected to undergo declines in clonal diversity under conditions where recruitment from seed is temporally and spatially restricted. Localized seedling recruitment in clonal populations may also affect spatial genetic structure due to the clumping of genetically related genets. Our major objective was to determine the effect of the degree of canopy closure on clonal diversity and spatial genetic structure in the rhizomatous, dioecious forest perennial Mercurialis perennis. As the distribution of the male and female shoots has been shown to be influenced by canopy openings, we paid special attention to the mediating role of the varying sex ratio. We used genome-wide AFLP markers to fingerprint six populations of M. perennis along a light penetration gradient. The proportion of male shoots in a population increased from 0.51 to 0.81 and male genet diversity decreased from 0.72 to 0.21 with increasing site illumination, in agreement with earlier reports of superior male growth in canopy openings. The most illuminated population, with the highest proportion of male shoots, was dominated by a few outsized, largely aggregated male clones (largest clone spreading over 10 m). Overall clonal diversity (G/N: 0.31-0.74; mean: 0.52) and evenness strongly declined in well-lit sites, suggesting reduced sexual recruitment and the vast vegetative spread of a few locally well-adapted male genets under canopy gaps. Fine-scale genetic structure among genets was significant within all populations, but its degree tended to increase with an increased proportion of male shoots and reduced clonal diversity. Very localized recruitment due to a low seed dispersal capacity combined with the aggregated distribution of large gender-specific clones likely incurred this pattern. Synthesis. Forest management practices such as the cutting and removal of trees and the establishment of paths decreased the degree of canopy closure. Many understorey herb populations flourish in these canopy gaps, reflected in a higher clonal diversity. We demonstrated, however, that increased illumination negatively affects genotypic and genetic diversity in the dioecious understorey herb M. perennis.
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