Inter-annual variation in fine-scale spatial genetic structure and patterns of density-dependent mating in a population of self-compatible Arabidopsis lyrata

BACKGROUND AND AIMS: Mating patterns in plant populations are shaped jointly by the local availability and relatedness of potential mates, both of which can vary through time in disturbance-prone habitats. We aimed to investigate how a year-to-year change in local plant densities and spatial genetic structure (SGS) influence opportunities for inbreeding and outcrossing in a predominantly self-compatible population of Arabidopsis lyrata subsp. lyrata located in a fore-dune habitat at Rondeau Provincial Park, Ontario. METHODS: We mapped plants in 40 patches, each centred on a focal plant, over two consecutive years, quantified SGS with kinship-distance analyses of 27 and 34 focal plants in the first and second years, respectively, and estimated multilocus rates of outcrossing and biparental inbreeding from 128 and 177 progeny, respectively. KEY RESULTS: Across surveyed patches, plant density increased more than five-fold between years, coinciding with a marked decline in fine-scale SGS (Sp = 0.130 in year one; Sp = -0.014 in year two). Despite similar population-wide outcrossing rates in both years (tm = 0.45 in 2021; tm = 0.49 in 2022), outcrossing was positively associated with neighbourhood density in the lower-density year but showed no density dependence when densities were high in the second year. In both years, 15-20% of outcrossed seeds were produced by mating among relatives, indicating substantial biparental inbreeding. CONCLUSIONS: Our results suggest that recruitment pulses in dynamic dune habitats can rapidly weaken SGS while simultaneously altering the scale at which density governs mating patterns. More generally, they highlight how short-term temporal variation in population size can modulate the local availability and relatedness of mates within plant populations.