The genetic structure of a <i>Brachypodium hybridum</i> population in a patchy arid landscape is independent of neighboring perennials and stable over two consecutive years

In arid landscapes perennials can provide protection for annual plants against environmental stress such as drought. This could lead to population sub-structuring of the annuals, because of position dependent reproductive success, and/or competition for association with perennial shrubs. Although such interactions are well known drivers of plant community dynamics, little is known about the local and temporal effects of perennial presence on the genetic structure of annuals. As a first step to address this, we used a set of microsatellite markers to assess the genetic structure of the annual grass <i>Brachypodium hybridum</i> (L.) P. Beauv. growing underneath and outside the canopy cover of perennials at an arid site in southern Spain over two consecutive years (2018 and 2019). Upon sampling, we observed clear but inconsistent seasonal differences in phenology between individuals underneath or outside canopies. However, we did not find genetic differentiation between sampling location or sampling year (four populations: overall <i>F</i> _ST = 0.024). An analysis of the overall genetic structure inferred three putative clusters, but these clusters were neither associated with location nor with year of sampling. We conclude that the genetic structure of this <i>B. hybridum</i> population is independent of neighboring perennials and stable over two consecutive years and is not associated with phenological differences. The data further show a high level of homozygosity in the population, and the recurrent presence of identical genotypes, both with respect to location and year, indicating a major role of selfing in <i>B. hybridum</i> reproduction. However, the samples collected outside the canopies in 2019 show a slightly higher value of estimated heterozygosity. This may indicate the effect of immigration from other <i>B. hybridum</i> populations. These results show that neighboring perennial plants do not affect the stable genetic structure of the <i>B. hybridum</i> population within this arid site, but that other factors such as immigration, could eventually result in local and temporal differences. These results also indicate a possible patchy distribution of identical genotypes of the annuals, which should be considered when sampling such populations.