Population size affected by environmental variability impacts genetics, traits, and plant performance in Trifolium montanum L.

Population size, genetic diversity, and performance have fundamental importance for ecology, evolution, and nature conservation of plant species. Despite well‐studied relationships among environmental, genetic, and intraspecific trait variation (ITV), the influence of population size on these aspects is less understood. To assess the sources of population size variation, but also its impact on genetic, functional trait, and performance aspects, we conducted detailed population size estimations, assessed 23 abiotic and biotic environmental habitat factors, performed population genetic analyses using nine microsatellite markers, and recorded nine functional traits based on 260 Trifolium montanum individuals from 13 semi‐dry grassland locations of Central Europe. Modern statistical analyses based on a multivariate framework (path analysis) with preselected linear regression models revealed that the variation of abiotic factors (in contrast to factors per se) almost completely, significantly explained fluctuations in population size (R (2) = .93). In general, abiotic habitat variation (heterogeneity) was not affected by habitat area. Population size significantly explained genetic diversity (N (A): R (2) = .42, H (o): R (2) = .67, H (e): R (2) = .43, and I: R (2) = .59), inbreeding (F (IS): R (2) = .35), and differentiation (G (ST): R (2) = .20). We also found that iFD(CV) (ITV) was significantly explained by abiotic habitat heterogeneity, and to a lesser extent by genetic diversity H (e) (R (2) = .81). Nevertheless, habitat heterogeneity did not statistically affect genetic diversity. This may be due to the use of selectively neutral microsatellite markers, and possibly by insufficient abiotic selective pressures on habitats examined. Small T. montanum populations in nonoptimal habitats were characterized by reduced genetic and functional trait diversity, and elevated genetic inbreeding and differentiation. This indicates reduced adaptability to current and future environmental changes. The long‐term survival of small populations with reduced genetic diversity and beginning inbreeding will be highly dependent on habitat protection and adequate land‐use actions.