Exotic gene flow affects fitness trait values but not levels of heritable trait variation in the southernmost population of Scots pine (Pinus sylvestris L. var. nevadensis)

Large-scale human-mediated translocation of plants is a widespread phenomenon throughout the world. In this study, we examine the extent to which exotic gene flow from conspecific plantations of mesic origin impacts pre- and post-dispersal fitness, seedling phenotype, and quantitative genetic variation of the relict southernmost native population of Scots pine (Pinus sylvestris var. nevadensis Christ). For this purpose, we conducted controlled pollinations to obtain native, exotic and intraspecific hybrid progenies for testing seed viability and seedling performance under experimental common-garden conditions mimicking the severe summer drought in the native population's environment. Proportion of aborted seeds was markedly higher in exotic (> 60%) than in intraspecific hybrid (~24%) and native (18%) progenies. Seed germination time and rate were similar across genetic crosses. Hybrid and exotic seedlings died on average 1.2 and 0.8 days earlier than local progenies when exposed to the drought treatment. Cumulative mortality rate was positively associated to biomass allocation in needles, suggesting that higher investment in needles is maladaptive in dry environments for P. sylvestris, probably because of increased total transpiration. Native progenies harbored higher genetic variation than exotic progenies, but similar to hybrids for most phenotypic traits, suggesting low impact of exotic gene flow on the levels of heritable genetic variation of the relict population. To the extent that our results held in natural conditions, they suggest that natural selection in the relict's dry natural environment could potentially hamper the effective establishment of exotic genes via seed and pollen dispersal from the plantations, and that exotic hybridization would not significantly affect the potential of early recruits in response to selection.