Geographic distance is more relevant than elevation to patterns of outbreeding in Ranunculus bulbosus

Summary Mountain ecosystems can exert different selection pressures on plant populations over small scales due to steep environmental gradients. Gene flow between lower- and higher-elevation plant populations could lead to the increased integration of potentially adaptive genes and thereby to outbreeding vigour under current rapid climatic changes. Alternatively, gene flow across elevation could disrupt gene complexes leading to outbreeding depression. We explored the responses of higher (1800 m a.s.l.) and lower (1200 m a.s.l.) Ranunculus bulbosusL. populations to intra- and interelevational and short- as well as long-distance gene flow by means of controlled crosses and assessment of the offspring's germination, growth and fitness. Outbreeding differed between elevations of origin: higher populations were negatively affected by pollen flow from lower populations at the earliest life stages, but tended to be positively affected in terms of growth later on while gene flow across small vs. large distances had no impact on growth traits. Lower populations were not significantly affected by the elevational origin of pollen per se. However, fitness of offspring of populations from both higher and lower elevations was jointly affected by elevational and regional origins of pollen, in which case the gene flow distance had a comparatively stronger positive impact on fitness (outbreeding vigour) than elevational origin. Synthesis: Our results indicate that compared to gene flow at regional scales, gene flow across elevational gradients has a minor effect on R. bulbosus. The lack of outbreeding depression suggests that potentially adaptive genes might well be integrated across populations resulting in an increased resilience of R. bulbosus and potentially similar montane plant species in changing climatic conditions.