Relative importance of range position, seed size, and genetic diversity for tree recruitment at alpine range edges

Abstract Climate change is forcing many plants to shift their distributional range to higher latitudes or higher elevations to keep up with their climatic niche. Seed dispersal is the only mobile phase in most plants' life cycles and determines where seedlings may recruit. Seedling recruitment, in turn, can be influenced by various factors such as the location of the mother plant within its local distributional range, reproductive traits, the genetic diversity of the mother plant, or local micro‐climatic conditions. So far, we lack knowledge about the relative importance of these factors in shaping seedling recruitment across and beyond plant distributional ranges. Using the model system of Swiss stone pine ( Pinus cembra ) and its main seed disperser, the spotted nutcracker ( Nucifraga caryocatactes ), we examined how the elevational range position of mother trees (i.e., range centre versus range edges), intraspecific variation in reproductive traits (i.e., crop size, proportion of infertile seeds, seed size), and genetic diversity (i.e., heterozygosity) along with micro‐climatic conditions of the seed location affect seedling establishment and survival. We conducted a transplant experiment by harvesting 3600 seeds from three different elevations and sowing them across 10 destination elevations spanning and exceeding the species' elevational range. Our results reveal that pine seedling establishment depends strongly on reproductive traits and on seed origin and destination elevations. Seeds from the range centre established best at high elevations outside the pine's range, while those from upper and lower range edges established best at the pine's range centre. Large seed size and a high proportion of fertile seeds promoted seedling establishment. In turn, micro‐climatic conditions at sowing destinations played a minor role, and the heterozygosity in mother trees did not significantly affect seedling establishment and survival. Synthesis . Our study highlights the crucial role of the range position in concert with reproductive traits for seedling establishment at distributional range edges, potentially outweighing the effects of micro‐climatic conditions and genetic diversity. Understanding the interplay between plant reproductive traits, genetic diversity, and dispersal ability will be essential for projecting future species range limits and informing conservation strategies in the face of climate change.