Seedling emergence and establishment of Pinus sylvestris in the Mongolian forest-steppe ecotone

The potential of Scots pine (Pinus sylvestris) for regeneration and encroachment onto dry grasslands in the forest-steppe ecotone was experimentally studied at the south-eastern distribution limit of the species in Mongolia. The experiment consisted of a sowing and planting (2-year old seedlings) assay at two different distances from the forest line and manipulation of the water supply by irrigation in one half of the replicate plots. Seedling emergence was strongly inhibited by post-dispersal seed predation (between 50 and > 90 % of the seeds were apparently consumed) and by drought. Seedling survival was limited by drought and phytopathogenic fungi that infected the needles. Herbivory by insects or rodents, a key factor for seedling mortality in Larix sibirica, the most frequent tree species in Mongolia's forest-steppe ecotone, was of little importance in Scots pine, probably due to the production of efficient allelochemicals. The potential of Scots pine to regenerate in Mongolia's forest-steppe ecotone and to encroach onto the steppe is very limited and mostly restricted to the immediate vicinity of the forest; it might even decrease in future in the face of climate warming. The observed dependence of seedling emergence and survival on soil moisture suggests that regeneration outside the forest may only be successful in exceptionally moist years. Livestock grazing is certainly an additional limiting factor for Scots pine regeneration in Mongolia, but was not relevant in the present study area.