Which generative reproduction characteristics determine successful establishment of the subalpine shrub Pinus mugo?

QUESTIONS: Shrub expansion in arctic, alpine and subalpine areas has been occurring recently due to warming, land abandonment and increasing nitrogen deposition. However, shrub expansion is strongly affected by largely unknown site‐specific regeneration patterns. We evaluated the importance of seed production, suitable microsite availability and seedling survival for seedling establishment of the subalpine conifer shrub Pinus mugo. LOCATION: Hrubý Jeseník Mts., Czech Republic (50° N, 17° E). METHODS: We collected data on: (a) seed quantity and quality; (b) seedling microsite preferences; and (c) seedling survival during initial ontogenetic stages. From 2006 to 2016 we assessed cone production annually and tested seed viability every second year. Microsite preferences were analyzed for 650 seedlings at six sites. The following microsite characteristics were recorded for 1‐m² around each seedling: percentage of bare surface; surrounding plant species cover; surface shape; and organic horizon thickness. Additionally, distance and direction to the nearest adult pine were recorded. Finally, we tested survival of 200 newly established seedlings in relation to microsite type. RESULTS: Seed source and viability were not critical factors for seedling establishment. Far more important was the presence of suitable bare‐surface microhabitats with thin or no organic layer near mature pine stands. Vegetation communities dominated by Avenella flexuosa and Nardus stricta facilitated seedling establishment; communities dominated by Vaccinium myrtillus showed strong competitive effects on seedlings. Vegetation's effect on seedling establishment was closely related to disturbance regime, which influences vegetation type. Seedling mortality was highest in the first year of life irrespective of microsite vegetation type. Highest mortality was associated with open mineral‐soil microsites, while microsites with dense herbaceous vegetation showed the lowest. CONCLUSIONS: Potential Pinus mugo expansion will occur in close proximity to existing mature stands, and its intensity will be strongly modified by site‐specific density of suitable microsites and/or dominant species of host communities.