Colonization of experimentally created gaps along an alpine successional gradient

The colonization of artificially created gaps was analyzed along an alpine successional gradient from pioneer to early, late, and old successional stages. The presence/absence of species and the abundances of seedlings and adults in the gaps were recorded and compared with those of the surrounding areas. We hypothesized that in the older successional stages, the gaps were likely to be colonized by clonal ingrowth of the surrounding species. In the younger stages, we expected to find a high presence of seedlings and adults recruited by seeds. Micro-succession in the gaps occurred at each successional stage, with all life forms among the colonizers. The abundance of seedlings was significantly higher in the gaps compared with the surrounding area. At the early and late successional stages, the surrounding areas provided safe sites for seedling establishment, with the abundance of adults recruited by seeds higher at the gap edges than in the gap centers. We can confirm the first hypothesis of a higher clonal ingrowth in the old successional stage. Clonal ingrowth also occurred in the younger successional stages. Despite the lower species richness in the gaps, a positive correlation was found between gap and surrounding species frequencies, which were the highest in the pioneer and the lowest in the old successional stage. We conclude that gaps are relevant for seedling recruitment along the entire primary succession gradient. New species invasions from greater distances were not observed in the gaps. The dominant species on each site were identified to be successful gap colonizers.