Gap dynamics in perennial subalpine grasslands: trends and processes change during secondary succession

1 Revegetation of gaps may affect the floristic composition of subalpine grasslands during secondary succession. Six hypotheses concerning overall effects, gap-size and edge effects, and changes in these during secondary succession, are tested using principal response curves (PRCs), a recent derivative of partial redundancy analysis. 2 In a field experiment, large (625 cm(2)) and small (39 cm(2)) gaps and controls were established in a replicated successional series (0, 10 and 40 years after abandonment). Colonization modes were quantified in year 1, and the floristic composition was monitored twice yearly over four growing seasons. 3 Micro-successions in gaps account for 8.8-12.2% of the compositional variance. Size effects add 3.3-5.5% per successional stage, while edge effects appear only in the mid-successional stage (adding 1.6%). Seed colonization rates are comparable with those observed in temperate systems, and the effects on community composition are considerable (initial gap response, size effect). In contrast, changes in local competitive regimes have weaker effects (revegetation dynamics after the first year, edge effects). This may be attributed to the slow growth rates of alpine plants and the short growing season in subalpine climates, as well as to the infertile soils of the semi-natural grasslands. 4 majority (74%) of the species are affected by gaps, but their responses are not consistent across successional stages. Species that recruit into gaps primarily as seedlings, and locally rare species, become relatively more dependent on gaps for local population persistence during succession. 5 These experiments give insight into the effects of gap revegetation processes for community composition and population persistence in these perennial grasslands. During secondary succession, changes in disturbance regime affect the gap revegetation processes and the probability for local population persistence for gap-enhanced species.