Seed rain on alpine ski runs in Switzerland

Spatial and temporal variation in seed rain was studied in two un-restored ski runs at the Jakobshorn Mtn. (NE Swiss Alps), at ca. 2500 m a. s. l. The study carried out from 1996 to 1998 included density per m(2), alpha diver sity (species richness), and species composition of the seed rain vs. that of the resident vegetation. This is the first detailed study of seed rain o n machine-graded ski runs at high elevation. Density of seed rain peaked mostly in late summer; on the other hand, alpha diversity varied most frequently with one peak after spring snowmelt and a nother towards the end of the growth period. Distribution of seeds among dr y traps was patchy. Between-plot differences within one year, and also differences between year s were not always significant. Between-slope differences were very clear. S eed rain in 1998 was significantly higher in the SW ski run than in the NE run (2467+/-418 vs. 900+/-202 seeds/m(2), 3.1+/-0.3 vs. 1.9+/-0.2 species p er trap). Seed rain on the SW ski run totalled 23 species; on the other han d, total alpha diversity of seed rain on the NE run amounted to only 16 spe cies. Only few species prevailed in seed rain, the others provided small co ntributions. Seed rain was largely dominated by forbs, whereas graminoids a mounted to only ca. one-third of all species and less than 10% of all seeds . Species included in seed rain represented mostly seed shadow and bore no clear relationship to successional stages. The results suggest that seed rain in high-alpine ski runs strongly depends on local conditions such as neighbouring vegetation, slope incline and mic rorelief; general landscape context is apparently important, too. Data on s eed rain in disturbed sites show potential for assessment of impact levels, and are thus relevant for the planning and implementation of restoration s chemes.