Experimental studies of complex crater formation under cluster implantation of solids

The results of a systematic study of surface defect formation after energetic Ar$_n^+$ (n = 12, 22, 32, 54) and Xe$_n^+$ (n = 4, 16) cluster ion implantation into silicon and sapphire are presented. Implantation energies vary from 3 to 18 keV/ion. Two cases of comparative studies are carried out: the same cluster species are implanted into two different substrates, i.e. Ar$_n^+$ cluster ions into silicon and sapphire and two different cluster species Ar$_n^+$ and Xe$_n^+$ are implanted into the same kind of substrate (silicon). Atomic force, scanning electron and transmission electron microscopies (AFM, SEM and TEM) are used to study the implanted samples. The analysis reveals the formation of two types of surface erosion defects: simple and complex (with centrally positioned hillock) craters. It is found that the ratio of simple to complex crater formation as well as the hillock dimensions depend strongly on the cluster species, size and impact energy as well as on the type of substrate material. Qualitative models describing the two comparative cases of cluster implantation, the case of different cluster species and the case of different substrate materials, are proposed

Seed bank and vegetation development of sandy grasslands after goose breeding

Four hypotheses were tested using long-term observations of vegetation development (12 years) and present-day seed bank data in a sandy grassland area overgrazed by domestic geese: i) Gap regeneration is crucial in maintaining species richness; thus, closed vegetation of the lower sites prevents continuous establishment of short-lived species. ii) Short-lived, early successional species comprise most of the seed banks and late successional perennials have at most sparse seed banks. iii) Composition of seed banks is more similar to pioneer vegetation than to later successional stages. iv) The similarity is higher between vegetation and seed banks in the upper-positioned plots than in the closed, lower-positioned ones. Two sites, located in the upper part of dune slopes, and another two, positioned on the lower part, were studied. In each site five 2×2 m permanent plots were surveyed between 1991 and 2002. Percentage cover was estimated three times a year. In the last study year, soil seed banks were sampled. Two vertical segments (0–5, 5–10 cm) were separately analyzed. The seedling emergence method was applied on concentrated samples. We found that the vegetation developed from open, annual dominated weedy assemblages to grasslands dominated by perennial graminoids. In the lowerpositioned sites perennial clonal grasses (Cynodon dactylon, Poa angustifolia and P. pratensis) formed more closed vegetation, which was accompanied by lower species richness compared to the upper-positioned sites. Seed density varied between 10,300 and 40,900 seeds/m2. Significantly higher seed densities were found in upper sites than in the lower ones. Annuals and short-lived perennial dicots comprised most of the seed bank. The dominant perennial graminoids also built up dense seed banks