Soil seed banks of pure spruce (Picea abies) and adjacent mixed species stands

The soil seedbank of long living seeds of herbs, graminoids and shrubs can survive several decades in the soil and germinate after disturbances like windthrow or clearcutting. The main goal of this study was to evaluate the risk of weeds, which may limit the success of conversions of secondary pure spruce stands (Picea abies) to mixed species stands. In a first step, germination experiments were performed in the greenhouse on soil samples collected under adjacent pure spruce and mixed species stands (mainly mixtures of spruce and beech - Fagus sylvatica) on two different soil substrates (Flysch: nutrient rich, basic soil; Molasse: nutrient poor, acidic soil). Seedling density and species richness were higher on the nutrient rich soil on Flysch. Comparisons between seedlings that emerged from soil samples collected at the end of the vegetation period and in spring justify the statement of the hypothesis that mixed spruce-beech stands advance the transient seed bank while pure spruce stands stimulate the persistent seed bank. In a second step, the seed banks of different soil horizons down to 35 cm soil depth were studied in a multivariate statistical design for the most dominant species J effusus, C. pallescens and R. idaeus, which are known to form long-term persistent seeds. Effects of bedrock material (Flysch, Molasse), species composition (pure spruce, mixed species) and treatment (control, nitrate) were tested. The total sum of these three species was significantly higher on Flysch than on Molasse. However, species composition indicated no significant differences, although there was a trend of higher amounts of germinating seeds under pure spruce. Nitrate treatments did not promote germination of viable buried seeds, indicating that the number of emerged seedlings is a realistic indicator of the seed bank density for the studied stands. It is concluded that overstorey tree species composition is not an important controlling factor for seed germination of the studied species after disturbances. The majority of emergents are the graminoids J. effusus and C. pallescens which were not present at all in the aboveground vegetation. Viable seeds were found down to 35 cm soil depth, although most seeds were concentrated in the upper 10 cm soil. Hence, care should be taken if management strategies create conditions that are generally favorable to germination. The success of forest regeneration or a conversion of pure spruce to mixed species stands could be endangered by any disturbance, which causes an immediate increase of light levels.