Environmental factors driving seed bank diversity in alkali grasslands

For an effective conservation and management in grasslands it is essential to understand mechanisms sustaining biodiversity. To gain knowledge is especially crucial in stressed grasslands harbouring a unique flora and fauna, like alkali grasslands. Aboveground vegetation, seed bank and environmental factors were studied in three stands of the following alkali grassland types: (i) Artemisia dry alkali grasslands at highest elevations; (ii) Puccinellia high and (iii) Puccinellia low grasslands at medium to low elevations, and (iv) Juncus wet alkali grasslands at the lowest elevations. We tested the following hypotheses: (i) Seed bank species diversity and density are the highest in the most stressed grassland types, where regeneration by seeds could have a major importance in sustaining vegetation diversity. (ii) Seed bank density of hygrophytes increases with decreasing elevation, because the cover of hygrophytes in the vegetation increases with decreasing elevation. The mean seed bank density ranged from 30,104 up to 51,410 seeds/m2, which is higher than in most dry grasslands. Both the lowest seed bank density and diversity were detected in the most stressed Puccinellia high grasslands; Spergularia salina was the only abundant seed bank species (possessing at least 1000 seeds/m2). These results not supported our first hypothesis. We detected the highest seed densities of almost all hygrophyte species in the lowest-elevated Juncus grasslands. But, we did not find a significant monotonous correlation between elevation and the overall hygrophyte seed bank density; because most of the hygrophyte species were missing from the seed bank at the medium-elevated, but most saline Puccinellia grasslands. Thus, our results only partly supported the second hypothesis. In total we detected more species in the seed bank than in the aboveground vegetation which emphasises that seed bank plays an important role in sustaining the diversity of alkali grasslands. However, characteristic graminoids possessed no considerable seed bank, except for Juncus compressus (up to 38,619 seeds/m2). We can conclude that persistence and establishment of most alkali grassland species are not supported by the local persistent seed bank.