Seed bank composition and seedling survival in forest soil polluted with heavy metals

Seedling recruitment from forest soil polluted with heavy metals was studied in order to determine the revegetation potential of the seed bank. The soil samples were collected from untreated and fertilised Scots pine stands along a 8 km transect running SE from a copper-nickel smelter in SW Finland. The composition and size of the active seed bank and the survival of the germinated seedlings were studied in greenhouse conditions. The average densities of germinated seeds of 15 species ranged from 250 to 4750 plants per m(2) at the six sites. Although vegetation was almost totally absent near the smelter, germinable seeds of Betula pubescens, Calluna vulgaris, Pinus sylvestris and Vaccinium uliginosum were found in the most contaminated soil. The number of Calluna vulgaris seedlings increased with increasing distance from the smelter, but no such trend was found for the other species. The mortality rate of the seedlings was highest in the soil samples collected near the smelter. Nutrient addition (stand-specific fertilisation with N, Ca, P and Mg) did not affect the number of germinated seeds, but liming slightly reduced heavy metal induced death of the seedlings. The survival probability of Calluna vulgaris seedlings decreased with proximity to the smelter. The results suggest that the recovery of Calluna vulgaris near the smelter was prevented by a low availability of seeds in the soil, unfavourable germination conditions and unsuccessful seedling establishment. We conclude that, despite the presence of viable seeds in polluted soil, revegetation from seed banks is not successful without soil mitigation to immobilise heavy metals.