Changing perspectives on the role of the soil seed bank in northern temperate deciduous forests and in tropical lowland rain forests: parallels and contrasts

In the early 1990s a simple view of forest dynamics and the role of the soil seed bank in forests was widely accepted. Plants could be divided into ‘pioneers’ that need canopy gaps for establishment, and ‘non-pioneers’ that are able to establish in shade. The species in the persistent seed bank were believed to be ‘pioneers’, awaiting formation of a gap in the canopy. It was also widely believed that seed bank species need (a) light, and (b) a high red: far-red (R:FR) ratio for germination, and that some need marked diel fluctuation in temperature. We review the changes in perspective during the last 15 years for the two forest-types best studied in this respect: northern temperate deciduous forest (NTDF) and tropical lowland rain forest (TLRF), and concerning NTDF we place especial emphasis on studies in nearly natural stands in the Białowieża Forest in Poland. It is now known that ‘pioneers’ and ‘non-pioneers’ merge in the sense that in NTDF there are species that utilize gaps in the herb layer and litter under an intact tree canopy, and in TLRF there are species which need only ‘micro-gaps’ in the tree canopy caused by the fall of a single branch or of a whole crown a little way off. It is also known that the soil seed bank of both these forest types may contain a substantial number of species that are to a significant degree shade-tolerant; in NTDF there are a few species able to establish in the deepest shade and a greater number able to utilize gaps in the herb layer and litter, while in TLRF there are both species able to establish in the deepest shade and species needing only micro-gaps in the tree canopy. In both forest types a requirement for light and a high R:FR ratio is now seen to be characteristic of the small-seeded canopy-gap-demanding species in the seed bank; small-seeded species able to grow in an appreciable degree of shade do not need a high R:FR for germination. There is emerging evidence that (a) stimulation of germination by diel temperature fluctuation is characteristic of larger-seeded species in the seed bank, and (b) that risk-spreading dormancy is overlain on the dormancy related to awaiting an appropriate signal for germination. TLRF seems to differ from NTDF in that larger-seeded species rather than smaller-seeded last longer in the soil.