Soil seed banks of pioneer tree species in European temperate forests: a review

The ability of short-lived tree species such as birch, alder, willow, poplar and rowan to form even a short-term soil seed bank is discussed controversially in the literature. Soil seed banks are an important component of succession and regeneration in ecosystems. Following disturbance, buried viable seeds germinate and the seedlings that establish cover the disturbed, exposed soil surfaces. The objective of this study was to undertake a literature review of soil seed bank research carried out in central and north-west European temperate forests to provide an overview of the ability of pioneer tree species to form a viable seed bank. The review of 33 publications revealed that birch is the only pioneer tree species of temperate forests with longer-lived seeds, persisting in the soil for 1 - 5 years. Birch seeds remain viable in deeper soil layers (5 - 10 cm), so birch may be assigned to the short-term persistent soil seed bank type. The seeds of alder, willow and poplar would appear to be short-lived. Maximum seed densities of all tree species were found in the upper soil layers. With increasing soil depth, seed density declined. Viable seeds of rowan were not detected in any of the soil seed bank studies, although seed trees were present. We found that in spite of the capacity for long seed dispersal distances, high densities of birch, alder and willow seeds were only observed in close proximity to seed trees. The higher the numbers of seed trees, the higher the seed densities in soils. Maximum seed densities were recorded during and shortly after seed rains had occurred. Our results reveal that a birch seed bank may compensate for years with lower levels of seed production. However, as the seed bank is only short-term persistent, it must be supplemented by fresh seeds from surrounding seed trees as often as possible to guarantee a continuous capacity for regeneration.