A seed bank is the reservoir of viable seeds present in a plant community. Seed banks are evaluated by a variety of methods. For some species , it is possible to make careful, direct counts of viable seeds. In most cases, however, the surface substrate of the ecosystem must be collected and seeds encouraged to germinate by exposure to light, moisture, and warmth. The germinating seedlings are then counted and, where possible, identified to species.
In most cases, the majority of seeds are found in surface layers. For example, the organic-rich forest floor contains almost all of the forest's seed bank, with much smaller numbers of seeds present in the mineral soil .
The seeds of some plant species can be remarkably long-lived, extending the life of the seed bank. For example, in northeastern North America, the seeds of pin cherry (Prunus pensylvanica ) and red raspberry (Rubus idaeus ) can persist in the forest floor for perhaps a century or longer. This considerably exceeds the period of time that these ruderal species are present as mature, vegetative plants during the initial stages of post-disturbance forest succession . However, because these species maintain a more-or-less permanent presence on the site through their persistent seed bank, they are well placed to take advantage of temporary opportunities of resource availability that follow disturbance of the stand by wildfire , windstorm, or harvesting.
The seeds of many other plant species have only an ephemeral presence in the seed bank. In addition to some tropical species whose seeds are short-lived, many species in temperate and northern latitudes produce seeds that cannot survive exposure to more than one winter. This is a common trait in many grasses, asters, birches, and most conifers, including pines, spruces, and fir. Often these species produce seeds that disperse widely, and can dominate the short-lived seed banks during the autumn and springtime. Species with an ephemeral presence in the seed bank must produce large numbers of well-dispersed seeds each year or at least frequently, if they are to successfully colonize newly disturbed sites and persist on the landscape.
Although part of the plant community, seed banks are much less prominent than mature plants. In some situations, however, individual plants in the seed bank can numerically dominate the total-plant density of the community. For example, in some cultivated situations the persistent seed bank can commonly build up to tens of thousands of seeds per square meter and sometimes densities which exceed 75,000 seeds per square meter. Even natural communities can have seed banks in the low tens of thousands of seeds per square meter. However, these are much larger than the densities of mature plants in those ecosystems.
The seed bank of the plant community is of great ecological importance because it can profoundly influence the vigour and species composition of the vegetation that develops after disturbance.
[Bill Freedman Ph.D. ]