Natural Seed Banks
There are manmade seed banks in the news lately, but nature has had its own seed banks for millions of years. A natural seed bank is composed of the dormant seeds that lie in soil or sediment that have the ability to germinate and become healthy adult plants once conditions become favorable. Seedbanks in lakes are found in the sediment, and contain submergent, emergent and floating-leaf plants.
Counterintuitively many aquatic and wetland plants require drought conditions to reproduce sexually via seed. In terrestrial environments the seedbank can be activated when soil is disturbed through burrowing animals, fire, landslides and other disturbances. The seed bank of agricultural fields is constantly activated through farming activities. Fighting weeds in farm fields is a constant battle because the field may contain 1,800 -496,000 weed seeds per square meter! Number of seeds varies widely depending on farming practices and the crops grown. Not all the seeds are viable though.
Seeds “know” when environmental conditions have become favorable for growth through a combination of natural ques that can include: temperature (both day and seasonal cycles), light intensity, light wavelengths, oxygen concentration, and hormones. The breaking of seed dormancy is poorly understood for most undomesticated plant species. However, it is known that temperature plays and import role in most seed germination. The seeds of many temperate plant species require a period of freezing. Warmer winters brought on by global climate change could prevent many species from breaking dormancy in the southern portions of their ranges.
For most species we do not know how long seeds remain viable, but we know that viability can vary greatly. Wild Rice (Zizania spp.) seeds remain viable in the seed bank for only 5-7 years. In the same seed bank, the American Lotus (Nelumbo lutea) seed may lie dormant for a century before sprouting into a new lotus plant.
The seeds of Hardstem bulrush (Schoenoplectus acutus) are known to remain viable in sediments of lakes and wetlands for 40 years. They lie in wait for a drought, because they can only germinate and grow on moist soil and sand. Those seeds may have to last decades before the right conditions occur. This just one reason why maintaining stable water levels in reservoirs is detrimental to aquatic and wetland plant life. Reservoirs that have experienced draw downs after decades of having stable water levels may suddenly colonized by species that haven’t been seen in many years because of the seeds that have remained viable in the seed bank.
The maximum amount of time a seed can remain locked away in a seed bank is not really known because it can be difficult to age seeds. The oldest confermed, naturaly germinated mature seed comes from a 2,000 year-old Judean Date Palm (Phoenix dactylifera) found in Herod the Great’s Palace, but an archeological dig is not a seed bank. Other seeds germinated have been claimed to be much older, but some of those had to be grown in conditions that can only be created in the lab and not nature.
Seeds become damaged through time from changes in temperature and moisture found under most natural conditions. Permafrost is probably the best place to find the oldest seeds. Perhaps as the climate warms and melts the permafrost we will see species that haven’t been encountered in thousands of years, but it doesn’t seem likely. Eventually the earth’s natural background radiation damages the DNA of the seed’s embryo causing damaging mutations that prevent growth.
Man-made Seed Banks
Today we use seed banks to preserve diversity of agricultural crops and non-domesticated plants. The huge Svalbard Global Seed Vault in Norway is a so called doomsday vault, because it hold seeds that could provide unique genes if some catastrophe wipes out a species of variety of plant. There are many other seed banks are used by plant breeders and researchers on a continual basis.