The seed bank of a forest

Much of our wooded land has been forest for up to 100 years or so. Over that time trees, shrubs, and other plants have gone through their yearly cycle of growth including dispersal of seed.

The seed that is produced is sometimes lost because it is eaten by wildlife, germinates, etc. but can also be deposited on the ground and become part of the seed bank. Most seed that finds its way into the soil remains dormant for a time. The seed may remain dormant for extended periods to help ensure that germination occurs when conditions are suitable for the survival of the young seedling.

The dormancy may be in response to a hard seed coat that has to be broken down before water and oxygen can promote germination, or it might be chemical: the hormone that is inhibiting germination needs to be degraded before germination can take place.

Some seeds such as those of the white oak have a very short period of dormancy; they begin germinating soon after they fall off the tree. Other trees like the black birch have seed that can remain dormant in the understory of a forest for a number of years until it is sufficiently warmed by sunlight to allow it to germinate.

However, once a sufficient opening occurs that allows sunlight to reach the forest floor, the dormant seed may germinate and a young seedling will develop. The young birch seedling might be able to take advantage of the light gap opening and grow to a place in the canopy. Most of the invasive plants have seeds with a lengthy viability period. The autumn olive can stay viable in the soil for up to 15 years if necessary. These plants can wait out undesirable conditions, then grow quickly once they improve.

A seed bank can be found throughout local forests. Often, seeds of invasive plants can survive for many years.

Credit: Unknown

The amount and kind of seeds found in a seedbed are dependent on a number of factors such as the type of plants present in the forest, amount of shade present, the slope of the ground, disturbances to the forest, history of fires, and wildlife present. In general, sites that are dry and steep do not have an extensive seed bank since the force of gravity pushes the seeds down slope.

On the other hand, level areas at the foot of the slope have more moisture and a larger seed bank. These extensive seed banks often have an abundant supply of invasive plants that are waiting for optimal conditions before they germinate and grow. That is why disturbances in optimal growing sites often result in very lush growth of plants, many of which might be invasive.

Recent studies show that the vegetation present may impact the seed bank. In particular, forests that are overrun with stilt-grass have a greater probability of higher intensity ground fires during a forest fire due to the flammability of the dried grassy fuel during the dormant period. Researchers have found that surface fires exceeding 300 degrees kill most seed in the seedbed except for fire-dependent serotinous seed and hearty seed such as autumn olive. The forest is a complex ecosystem with many layers of growth in the overstory, understory, on the surface, and in the soil.

Article by FCFCDB

Nature Note for 11/23/14