14 April 2008
Seedling
Posted by bryanc under: biology; nature .
During the cold winter months, below the snow layer, there exists a cache of seeds frozen in the soil. Many of these seeds are small and frail and may stay dormant for many years. Others will experience certain environmental cues that will cause a cascade of chemical and biological transformations leading to a life full of competition for energy and nutrients in order to survive and reproduce. Our subject was recently one such seeds and is now in it’s first and most vulnerable developmental phase; the seedling.
As a result of millions of years of evolution of fine tuning seed dispersal mechanisms; geological uplift, erosion, and glaciation; and a bit of dice rolling by mother nature, our subject was placed on a southern facing slope in a soil bed of glacial till. This is the exact spot where our subject will stay until its death and it is the exact spot where it will need to use all of the tools evolution has equipped it with in order to survive long enough to contribute its own seeds into the soil. This is no simple matter and in fact our subject is at a phase where the death rate is at its highest.
The reason survivorship is so low is because the seedling must establish itself, in a very unforgiving environment, with a limited food supply proportional to its seed size. Our subject came from a small seed. This was not a result of some error or misfortune but rather a result of its ancestor’s life history strategy – to produce many small seeds instead of a few large seeds in hopes that some, being randomly disbursed, will reach an easily habitable environment. In our subject’s case it seems as though the strategy worked. Although the seedling was left with little food, so it cannot cope with strong competition, it was placed in an open area rich with sun and nutrients, thus competition may not be a large factor.
Although competition may not be a large obstacle to success there are still plenty that our subject must overcome such as environmental stress, herbivory and disease. Environmental stress includes frost damage and dessication, or drying out of the plant tissue. In order to get a head start on the competition the seedling needed to germinate and start growth as early as possible. In a dense population of seedlings a day or two head start can mean life or death. The risk of growing early is that the days are shorter and the temperatures are colder. If the seedling grows too early it may experience too many cold nights where ice will form in its cells and will tear and rupture the walls causing certain death. Dessication is another stress and to overcome this the seedling not only needs to put its limited resources into shoot and leaf growth but also root growth. Soil moisture can be rare here in early spring where the majority of it is still locked up in ice crystals at higher elevations so our subject needs to send its roots far into the earth in hopes to siphon up as much moisture and nutrients as possible.
Growing a stem, leaves, and a root system is no easy task especially from a small reserve of food donated by the endosperm of the seed. Our subject must devote all of its initial energy into growth to rapidly gain access to sunlight, nutrients, and water. This leaves little to allocate to defense and so herbivory and pathogens are very real factors that are responsible for many seedling deaths. The seedling cannot move or run so cannot escape approaching herbivores or parasites. It is stuck in one place where it devotes all of its energy and reserves to growth so it can survive another day but could be all for naught if a single deer decides to take a bite in its location or a turkey decides to scratch the ground looking for insects. Our subject is vulnerable and just as many millions of years of random events led up to its position in this world a single random event can cause its doom.
