Dispersal Mechanisms  

            One important aspect of succession is dispersal, the process by which plant species move into a new area.  It would not be advantageous for plants to simply drop their seeds; the new seedlings would then have to compete with the parent plant and with each other.  Plants have developed a number of strategies to enhance the transportation of their seeds away from the parent.  Many species have lightweight seeds which are easily carried by the wind: many grasses and annuals disperse in this way.  Some plants add special structures to aid wind dispersal, for example the "wings" of maple seeds.  Other plant species depend on animals to move their seeds to distant areas.  Many species surround their seeds with fruits which are attractive to birds and mammals; the seeds are not digested and pass out with feces, usually at a site far removed from the parent plant.
           
Fruiting plants can be classified into three based on the time the fruit is available and the type of fruit; summer fruiting species, fall high-quality fruiting species, and fall low quality fruiting species (sometime referred to as winter fruits). Summer fruiting species usually surround the seeds with berries high in carbohydrates and time fruiting so that it occurs late in summer when insects are less numerous.  These fruits are usually eaten by mammals or resident birds, not migrators, and are dispersed within the same general area.  Many of these plants produce small seeds inside of a relatively large fruit (think about strawberries or raspberries) so that the seeds are too small to be a food source for seed-eating mammals and birds.
           
Fall high-quality fruiting species produce fruits that are rich in lipids; these fruits usually do not taste sweet.  Lipids provide more calories per gram than carbohydrates ; these fruits are usually eaten by migrating birds, and may be carried long distances since they ripen just before the fall migration.  These fruits are not attractive to mammals. Because lipids decompose rapidly, any fruits which are not taken by birds tend to rot and fall off the parent plant, so that they are not available during the winter.  in contrast, low-quality fall fruits have a low lipid content, and hence little nutritive value, but are decay-resistant.  They are not eaten by birds in the fall when high-quality fruits are available and remain on the plants until winter.  these fruits are an important food source for resident birds in the winter when other foods are scarce and during the spring return migration.
           
Very few plants produce fruits high in lipids, compared with the number which produce low-quality fruits.  Production of lipids is expensive for the plant, and it requires large amounts of energy to produce the fruits.  Plants which produce high-quality fruits invest their energy to make their fruits very attractive to migrating birds, so that hopefully all of most of the fruits will be eaten and dispersed.  Since each fruit costs a great deal to produce, the plant's strategy is to try to make each one "count".  Low-quality fruits cost the plant little to produce, so a plant can produce more of them.  Low-quality fruits are less attractive to birds and will be rejected in favor of high-quality fruits while the high-quality fruits are available, but many will eventually be eaten during the winter.  Since the plant was able to produce many low-quality fruits, the chances are that at least some of them will be eaten and dispersed.  These two strategies represent two different ways to invest energy to ensure that seeds get dispersed.
           
Please seeds also contain energy to nourish the seedling during germination.  This energy makes seeds attract food sources for birds, mammals and insects.  Plants have a number of strategies to avoid having all of their seeds become someone's meal.  As mentioned above, some species have small seeds which are less attractive to seed-eaters, but many tree species must produce large seeds to nourish the growing seedlings.  Many tree species ensure that some of their seeds will survive by using a strategy known as masting.  These species may go several years producing few or no seeds.  At random intervals, the parent plants will produce a "bumper crop" of seeds.  In many masting species all the individuals in an area will mast synchronously.  The enormous numbers of seeds make it unlikely that all will be eaten by seed predators.  In addition, since few seeds are available most years population numbers of seed-eating animals stay low, so there are too few of them to eat all of the seeds during mast years.  Many conifers, hickories, beeches and oaks are masting species.  During mast years for oaks, squirrels and blue jays cache many acorns, but they are unable to eat all they hide, so that the plant disperses many acorns.

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