what will happen if all the seeds of the plant falls under a parent plant

If all the seeds of a plant fall under the parent plant, there will be shortage of space for the germination of the seeds. Even if they germinate, they cannot grow properly due to scarcity of food resources and also space. Thus some of them will fail to grow. Moreover if the seeds fall under the large parent tree, they will also not receive adequate sunlight due to the shadow of the parent tree. Without adequate sunlight, they cannot perform photosynthesis properly. For all these reasons, seed dispersal is necessary.