Concept:Ecological succession is the natural, gradual replacement of one plant community by another over time.
Explanation:When a farmland is left fallow and abandoned, it does not remain bare for long.
First, grasses and weeds colonise the exposed soil.
Over the years, shrubs begin to grow among the grasses.
Later, seedlings of trees appear and gradually form a young forest.
This step-by-step, orderly change in the plant community continues until a stable climax forest develops.
Because this process involves a series of communities replacing one another in sequence, it is known as ecological succession.
The entire process from abandoned field to mature forest is a classic example of old-field succession.
Answer:The phenomenon is called succession.
Therefore, the correct option is A. Succession.