Answer:
change in environment
Step-by-step explanation:
During ecological succession, short type of tree get replaced by taller tree because the environment is in favor of the taller trees. The suddenly and drastically change that occurs in an ecosystem is the main reason for ecological succession. In this ecological succession, the old species becomes vanished and replace by new species which has the ability to survive in that changed environment. The short type of tree get replaced by taller tree because the shorter trees did not get sunlight and nutrients from the soil so tall trees takes their place. The tall trees are more competitive that short trees.