Which type of wetland is constantly submerged in water and contains trees ?

... Swamp ... (Apex)

A wetland refers to a different ecosystem, which is flooded by water, either seasonally or permanently, where the oxygen-free procedures exist. The main element, which separates wetlands from the other water bodies or landforms is the characteristic vegetation of aquatic plants, amended with the specific hydric soil.  

Swamp refers to a wetland ecosystem featured by mineral soils with poor drainage and by plant species dominated by trees. The swamps are found all through the world, most usually in low-lying regions with poor drainage adjacent to rivers that supply the swamp with water. Some of the swamps develop from marshes, which gradually fill in, permitting woody shrubs and trees to develop.  

Usually, Populations have a genetic variation among individuals, which is important to the population's ability to survive in different situations that have an effect on the natural selection. There are different factors which can increase or decrease the genetic diversity, and influencing natural selection.

Explanation: These factors include an environmental factor, one of the major factors, which may cause changes in the genetic variation of a population and influence the natural selection.