Thicket
Coppice
A synonym for the word "dense" is the word mass or the word volume
Rainforest
A dense forest is an area of land covered with thick vegetation, typically comprised of closely packed trees, bushes, and other plant life. This type of forest is characterized by limited sunlight penetration due to the dense canopy, creating a dark and humid environment with rich biodiversity.
Heavy - or - dense
The opposite of dense could be thin, sparse, uncompressed, lightweight, or rarified (in the case of gasses).For populations, the opposite could be scattered or meager.
The forest was densely populated with tall trees, creating a dark and mysterious atmosphere.
The trees are hot
The word coppice means a dense area of bushes or trees. It often refers to a field or area of trees that is routinely trimmed down to ensure a continuous supply of growth.
The word “jungle” is popularly used to refer to a dense, tangled mass of tropical vegetation. It usually means a dense tropical growth with trees, vines, shrubs and undergrowth that can be difficult to get through. Jungles are typically located in warm, humid regions and are renowned for their rich biodiversity and verdant surroundings.
tropical rainforest
It means that there is much mass per unit volume. (in relation to density)