Because the trees are close together and there are many other plants in between the trees, which block out a lot of light from reaching the ground.
Dark and gloomy
dark and gloomy
Depressing
The chambers of the castle were dark and gloomy. Just because our team lost, there is no need to be gloomy and dejected.
This rainy weather has made everything so dark, depressing and gloomy.
gloomy, dark
bleak
Stygian is a Dark, Gloomy place
black, gloomy,
gloomy, dark, dull, bleak
dark and gloomy
dim, shadowy, cloudy, dark