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.
Depressing
dark and gloomy
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.
bleak
Stygian is a Dark, Gloomy place
gloomy, dark
Stygian.
lowering
dark and gloomy
gloomy, dark, dull, bleak
dim, shadowy, cloudy, dark