There is basically only one filter you really need when photographing in the forest and that's the polarizing filter also called CPL. It's especially useful when reflective surfaces appear. After rain or in sunny forest conditions leaves can turn into shiny surfaces that lose all visible detail in the image. Wet leaf-covered ground also reflects light. To avoid this effect you simply rotate the polarizer until the reflections fade or disappear (never more than 90 degrees as rotating it 180 degrees will bring you back to the original effect). It also shines when water surfaces or droplets are present in the scene.
Another welcome effect of a CPL is the boost in color saturation. Especially in green spring forests or colorful autumn woods the polarizer can bring out richer tones. Polarizers that are designed specifically for this purpose like the “Landscape CPL” (included for example in the V6 filter holder) enhance this effect even more. The “True Color CPL” ensures a neutral color reproduction.
However always check whether you actually like the effect a polarizer creates. Sometimes it can remove the glow or light play in a scene that you might want to keep. You might also prefer a more purist look without any added saturation.
There are conditions in which you don't need a polarizer in the forest. When no sunlight hits the forest floor the leaves are dry or you're shooting in overcast snowy or foggy woods using a polarizer only makes sense in rare cases. Since it reduces the amount of light reaching the sensor and longer exposure times in the forest can be problematic due to wind-blown leaves you need to decide case by case whether a polarizer will actually improve your image.
You won't need a graduated filter in the forest. It would darken the trees and branches in the upper part of the frame but not the lower areas which would look unnatural. In post processing it would be quite difficult to remove that unwanted effect. However there's another filter I've recently discovered for forest photography that I'd like to introduce to you as well.
The Black Mist filter produces effects that depending on personal taste can be helpful for forest photography. It adds a dreamy glow to the highlights in a forest scene and reduces contrast. Especially when there is no natural haze or fog in the forest this filter can enhance the mood of your images. These filters come in different strengths: one eighth one quarter and one half.
