Chainsaw.
As said above misty morning with a thin canopy.
Don't do the smoke machine idea said above. A great quote is "leave nothing but footprints, kill nothing but time".
You don't know what effect the smoke has on the bird life and insect life etc. May sound trivial but it's unethical photography imo.

Hi Guys, i'm trying to take some photos in woodland but struggle with getting the sunrays through the trees. Any tips?
Thanks
Later in May, a verdant woodland floor will give up an awful lot of pollen, wild garlic etc if you can find woods near a river or stream.
The backlit pollen is enough 'pollution' to show up the sunlight.
I am pleased no-one has suggested a 'plugin' though.
That's an interesting one, I'll look out for that or is not visible to the naked eye and can only be seen by the camera?