And here's a simple screenshot of the lighting setup (Fig.18).
For the Global Illumination I used Irradiance and Light Cache. The final image took roughly six hours to render at 1920 x 1080.
I've also included an alternate render in which the lightning was also modeled in 3D, but which I didn't use (Fig.19).
After the render, I did a fair bit of painting in Photoshop to achieve the final look. The lighting generated by the trees was the major challenge for me. Originally I wanted to try and do it in 3D but eventually realized that it would be better if I painted it. I also painted some additional fog and floating particles in the air.
Since I am not very good at describing the painting process, I've included a strip of images to show the progression (Fig.20 - 21).
Well, that was how I went about creating the image. I hope the information was helpful. If you have any further questions feel free to contact me at: firstname.lastname@example.org