The rimlighting on the trees makes the leaves shinier along the "sunny trail". This simulates a poor man's subsurface scattering; basically, light that hits a semi-translucent surface will have some light pass through while some will bounce off as specular shine and diffuse color. So, the brightened leaves act like the light behind the leaves passing through. Had to tweak and tone this down some. The problem is while I was able to isolate just trees from the grass/tree shader, the tree section impacts both tree trunks and branches/leaves. Also, the masking I use to highlight a bit of the leaves while not doing all of it uses a grayscale of the leaf color. So, pine trees are dark, and needed a bit of punch-up while bright green birch leaves can be too bright. And, didn't want tree trunks to look too bright along edges (esp since birch trees are white, and create a very bright grayscale). So, had to mess with it a bunch. It's basically trying to weave straw into gold, and ending up with brass. But, it looks nice in some places. With a bit of bloom added in, it looks like sunlight is diffusing through trees into the shadows... and the vehicle having some rimlighting happening, even in shadows, helps sell the effect.