Definitely some form of HDR or extreme shadow recovery.

Look at the shadow of the tree on the grass, in a regular photograph it would be rendered almost pure black if you were shooting into the sun like that, instead it is only a few shades darker than the grass next to it under the sun. Now some fill light will be hitting the shadow area via the leaves, but not enough to get the result seen.

The same is true of the unlit side of the branches/trunk.

You don't need a variable ND filter - you're not aiming for any sort of long exposure. All you need is a tree, sun low in the sky (morning or late afternoon) multiple exposures and software to merge them.

There is no filter that can tame a wide dynamic range in all cases. A graduated ND wouldn't be of much use as it's not just the sky you want to darken, you want to darken the sky around the tree branches leaving detail in the branches themselves. This would be almost impossible to pull off without digital post production, even with dodging and burning in the darkroom.

So you are left with some sort of local brightness enhancement or exposure blending in the digital darkroom as the only option.