I am not quite sure if the Game Developers Conference led to this video being released, or if it was just a coincidence. This is the sole work of Alexander Dracott, a visual effects, lighting, and shader artist who has been employed at Sucker Punch and Sony Online Entertainment. He works for a studio in Bellevue, Washington, USA doing VR demos, which sounds like Valve but is probably someone else entirely.
Basically, it is a forest scene that is rendered in Unreal Engine 4. It is convincing, despite a little macroblocking from Vimeo compression (or its source). Even the falling leaves cast appropriate shadows. Granted, he never mentions his computer's specifications, which could make a difference in how many features he could get away with enabling. Either way, the art would even be amazing in a non-realtime scene, let alone Unreal Engine 4.
A couple of days later, he posted pictures of the same scene in an autumn time frame (same link). I guess that I should keep coming back to this thread, just in case it gets a Winter update or something. Awesome work!