Tip of the day: Although "Intersect Underlying" doesn't work for distributing populations [EDIT: actually it does, but is tricky to set up], you can create your own "favour rises" or "favour depressions" masks using just 3 nodes. Simply subtract "Get altitude in texture" from "Get altitude". This measures how much the terrain rises above a smoothed version of itself. The smoothing scale is determined by the "patch size" in the Compute Terrain node. To select depressions/gullies instead of rises/ridges, swap the inputs to the Subtract node.
Bonus tip of the day: Rock populations are an easy way to create distant vegetation without fiddling about with more complex objects and materials.
CORRECTION TO SCREENSHOT: The setup should use a Distribution Shader V4, not a Surface Layer, because the distribution shader's colour defaults to white (1). If you don't care about the altitude and slope controls you can just pipe the blue nodes directly into the populator.