When I look at so many maps, I see areas that are forests, areas that are mountains but so many maps (though not all and maybe it is not the majority) don't have an area that is forested mountains. Why is that? How am I supposed to know if a mountain area is forested? Or which part of it is forested? I kind of guesstimate that if it is adjacent to a forest, then part of that mountain area is forested.

I guess the same point could apply to hills and forests and maybe a couple of other terrain combos, but I can't think of any more prominent examples.

Is there something I'm missing? A convention I don't know about?