Well I think it depends on how anal you are about definitions... Actually, this is probably a good subject for Michael -- similar to snapshot/postcard/fine-art... And to be sure, there is a lot of room for cross-overs
IMO only and would like to hear other opinions:
Landscape implies features of nature in a broad spectrum, but can include cityscapes, seascapes, sunsets, single trees, boats in a harbor, folks enjoying the outdoors or groups of anmals in their outdoor habitat.
OTOH, nature implies closer-up shots of trees, plants and animals, including macros of our natural environment. Of course there is pure macro too, which I guess would be anything shot close up...
My .02 only, and ultimately I'm not sure it matters