I haven't seen a formal definition, but I think most would consider landscapes to be largely a subset of nature photography.
The part of landscapes that would not apply would be city scapes or other urban and suburban architectural landscapes, but even these get into a gray area.
Competitions frequently have to define something more specific, so whether or not manmade structures, wildlife, and digital creations are included in landscapes is a matter of choice. I've seen places where HDR and Panos were not considered part of the landscape criteria. I've even seen infrared landscapes excluded. And I don't necessarily agree with these distinctions, but they may represent other categories.
Thank you, Eric. This question came up because I was going to give a program on nature and landscape photography at a wildlife refuge and while I had my own ideas of the differences and similarities between the two, I was looking around to see if there were actually specific definitions in use. The only specific definitions I found were with the Photographic Society of America (PSA), but they were in relation to PSA competitions and seemed a bit too specific and strict for my purpose and philosophy.
I was surprised to find NANPA didn't define nature photography. I've been a member for years and hadn't looked for a statement before. I supposed we all have our concept of what nature photography is and an "official" definition could create confusion and unnecessary debate.
I think your comment about unnecessary debate is on target. Much of nature photography is pretty obvious, but the edges and areas of debate probably detract from the main idea which is to create great photos. Invariably the definition would come down to the size of the manmade structure in the image, or the size of the person.
There is some specific information on ethics and ethical captioning. It's probably more important to represent the facts behind an image. I've seen some fabulous images in PPA competitions that are purely digital creations - and correctly represented as such.
We have enough debate on whether captive animals or captive animal photos are okay.