It’s bad, but not that bad! Yes, central Oregon is famed for its species “diversity” or perhaps species intergrading. Intergraded species have driven botanists to drink for more than a century. Species require that we wrap our minds around the notion that a species is a population of individuals. Individuals are not identical to one another (except, sometimes, if they are in the same thicket). They aren’t cookie cutter clones. Think of a range of individuals distributed on a bell curve. Some species are more variable that others. Some will jump out, others will have so much variation all you can do is rule out that is isn’t something else, and what is left is this species.
Rosa gymnacarpa is completely distinct. You can ID it or rule it out. It doesn’t form a thicket and lives in the understory. The buds are very distinct and tiny, and the hips are unmistakable, small, tannish orange, and with deciduous sepals. HMF has good pictures.
Although the blooms are helpful for identification, it isn’t because of their outward appearance, the way we can distinguish cultivars. It’s more about the way the sepals coat the calyx tube (do they extend beyond the bud? Are they long and do they cross? Do they bloom in clusters? Of how many? When do they start to bloom and when do they stop?) In my brief experience documenting our local species, you need to observe (ideally photograph) all of these:
plant habit
foliage top and bottom, surface texture, leaf margin and glandularity
buds and form of inflorescence
armature - on laterals, on old basals and on new basals.
bloom scent
hip shape and timing of formation
Rather than despair, it’s much easier to use the tools of native flora groups, herbaria and botanical gardens and rule out what is and isn’t found in that area. Look for a native plant society plant list in your area. That will narrow the field, so you’re not looking for a species that no one has found in that area in 140 years of looking. I have used local native plant lists in two counties to locate stands of species roses, and they are reliable and helpful.
I particularly like the herbarium descriptions linked below and the Oregon State plant list:
Link: biology.burke.washington.edu/herbarium/imagecollection.php