I would get rid of the water hogging Acer macrophyllum right off the bat. Wild Hazel next. Blackberries, of course. Then plant some evergreen trees.
Our native rhododendrons grow under a partial canopy of Doug firs and hemlocks.
Those areas of 'naturalized' rhododendrons growing in English woods have had a lot of woods editing to achieve what looks like they're naturalized.
The variety R. ponticum is considered an invasive weed in most parts of England.
Mike

will you ever be able to water them???? .. do you live there??? .... have water available ....

or are we trying to talk about a plant and forget system????

ken

Did you plant outside or inside the DougFir dripline?

And the blackberry is (!) because there are no others anywhere nearby? Are these Himalayan-type blackberries or are they the trailing dewberry?

Available water would make things a lot easier to get new plants established. I have a waterline about 900 ft long. I don't use it much, but it's there when I need it.
Mike

Plant in fall and mulch with wood chips. Choose locally adapted native species that have demonstrated an ability to be easily cultivated. Independent garden centers here continue to devote space to specific sections of native plants, albeit of varying ease of cultivation and horticultural interest - look around in nearby natural areas and parks for possible choices.

I was up near west side Mt Hood a few weeks ago for a few days. Rhododendrons were the main understory for a few miles along Hi-way 26. It's just past a place called ZigZag and it's called...Rhododendron! I wish now I had taken a few pictures. Drive up there and take a look to see the environment in which native rhododendrons thrive.
I suppose it's a little damper and cooler than where you are, but way rockier. Those rhododendrons are growing in duff on old gravel bars in the shade of firs and hemlocks. The only maples are vine maples.
Mt. Hood sure is a beautiful mountain.
Mike

zigzag/rhododendron get 70 inches plus of precipitation, i think yamhill county might get slightly more than half of that.

you could naturalize rhodies but they will probably need more than a year or so of help with supplemental water.

i would be inclined to not think of the contemporary native plant choices, but think ahead to the future- slightly drier and slightly warmer. i would try to get a oak savanna or klamath mountain type plants in there.

to get the culture going i would selectively thin out the existing forest, secondary growth is usually a bit too crowded and it will make establishment easy. rent a chipper and keep all the material on site. cut the logs to manageable size and keep as much as you can there.

some plant species off the top of my head

big trees trees

garry oak, madrone, incense cedar, brewer spruce, myrtle.

small trees/large shrubs

garrya, evergreen shrubby oaks, various ceanothus, big manzanitas

shrubs/understory

mahonia (native and exotic), huckleberries, various ferns, smaller manzanitas, salal

Thank you everyone. To answer a few questions: yes we live on site, but no, there is no water to this steep area. We get about 62 inches of rain a year, more than the valley floor by quite some. The blackberries are Himalayan.

Trails are the first thing you build.

Is it steep up or steep down from where you start? What direction does the steep face? It makes a big difference in what you can plant without available water. That, and the amount of shade or sun.
My property gets pretty steep down from the house in a few places and faces north and east.
Mike

This is the time of year to consider buying bare root plants from your local conservation district. I did a quick search and there is one near Portland if not closer to you.

In my situation, it worked best to pot grow some things for a year to two to get them large enough to compete with the other native trees and shrubs. If you are planting in quantity you will save a lot of money too.

you can also buy seed mixes for naturalizing. like this one and below
http://sunmarkseeds.co/fir-forest/

Here is a link that might be useful: like this one