Brooke,
I'm going to speak from my
experience in the Arizona mountains, which is high desert but probably not quite as cold and at a lower latitude. I'd think about introducing some drought tolerant nitrogen fixing shrubs in the understory as a start: False Indigo (Amorpha fruiticosa), your
native ceanothus are a couple. But you'll need water to plant, or plant with the rain.
Some tough shrubs for understory: Golden Currant (Ribes aureum), Wild
Rose (Rosa spp.)
Tough flowers for sunnier margins: Red Hot
Poker Flower (Kniphofia Uvaria), Clary Sage, Red Tipped Yucca
HEAVY mulch: woody debris will do. You can place it on contour, like a
swale above or below the tree, and seed native wildflowers and shrubs, even squash seeds where the water will collect when it rains.
How about Wolfberry (Lycium pallidum)? It's like a native version of the Goji Berry. That, and even cactus (prickly pear, agave) could grow on the sunnier margins of the orchard, for windbreak and soil coverage. Once you can establish protected microclimates with the hardier species mentioned or their relatives, then you will have the conditions for more tender plants.
Good luck!
p.s. Are you the Brooke Medicine Eagle that had an album of lullabies? If so, my daughter used to listen when she was little
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