I just thought I'd come back to this very important subject and explain what I meant by saying, "As gardeners rather than farmers."
The horticulturalists talking about diversity brought to mind the fact that bees may have to fly for miles to find diversity in farming environment. And, perhaps in yours as well Beak, with only "prairie grass and pine trees."
On that UC Berkeley webpage however, note that having attractant plants in,
at least, 1 square meter, area made a large amount of difference. Where there was only a tiny "spot" of something flowering that the bees appreciated, the flowers went ignored by our bee friends, for the most part.
It shouldn't take a huge area to bring in the bees but I can just imagine a patch of beebalm, ammi major, salvia and allyssum absolutely aswarm on a warm Summer day

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Coming back today also gives me the opportunity to use these smilies: :rainbow-sun :bee

Steve's digits