Disclaimer: am not a biologist :)
Interesting! My wife loves lupins, and we're on the west coast of Canada where they're native as I understand it -- but apparently they can steal resources from other plants where they aren't native (thought they do indeed fix nitrogen)...
This link describes some of the good and bad, only hinting that they're not as good for eastern ecosystems; nothing specific about how they might affect birch or larch:
https://www.cbc.ca/news/canada/new-brunswick/lupins-invasive-species-1.6488784
..and yet, this page here says lupine and birch do better when together (though I just skimmed the article, perhaps Alaskan lupine is different):