I am in Iceland, here downy birch is the native tree. Next to it grow dwarf willows, lady mantles, meadowsweet, angelica, sometimes nettles and valerian, currants, crowberries and blueberroes, and lupins.
Birches do not have big roots, so they are good to grow around them. They also do not shade much, so another nice thing for your vegetables.
I have a birch on the edge of my garden. Plants grow fine nearby.
They probably accumulate nutrients but I am not sure if they are nitrogen fixers. Probably not. They are however pioneer trees, after the other smaller vegetation has settled.
in Portugal, sheltered terraces facing eastwards, high water table, uphill original forest of pines, oaks and chestnuts. 2000m2
in Iceland: converted flat lawn, compacted poor soil, cold, windy, humid climate, cold, short summer. 50m2