Plantings under Alder Trees

I am looking at trying to grow ground cover under a alder stand that I have. Purpose would be for running chickens on it. size is about 1/2 acre. Tree would be thin over the years for fire wood. Location is PNW Washington Hood Canal area/ Any ideas what would grow under them.
Thanks in advance for the help.

I found not much grew under them, grass did but wasn't as good as the grass out in the open, red currents managed but again not as well as those in the open, and alder suckers like mad so it makes it's own groundcover.

In my experience, not a lot grows underneath them naturally, in the PNW. That said, huckleberries and salal do well around the margins of alder stands. Claytonia covers a lot of ground underneath too!

Alders are our primary native nitrogen fixators, and host fungus beneficial to most native conifers. Thimbleberries, ferns, wild blackberries, salmon berries and raspberries, salal, currants, elderberry, nettles, devils club, sorrel, gooseberry, hazelnut, are edibles that grow underneath in the wild, doing best in edges where they get some sun. Here in nw California, tanoak, madrone, big leaf maple and vine maple all succeed alder as understory trees as the shade intolerant alder it gets displaced  by conifers.