Thank you all for your replies. This variety of Elderberry indicated that it wanted it sunny and moist. It could use a bit more sun, but it's definitely not dry. I did wonder if it might be voles, since I've had them in that area before, but I haven't seen any of their classic signs. The mulch is fairly thin (1" organic bark type). I've looked at the bark around the base, and it's completely normal, so I'm leaning towards something like (either small root boring or furry root eating) underground pest.
I must say that I love the user name, if I've translated it right..."Evil Chicken"?
Thanks again,
Jennifer
