Willow, and yes birch ('canoe birch' = white). Each is probably preferred where it's more common. Fur trappers knew about birch where willow was rare, and BTW long before Herr Bayer was born.

EG https://www.survivopedia.com/how-to-make-your-own-aspirin-fo...

EG https://www.anniesremedy.com/betula-alba-birch-bark-leaf.php