Just like most of the shards that are set outside, if you are aware of the map marker for this one, you’ll find it easy. The lake we are looking for is the largest one in Vvardenfell. It is also south from the Morrowind expansion “landing point” of Seyda Neen.
Once you find yourself at Lake Amaya, head to its northeastern part. There is a small road following its shoreline. The stump with the shard is just next to another tree, one with a weird looking trunk. Some of its parts almost branch out at a right angle. From this place, you can spot the house with four floors on the other side of the lake. The rooftop of this house almost reflects the image of the large mushrooms found nearby.
Next to the strange tree.
Sand Kingdom Moons - Super Mario Odyssey Power Moons