Otters secrete thick oils in their fur that help them to be more hydrodynamic and water resistant. Without some kind of cleaning regime, they would like sloths grow plant matter in their persistently moist fur. Whether it be algae, fungi, moss, or other kinds of small plants.
The tree has a circle of absence around it in part because the tree excretes an herbicidal resin; this is a common property of trees particularly conifers.
The otters are rolling around in the dirt around the tree to clean themselves of microscopic insects with the coarse silicone (like with diatomaceous earth) and to coat themselves in herbicidal resins to keep them from growing things.
