rush or sedge
菼 refers to a type of grass-like plant: a rush or sedge, typically found in wet areas.
菼 combines the grass radical 艹 with 炎, which likely contributes a phonetic element. The exact historical development is uncertain, but the structure suggests a plant name.
The grass radical 艹 sits above 炎 (flames). Imagine a tough rush or sedge growing in a marsh that can survive even when the area is set ablaze.
For タン, picture a tan-colored rush plant: tan -> タン, and the plant's color matches the dry marsh grass.